A method and system for calculating current-limiting virtual impedance parameters of a droop control inverter

Abstract

The present invention belongs to the technical field of grid-connected converters and discloses a method and system for calculating virtual impedance parameters for current limiting in a droop-controlled inverter. The method comprises the following steps: S1: presetting fault parameters of a grid fault and an initial value of a virtual impedance used by the droop-controlled inverter to control the fault current; S2: calculating the fault current of the droop-controlled inverter based on the fault parameters and the initial value of the virtual impedance; and S3: obtaining a fault current limit value corresponding to the grid fault, comparing whether the fault current value is less than the fault current limit value, and if not, increasing the initial value of the virtual impedance and repeating step S2 until the obtained fault current simulation value is less than or equal to the fault current limit value. The present invention improves the success rate and accuracy of fault current and current limiting virtual impedance parameter calculations, reduces algorithm complexity, and ensures the safe and stable operation of the droop-controlled inverter.

Description

Technical Field

[0001] The present invention belongs to the technical field of grid-connected converters, and in particular relates to a method and system for calculating current-limiting virtual impedance parameters of a droop control inverter. Background Art

[0002] As renewable energy sources such as wind power and photovoltaics are integrated into the grid through inverters (a phenomenon also known as renewable energy grid integration), the structure and characteristics of the power system are undergoing changes. Droop control inverters are widely used in scenarios with a high proportion of renewable energy grid integration due to their advantages such as no communication requirements, rapid voltage and frequency support, stable island operation, and high stability in weak grid conditions.

[0003] However, it's worth noting that compared to synchronous generators (which, as traditional power generation equipment, can handle large current surges), inverters have relatively weak overcurrent capabilities. In particular, during grid faults, the voltage source characteristics of droop-controlled inverters can expose them to severe overcurrent threats, potentially damaging the inverter itself. Although droop-controlled inverters exhibit similar voltage source operating characteristics, their fast dynamic response, driven by power electronics, significantly influences the fault current characteristics of their control structure. Specifically, the inverter's input power is closely related to the terminal voltage and its phase angle at the grid connection point. The grid connection point is the point where the inverter connects to the grid, and the phase difference between its voltage phasor and the reference voltage phasor is the phase angle. However, these two key factors are complexly affected by the control mechanisms of the droop outer loop and the voltage-current dual closed loop. In microgrids or distributed systems with multiple power sources in parallel, the droop outer loop regulates the output voltage or current of the power source to maintain system stability and power distribution. The voltage-current dual closed loop ensures the stability and accuracy of the power system's output voltage while limiting the current to a safe range. This complex control structure and its parameter variations significantly impact the fault current. Therefore, in-depth research on the fault current characteristics of droop-controlled inverters is crucial to ensuring the safe and stable operation of systems with a high proportion of renewable energy. Given the strong coupling between the input power and terminal voltage of droop-controlled inverters, analytically expressing the fault current becomes particularly difficult. To address this issue, some literature has examined the fault current characteristics of droop-controlled inverters. In Shuai et al. (Z. Shuai, C. Shen, X. Yin, X. Liu, and Z. S. Shen, "Fault Analysis of Inverter-Interfaced Distributed Generators With Different Control Schemes," IEEE Trans. Power Del., vol. 33, no. 3, pp. 1223-1235, June 2018), they proposed an iterative fault current calculation method that intentionally accounts for the strong coupling between the input power and terminal voltage of droop-controlled inverters. Through small-step iterations, the voltage and phase angle data before the fault are used to derive the voltage and phase angle at the next moment according to the droop control equation, thereby calculating the current magnitude after the fault.Similarly, Zhao et al. (H.Zhao, J.Ge, Z.Shuai, Y.Cheng, J.Lei and J.Shen, "An Asymmetrical Fault Current Iterative Algorithm of Droop-controlled Inverter," in Proc.IEEE Energy Convers.Congr.Expo., 2019, pp.2219-2223.) also used an iterative method to construct a current calculation method under asymmetric faults, and revealed the main influences of control parameters, line impedance and grid voltage on the steady-state component of the fault current, while the transient component is closely related to the line impedance ratio and the fault start time. Dag et al. (B. Dag, A. R. Boynuegri, Y. Ates, A. Karakas, A. Nadar, and M. Uzunoglu, "Static Modeling of Microgrids for Load Flow and Fault Analysis," IEEE Trans. Power Syst., vol. 32, no. 3, pp. 1990-2000, May 2017.) proposed a static model for analyzing the fault current characteristics of an inverter-based power supply. However, the impact of the droop control structure and parameters on the model was not explored in depth, so its applicability in droop-controlled inverters is questionable.

[0004] In summary, existing methods for calculating fault current in droop-controlled inverters still have certain drawbacks. First, most of these methods fail to consider the role of virtual impedance control in limiting overcurrent, limiting their applicability in fault situations. Second, although these methods analyze the impact of factors such as grid voltage, line impedance, and fault onset time on fault current, these circuit parameters cannot directly guide the design of virtual impedance parameters, resulting in poor practicality. Summary of the Invention

[0005] Based on the above-mentioned shortcomings and deficiencies in the prior art, one of the objects of the present invention is to at least solve one or more of the above-mentioned problems in the prior art. In other words, one of the objects of the present invention is to provide a method and system for calculating the current-limiting virtual impedance parameters of a droop control inverter that meets one or more of the above-mentioned requirements, so as to improve the success rate and accuracy of the calculation of fault current and current-limiting virtual impedance parameters, reduce the complexity of the algorithm, and ensure the safe and stable operation of the droop control inverter.

[0006] In order to achieve the above-mentioned object of the invention, the present invention adopts the following technical solutions:

[0007] In a first aspect, the present invention provides a method for calculating current-limiting virtual impedance parameters of a droop control inverter, comprising the steps of:

[0008] S1, preset fault parameters of the grid fault and the initial value of the virtual impedance used by the droop control inverter to control the current;

[0009] S2. Calculating a droop control inverter fault current based on the fault parameter and the initial value of the virtual impedance, wherein the step of calculating the fault current includes:

[0010] S2.1. Calculating a maximum virtual power angle based on the initial value of the virtual impedance;

[0011] S2.2. Calculate a virtual power angle based on the fault parameters until the obtained virtual power angle is no greater than the maximum virtual power angle, and record the value corresponding to the virtual power angle as the steady-state value of the virtual power angle;

[0012] S2.3. Calculating a fault current value when a power grid fault occurs based on the steady-state value of the virtual power angle;

[0013] S3. Obtain the fault current limit value corresponding to the grid fault, and check whether the fault current value is not greater than the fault current limit value. If not, increase the value of the initial value of the virtual impedance and repeat step S2 until the fault current value obtained is less than or equal to the fault current limit value, so as to obtain the virtual impedance corresponding to the droop control inverter and its corresponding fault current.

[0014] As a preferred solution, the preset fault parameters of the grid fault and the initial value of the virtual impedance of the droop control inverter for controlling the current in step S1 include:

[0015] Preset grid voltage U g , grid side resistance R g , grid side inductance X g , active power reference value P0, reactive power reference value Q0, voltage reference value V0, reactive-voltage droop coefficient n;

[0016] The preset virtual impedance initial value includes the virtual resistance R v0 and the initial value of virtual inductive reactance X v0 .

[0017] As a preferred solution, the step S2.1 of calculating the maximum value of the virtual power angle based on the initial value of the virtual impedance includes the following steps:

[0018] The expression for the output power of the droop control inverter is obtained as Equation 1;

[0019] An expression for the virtual internal potential amplitude is established based on the reactive-voltage droop coefficient n, the voltage reference value V0, the reactive power reference value Q0, and the equation 1, which is recorded as equation 2;

[0020] Based on the above formula 1 and formula 2, the expression of the virtual internal potential amplitude with respect to the virtual power angle is obtained, which is expressed as formula 3;

[0021] Substituting Equation 3 into Equation 1, we can obtain the virtual power angle characteristic expression considering virtual impedance, which is recorded as Equation 4. Based on Equation 4, we can calculate the maximum value of the virtual power angle δ. refmax .

[0022] As a preferred solution, step S2.2 calculates the virtual power angle based on the fault parameters until the obtained virtual power angle is no greater than the maximum value of the virtual power angle, and records the value corresponding to the virtual power angle as the virtual power angle steady-state value, including the steps of:

[0023] Obtain the output power reference value P0 of the droop control inverter;

[0024] Based on the output power reference value P0 and the expression of the droop control inverter output active power in the formula 1, the virtual power angle characteristic expression is obtained as P0=f(δ,R v ,X v );

[0025] The virtual power angle is solved based on the virtual power angle characteristic expression. If the obtained virtual power angle is greater than the maximum value of the virtual power angle, the virtual power angle is recalculated until it is less than or equal to the maximum value of the virtual power angle.

[0026] As a preferred solution, the step S2.3 of calculating the fault current value when the power grid fails based on the steady-state value of the virtual power angle includes the following steps:

[0027] Substitute the steady-state value of the virtual power angle into the formula (3) to obtain the internal potential amplitude, and then calculate the fault current I according to the phasor relationship. t Size:

[0028]

[0029] Where, E ref , δ ref are the amplitude and phase angle of the virtual internal potential, Z Σ is the equivalent total impedance modulus.

[0030] As a preferred solution, the formula 1 is:

[0031]

[0032] in, In formula 1, Pe , Q e are the output active and reactive power of the inverter grid-connected point under droop control, P i , Q i are the output active and reactive power of the virtual internal potential point, E ref , δ ref are the amplitude and phase angle of the virtual internal potential, U g is the grid voltage, R g 、X g are the grid side resistance and inductive reactance, R v 、X v are virtual resistance and virtual inductive reactance respectively, R Σ 、X Σ are the equivalent total resistance and total inductive reactance, Z Σ is the equivalent total impedance modulus.

[0033] As a preferred solution, the formula 2 is:

[0034] E ref =V0+n(Q0-Q e );

[0035] In formula 2, V0 is the voltage reference value, Q0 is the reactive power reference value, and Q e It is the reactive power output size of the inverter grid connection point for droop control, and n is the reactive-voltage droop coefficient.

[0036] As a preferred solution, the formula three is:

[0037]

[0038] Denoted as E ref =g(δ), where

[0039]

[0040] As a preferred solution, the formula (4) is:

[0041]

[0042] The formula 4 is P e =f(δ,R v ,X v ), the maximum value of the virtual power angle is obtained by the virtual power angle when the virtual power angle characteristic curve reaches the extreme value, that is:

[0043]

[0044] Where, δ refmax is the maximum value of the virtual power angle corresponding to the virtual power angle characteristic curve.

[0045] In a second aspect, the present invention provides a system for calculating a current-limiting virtual impedance parameter of a droop control inverter, based on a method for calculating a current-limiting virtual impedance parameter of a droop control inverter described in the first aspect:

[0046] It includes a preset module, a calculation module, an acquisition module, a fault current comparison module, and a virtual impedance iteration module; the preset module is used to preset the fault parameters of the power grid fault and the initial value of the virtual impedance used by the droop control inverter to control the power grid current;

[0047] The calculation module includes a virtual power angle calculation unit, a virtual power angle comparison unit, and a fault current calculation unit; the virtual power angle calculation unit includes a maximum virtual power angle calculation subunit and a virtual power angle calculation subunit;

[0048] The maximum virtual power angle calculation subunit calculates a maximum virtual power angle based on the initial virtual impedance value; the virtual power angle calculation subunit performs virtual power angle calculation based on the fault parameter until the obtained virtual power angle is no greater than the maximum virtual power angle, and records the value corresponding to the virtual power angle as a steady-state virtual power angle value;

[0049] The virtual power angle comparison unit is configured to compare the virtual power angle calculated by the virtual power angle calculation unit with the maximum value of the virtual power angle;

[0050] The fault current calculation unit calculates the fault current value when the power grid fails based on the steady-state value of the virtual power angle;

[0051] The acquisition module is used to obtain the fault current limit value corresponding to the power grid fault;

[0052] The fault current comparison module is used to compare the fault current value with the fault current limit value;

[0053] The virtual impedance iteration module increases the value of the initial value of the virtual impedance based on the fault current value being greater than the fault current limit value and jumps to the calculation module to recalculate until the fault current value is less than or equal to the fault current limit value, so as to obtain the virtual impedance corresponding to the droop control inverter and its corresponding fault current.

[0054] Compared with the prior art, the method for calculating the current-limiting virtual impedance parameters of the present invention significantly improves the accuracy and practicality of calculating the fault current and current-limiting virtual impedance parameters of a droop control inverter. Specifically, the present invention has the following beneficial effects:

[0055] 1. This invention innovatively incorporates virtual impedance as a key analysis factor, resolving the application challenges of traditional methods due to their neglect of virtual impedance. This not only provides a new perspective for fault current characteristic analysis, parameter setting, and fault ride-through control method optimization, but also promotes the advancement of related technologies and meets the needs of practical applications.

[0056] 2. This method is highly universal and unaffected by changes in the virtual impedance control structure. It can effectively evaluate the impact of virtual impedance on fault current. It is suitable for analyzing fault current magnitudes under various virtual impedance control structures, providing strong support for accurate fault current calculation.

[0057] 3. Through automatic iterative calculations, the present invention can quickly determine the virtual impedance parameter range that meets current limiting requirements, providing clear guidance for parameter setting. This ensures the safe and stable operation of the droop control inverter, bringing significant economic benefits and broad application prospects.

[0058] 4. Compared to traditional methods, this method directly solves the virtual power angle characteristic expression that takes virtual impedance into account, improving the calculation success rate and accuracy while reducing algorithm complexity. Furthermore, this method can also be used for virtual power angle stability analysis, providing an effective means for power system stability assessment.

[0059] Further or more detailed beneficial effects will be described in conjunction with specific examples in the specific implementation manner. BRIEF DESCRIPTION OF THE DRAWINGS

[0060] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0061] Figure 1 It is a schematic diagram of an application scenario of the method for calculating the current-limiting virtual impedance parameters of a droop control inverter provided in an embodiment of the present invention.

[0062] Figure 2 It is a flow chart of a method for calculating current-limiting virtual impedance parameters of a droop control inverter provided in an embodiment of the present invention.

[0063] Figure 3 It is an equivalent circuit diagram of a droop control inverter according to a method for calculating current-limiting virtual impedance parameters of a droop control inverter provided by an embodiment of the present invention.

[0064] Figure 4It is a specific operation diagram of the method for calculating the current-limiting virtual impedance parameters of a droop control inverter provided by an embodiment of the present invention.

[0065] Figure 5 This is a calculation result diagram of the fault voltage, phase angle and current when the grid voltage drops to 0.6pu when applying the method for calculating the current limiting virtual impedance parameters of a droop control inverter provided by an embodiment of the present invention, wherein a is the calculation result diagram of the voltage, b is the calculation result diagram of the phase angle, and c is the calculation result diagram of the current.

[0066] Figure 6 This is a simulation result diagram of the fault voltage, phase angle and current when the grid voltage drops to 0.6pu when applying the method for calculating the current limiting virtual impedance parameters of a droop control inverter provided by an embodiment of the present invention, wherein a is the simulation result diagram of voltage, b is the simulation result diagram of phase angle, and c is the simulation result diagram of current.

[0067] Figure 7 This is a calculation result diagram of the fault voltage, phase angle and current when the grid voltage drops to 0.5pu when applying the method for calculating the current limiting virtual impedance parameters of a droop control inverter provided by an embodiment of the present invention, wherein a is the calculation result diagram of the voltage, b is the calculation result diagram of the phase angle, and c is the calculation result diagram of the current.

[0068] Figure 8 This is a simulation result diagram of the fault voltage, phase angle and current when the grid voltage drops to 0.5pu when applying the method for calculating the current limiting virtual impedance parameters of a droop control inverter provided by an embodiment of the present invention, wherein a is the simulation result diagram of voltage, b is the simulation result diagram of phase angle, and c is the simulation result diagram of current.

[0069] Figure 9 It is a structural diagram of a system for calculating current-limiting virtual impedance parameters of a droop control inverter provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0070] The technical solutions in the embodiments of the present application will be described clearly and completely below in conjunction with the drawings in the embodiments of the present application.

[0071] In the following description, multiple embodiments of the present application are provided. Different embodiments may be replaced or combined, and therefore the present application may be considered to include all possible combinations of the same and / or different embodiments described. Thus, if one embodiment includes features A, B, and C, and another embodiment includes features B and D, then the present application should also be considered to include embodiments containing one or more of all other possible combinations of A, B, C, and D, even though such embodiments may not be explicitly described in the following text.

[0072] The following description provides examples and does not limit the scope, applicability, or examples set forth in the claims. Changes may be made to the function and arrangement of the elements described without departing from the scope of the present application. Various examples may appropriately omit, replace, or add various processes or components. For example, the described method may be performed in an order different from the order described, and various steps may be added, omitted, or combined. In addition, features described in some examples may be combined in other examples.

[0073] In order to facilitate a better understanding of the embodiments of the present application, before explaining the specific implementation methods of the present application in detail, its application scenarios are first described.

[0074] See also Figure 1 , Figure 1 A schematic diagram of an application scenario of the method for calculating the current-limiting virtual impedance parameters of a droop control inverter is shown. Figure 1 The droop control inverter is connected to the common coupling point (PCC) through an LC filter. The line impedance between the PCC and the grid is represented by Z g Indicates that a low-pass filter is often added to the droop control outer loop to suppress power fluctuations and provide a certain amount of damping. m is the active-frequency droop coefficient, n is the reactive-voltage droop coefficient, and P e , Q e are the active power and reactive power output by the inverter, P0 and Q0 are the active power reference value and reactive power reference value, respectively. v 、X v are virtual resistance and virtual inductive reactance respectively.

[0075] The method for calculating the current-limiting virtual impedance parameters of the droop control inverter described in the embodiments of this specification is applied to the fault current control process of a high proportion of new energy grid-connected. In these scenarios, the application of the method for calculating the current-limiting virtual impedance parameters of the droop control inverter is intended to make it more in line with actual needs, thereby making the calculated fault current and current-limiting virtual impedance parameters more accurate, which is of great significance for ensuring system safety and improving power supply quality.

[0076] Example 1:

[0077] like Figure 2 As shown, this embodiment provides a method for calculating the current limiting virtual impedance parameters of a droop control inverter, comprising the steps of:

[0078] S1, preset fault parameters of the grid fault and the initial value of the virtual impedance used by the droop control inverter to control the current;

[0079] S2. Calculating a droop control inverter fault current based on the fault parameter and the initial value of the virtual impedance, wherein the step of calculating the fault current includes:

[0080] S2.1. Calculating a maximum virtual power angle based on the initial value of the virtual impedance;

[0081] S2.2. Calculate a virtual power angle based on the fault parameters until the obtained virtual power angle is no greater than the maximum virtual power angle, and record the value corresponding to the virtual power angle as the steady-state value of the virtual power angle;

[0082] S2.3. Calculating a fault current value when a power grid fault occurs based on the steady-state value of the virtual power angle;

[0083] S3. Obtain the fault current limit value corresponding to the grid fault, compare whether the fault current value is not greater than the fault current limit value, and if not, increase the value of the initial value of the virtual impedance and repeat step S2 until the obtained fault current value is less than or equal to the fault current limit value, so as to obtain the virtual impedance corresponding to the droop control inverter and its corresponding fault current.

[0084] Specifically, this embodiment provides a preferred implementation of step S1, wherein the preset fault parameters of the grid fault and the initial value of the virtual impedance used by the droop control inverter to control the current include:

[0085] Preset grid voltage U g , grid side resistance R g , grid side inductance X g , active power reference value P0, reactive power reference value Q0, voltage reference value V0, reactive-voltage droop coefficient n;

[0086] The preset initial value of the virtual impedance includes the virtual resistance R v0 and the initial value of virtual inductive reactance X v0 .

[0087] Specifically, this embodiment provides a preferred implementation of step S2.1, wherein the calculation of the maximum value of the virtual power angle based on the initial value of the virtual impedance includes the steps of:

[0088] The expression for the output power of the droop control inverter is obtained as Equation 1;

[0089] An expression for the virtual internal potential amplitude is established based on the reactive-voltage droop coefficient n, the voltage reference value V0, the reactive power reference value Q0, and the equation 1, which is recorded as equation 2;

[0090] Based on the above formula 1 and formula 2, the expression of the virtual internal potential amplitude with respect to the virtual power angle is obtained, which is expressed as formula 3;

[0091] Substituting Equation 3 into Equation 1, we can obtain the virtual power angle characteristic expression considering virtual impedance, which is recorded as Equation 4. Based on Equation 4, we can calculate the maximum value of the virtual power angle δ. refmax .

[0092] Specifically, this embodiment provides a preferred implementation of step S2.2, wherein calculating the virtual power angle based on the fault parameter until the obtained virtual power angle is no greater than the maximum virtual power angle, and recording the value corresponding to the virtual power angle as the virtual power angle steady-state value, includes the steps of:

[0093] Obtain the output power reference value P0 of the droop control inverter;

[0094] Based on the output power reference value P0 and the expression of the droop control inverter output active power in the formula 1, the virtual power angle characteristic expression is obtained as P0=f(δ,R v ,X v );

[0095] The virtual power angle is solved based on the virtual power angle characteristic expression. If the obtained virtual power angle is greater than the maximum value of the virtual power angle, the virtual power angle is recalculated until it is less than or equal to the maximum value of the virtual power angle.

[0096] Specifically, this embodiment provides a preferred implementation of step S2.3, wherein the calculation of the fault current value when the power grid fails based on the steady-state value of the virtual power angle includes the following steps:

[0097] Substitute the steady-state value of the virtual power angle into the formula (3) to obtain the internal potential amplitude, and then calculate the fault current I according to the phasor relationship. t Size:

[0098]

[0099] Where, E ref , δ ref are the amplitude and phase angle of the virtual internal potential, Z Σ is the equivalent total impedance modulus.

[0100] As a preferred solution, the formula 1 is:

[0101]

[0102] in, In formula 1, P e , Q e are the output active and reactive power of the inverter grid-connected point under droop control, P i , Q i are the output active and reactive power of the virtual internal potential point, E ref , δ ref are the amplitude and phase angle of the virtual internal potential, U g is the grid voltage, R g 、X gare the grid side resistance and inductive reactance, R v 、X v are virtual resistance and virtual inductive reactance respectively, R Σ 、X Σ are the equivalent total resistance and total inductive reactance, Z Σ is the equivalent total impedance modulus.

[0103] Specifically, this embodiment provides a preferred implementation method, wherein the formula 2 is:

[0104] E ref =V0+n(Q0-Q e );

[0105] In formula 2, V0 is the voltage reference value, Q0 is the reactive power reference value, and Q e It is the reactive power output size of the inverter grid connection point for droop control, and n is the reactive-voltage droop coefficient.

[0106] As a preferred solution, the formula three is:

[0107]

[0108] Denoted as E ref =g(δ), where

[0109]

[0110] Where Q ref is the reactive power reference value.

[0111] Specifically, this embodiment provides a preferred implementation, wherein the formula (4) is:

[0112]

[0113] The formula 4 is P e =f(δ,R v ,X v ), the maximum value of the virtual power angle is obtained by the virtual power angle when the virtual power angle characteristic curve reaches the extreme value, that is:

[0114]

[0115] Where, δ refmax is the maximum value of the virtual power angle corresponding to the virtual power angle characteristic curve.

[0116] More specifically, this embodiment is based on Figure 1 The application scenario shown in Figure 1 is used to obtain the equivalent circuit of the droop control inverter considering virtual impedance. Figure 3 As shown, the potential amplitude E in the inverter ref and phase angle δ refis directly affected by droop control and indirectly by virtual impedance, and provides Figure 4 The specific operation process shown:

[0117] First, the virtual impedance value during normal operation, i.e., the initial virtual impedance value, is set. Then, the grid voltage, grid-side impedance, active reference value, reactive reference value, voltage reference value, and droop coefficient during a fault are set. Then, the maximum virtual power angle value that meets the static stability requirements, i.e., the maximum virtual power angle value, is obtained based on the virtual power angle characteristic expression. The steps for solving the virtual power angle characteristic expression are as follows:

[0118] according to Figure 3 The equivalent circuit diagram shows that the relationship between the inverter output power, virtual internal potential amplitude and virtual power angle is:

[0119]

[0120] in,

[0121]

[0122] Where, P e , Q e They are the output active power and reactive power of the inverter grid connection point under droop control, P i , Q i are the output active power and reactive power of the virtual internal potential point, E ref , δ ref are the amplitude and phase angle of the virtual internal potential, U g is the grid voltage, R g 、X g are the grid side resistance and grid side inductive reactance, R v 、X v are virtual resistance and virtual inductive reactance respectively, R Σ 、X Σ are the equivalent total resistance and equivalent total inductive reactance, Z Σ is the equivalent total impedance modulus.

[0123] Since the above equation contains the internal potential amplitude as a variable in addition to the virtual power angle, it is necessary to establish the relationship between the internal potential amplitude and the virtual power angle. According to the reactive power-voltage droop control, the expression is:

[0124] E ref =V0+n(Q0-Q e )

[0125] Where V0 and Q0 are the voltage reference value and reactive power reference value respectively, and n is the reactive-voltage droop coefficient.

[0126] Further combining the expression of output reactive power and reactive-voltage droop equation, we can obtain the expression of virtual internal potential amplitude with respect to virtual power angle:

[0127]

[0128] Denoted as E ref =g(δ), where

[0129]

[0130] Furthermore, by substituting the expression of the internal potential amplitude with respect to its virtual power angle into the active power expression, the expression of the virtual power angle characteristic considering the virtual impedance can be obtained as follows:

[0131]

[0132] Let the above formula be P e =f(δ,R v ,X v ), the maximum virtual power angle can be obtained from the virtual power angle when the virtual power angle characteristic curve reaches the extreme value, that is:

[0133]

[0134] Where, δ refmax is the maximum virtual power angle value corresponding to the virtual power angle characteristic curve.

[0135] According to the active balance constraint, the output active power P e Equal to the reference value P0, the virtual power angle characteristic expression is P0=f(δ,R v ,X v ), from the virtual power angle characteristic curve, we can know that the virtual power angle corresponding to this equation, that is, the internal potential phase angle, has two solutions, denoted as δ ref1 and δ ref2 According to the static stability condition, the virtual power angle working point δ of small disturbance stability is ref1 The conditions to be met are:

[0136] δ ref ≤δ refmax

[0137] Therefore, after obtaining the internal potential phase angle corresponding to different virtual impedance values, it is necessary to determine the relationship between the obtained internal potential phase angle and the maximum virtual power angle. When the obtained virtual power angle is less than the maximum virtual power angle, the static stability requirements are met. On the contrary, when it is greater than the maximum virtual power angle, it is necessary to discard and recalculate the virtual power angle until the conditions are met. If there is no solution to the virtual power angle characteristic expression, it means that the system has no steady-state equilibrium point under the corresponding virtual impedance parameters, and transient instability occurs. If the virtual impedance is further increased, the system will not be able to operate stably, so the calculation ends.

[0138] Furthermore, after obtaining the virtual power angle, the relationship between the internal potential amplitude and the phase angle E is introduced. ref =g(δ) to obtain the internal potential amplitude, and then the fault current can be obtained according to the phasor relationship:

[0139]

[0140] When the fault current is greater than the inverter current limit requirement value, that is, the fault current limit value I lim When , continue to increase the virtual impedance and repeat the above steps. Otherwise, output the virtual impedance value. When the calculation program ends, the feasible range of virtual impedance that meets the current limiting requirements can be obtained.

[0141] Example 2:

[0142] like Figure 9 As shown, this embodiment provides a system for calculating the current-limiting virtual impedance parameters of a droop control inverter, based on the method for calculating the current-limiting virtual impedance parameters of a droop control inverter described in Example 1. The system includes a preset module, a calculation module, an acquisition module, a fault current comparison module, and a virtual impedance iteration module. The preset module is used to preset the fault parameters of the power grid fault and the initial value of the virtual impedance used by the droop control inverter to control the power grid current.

[0143] The calculation module includes a virtual power angle calculation unit, a virtual power angle comparison unit, and a fault current calculation unit; the virtual power angle calculation unit includes a maximum virtual power angle calculation subunit and a virtual power angle calculation subunit;

[0144] The maximum virtual power angle calculation subunit calculates a maximum virtual power angle based on the initial virtual impedance value; the virtual power angle calculation subunit further calculates the virtual power angle based on the fault parameter until the obtained virtual power angle is no greater than the maximum virtual power angle, and records the value corresponding to the virtual power angle as a steady-state virtual power angle value;

[0145] The virtual power angle comparison unit is configured to compare the virtual power angle calculated by the virtual power angle calculation unit with the maximum value of the virtual power angle;

[0146] The fault current calculation unit calculates the fault current value when the power grid fails based on the steady-state value of the virtual power angle;

[0147] The acquisition module is used to obtain the fault current limit value corresponding to the power grid fault;

[0148] The fault current comparison module is used to compare the fault current value with the fault current limit value;

[0149] The virtual impedance iteration module increases the value of the initial value of the virtual impedance based on the fault current value being greater than the fault current limit value and jumps to the calculation module to recalculate until the fault current value is less than or equal to the fault current limit value, so as to obtain the virtual impedance corresponding to the droop control inverter and its corresponding fault current.

[0150] Example 3:

[0151] In order to verify the effectiveness of the method for calculating the current limiting virtual impedance parameters of the droop control inverter according to the present invention, this embodiment provides the following parameters based on the parameter design shown in Table 1: Figure 5 、 Figure 6 The voltage, phase angle and current calculation results and simulation results when the grid voltage drops to 0.6pu are shown in the figure. Figure 5 The part between the two curves in the calculation results is the feasible range of virtual impedance current limiting. In order to verify the effectiveness of the proposed calculation method, the virtual impedance values ​​adopted in the simulation are as follows: Figure 5 As shown in the calculation example point, Figure 6 The corresponding voltage, phase angle and current result diagrams are given. It can be seen that the simulation results are consistent with the calculation results, which proves the effectiveness of the proposed calculation method.

[0152] Furthermore, this embodiment also provides Figure 7 、 Figure 8 The calculated and simulated results for voltage, phase angle, and current when the grid voltage drops to 0.5 pu are shown. The simulation results are consistent with the calculated results, noting that the fault current level increases when the grid voltage drops more deeply. In summary, the comparison of the above calculation and simulation results confirms the effectiveness of the method for calculating the current-limiting virtual impedance parameters for droop-controlled inverters described in this invention. The resulting feasible range of virtual impedance current limiting can provide a new method for adjusting the current-limiting control parameters for droop-controlled inverters.

[0153] Table 1 Parameters of the grid-connected inverter system with droop control considering virtual impedance

[0154]

[0155]

[0156] It should be noted that for the aforementioned method embodiments, for the sake of simplicity, they are all expressed as a series of action combinations, but those skilled in the art should be aware that this application is not limited by the order of the actions described, because according to this application, certain steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also be aware that the embodiments described in the specification are all preferred embodiments, and the actions and modules involved are not necessarily required by this application.

[0157] In the above embodiments, the description of each embodiment has its own focus. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0158] The above is only an exemplary embodiment of the present disclosure and cannot be used to limit the scope of the present disclosure. That is, any equivalent changes and modifications made according to the teachings of the present disclosure are still within the scope of the present disclosure. After considering the specification and practicing the disclosure herein, those skilled in the art will easily think of the implementation scheme of the present disclosure. This application is intended to cover any variation, use or adaptation of the present disclosure, which follows the general principles of the present disclosure and includes common knowledge or customary technical means in the art that are not recorded in the present disclosure. The description and examples are to be regarded as exemplary only, and the scope and spirit of the present disclosure are defined by the claims.

Claims

1. A method for calculating the current-limiting virtual impedance parameters of a droop control inverter, characterized in that: Including steps: S1, preset fault parameters of the grid fault and the initial value of the virtual impedance used by the droop control inverter to control the current; S2. Calculating a droop control inverter fault current based on the fault parameter and the initial value of the virtual impedance, wherein the step of calculating the fault current includes: S2.

1. Calculating a maximum virtual power angle based on the initial value of the virtual impedance; S2.

2. Calculate a virtual power angle based on the fault parameters until the obtained virtual power angle is no greater than the maximum virtual power angle, and record the value corresponding to the virtual power angle as the steady-state value of the virtual power angle; S2.

3. Calculating a fault current value when a power grid fault occurs based on the steady-state value of the virtual power angle; S3. Obtain the fault current limit value corresponding to the grid fault, and determine whether the fault current value is not greater than the fault current limit value. If not, increase the value of the initial value of the virtual impedance and repeat step S2 until the obtained fault current value is less than or equal to the fault current limit value, so as to obtain the virtual impedance corresponding to the droop control inverter and its corresponding fault current.

2. The method for calculating the current limiting virtual impedance parameters of a droop control inverter according to claim 1, wherein: The preset fault parameters of the grid fault and the initial value of the virtual impedance of the droop control inverter for controlling the current in step S1 include: Preset grid voltage U g , grid side resistance R g , grid side inductance X g , active power reference value P0, reactive power reference value Q0, voltage reference value V0, reactive-voltage droop coefficient n; The preset virtual impedance initial value includes the virtual resistance R v0 and the initial value of virtual inductive reactance X v0 .

3. The method for calculating the current limiting virtual impedance parameters of a droop control inverter according to claim 2, wherein: The step S2.1 of calculating the maximum value of the virtual power angle based on the initial value of the virtual impedance comprises the following steps: The expression for the output power of the droop control inverter is obtained as Equation 1; An expression for the virtual internal potential amplitude is established based on the reactive-voltage droop coefficient n, the voltage reference value V0, the reactive power reference value Q0, and the equation 1, which is recorded as equation 2; Based on the above formula 1 and formula 2, the expression of the virtual internal potential amplitude with respect to the virtual power angle is obtained, which is expressed as formula 3; Substituting Equation 3 into Equation 1, we can obtain the virtual power angle characteristic expression considering virtual impedance, which is recorded as Equation 4. Based on Equation 4, we can calculate the maximum value of the virtual power angle δ. refmax .

4. The method for calculating the current limiting virtual impedance parameters of a droop control inverter according to claim 3, wherein: Step S2.2, performing virtual power angle calculation based on the fault parameters until the obtained virtual power angle is no greater than the maximum virtual power angle, and recording the value corresponding to the virtual power angle as the virtual power angle steady-state value, comprises the following steps: Obtain the output power reference value P0 of the droop control inverter; Based on the output power reference value P0 and the expression of the droop control inverter active output power in formula 1, the virtual power angle characteristic expression is obtained as P0=f(δ,R v ,X v ); The virtual power angle is solved based on the virtual power angle characteristic expression. If the obtained virtual power angle is greater than the maximum value of the virtual power angle, the virtual power angle is recalculated until it is less than or equal to the maximum value of the virtual power angle.

5. The method for calculating the current limiting virtual impedance parameters of a droop control inverter according to claim 4, characterized in that: The step S2.3 of calculating the fault current value when the power grid fails based on the steady-state value of the virtual power angle comprises the following steps: Substitute the steady-state value of the virtual power angle into the formula (3) to obtain the internal potential amplitude, and then calculate the fault current I according to the phasor relationship. t Size: Where, E ref , δ ref are the amplitude and phase angle of the virtual internal potential, Z Σ is the equivalent total impedance modulus.

6. The method for calculating the current limiting virtual impedance parameters of a droop control inverter according to claim 3, wherein: The formula 1 is: in, In formula 1, P e , Q e are the output active and reactive power of the inverter grid-connected point under droop control, P i , Q i are the output active and reactive power of the virtual internal potential point, E ref , δ ref are the amplitude and phase angle of the virtual internal potential, U g is the grid voltage, R g 、X g are the grid side resistance and inductive reactance, R v 、X v are virtual resistance and virtual inductive reactance respectively, R Σ 、X Σ are the equivalent total resistance and total inductive reactance, Z Σ is the equivalent total impedance modulus.

7. The method for calculating the current limiting virtual impedance parameters of a droop control inverter according to claim 6, characterized in that: The formula 2 is: E ref =V0+n(Q0-Q e ); In formula 2, V0 is the voltage reference value, Q0 is the reactive power reference value, and Q e It is the reactive power output size of the inverter grid connection point for droop control, and n is the reactive-voltage droop coefficient.

8. The method for calculating the current limiting virtual impedance parameters of a droop control inverter according to claim 7, characterized in that: The formula three is: Denoted as E ref =g(δ), where Where Q ref is the reactive power reference value.

9. The method for calculating the current limiting virtual impedance parameters of a droop control inverter according to claim 8, characterized in that: The formula 4 is: The formula 4 is P e =f(δ,R v ,X v ), the maximum value of the virtual power angle is obtained by the virtual power angle when the virtual power angle characteristic curve reaches the extreme value, that is: Where, δ refmax is the maximum value of the virtual power angle corresponding to the virtual power angle characteristic curve.

10. A system for calculating current-limiting virtual impedance parameters of a droop control inverter, based on a method for calculating current-limiting virtual impedance parameters of a droop control inverter according to any one of claims 1 to 9, characterized in that: It includes a preset module, a calculation module, an acquisition module, a fault current comparison module, and a virtual impedance iteration module; the preset module is used to preset the fault parameters of the power grid fault and the initial value of the virtual impedance used by the droop control inverter to control the power grid current; The calculation module includes a virtual power angle calculation unit, a virtual power angle comparison unit, and a fault current calculation unit; the virtual power angle calculation unit includes a maximum virtual power angle calculation subunit and a virtual power angle calculation subunit; The maximum virtual power angle calculation subunit calculates the maximum value of the virtual power angle based on the initial value of the virtual impedance; the virtual power angle calculation subunit performs virtual power angle calculation based on the fault parameter until the obtained virtual power angle is no greater than the maximum value of the virtual power angle, and records the value corresponding to the virtual power angle as the virtual power angle steady-state value; the virtual power angle comparison unit is used to compare the virtual power angle calculated by the virtual power angle calculation unit with the maximum value of the virtual power angle; The fault current calculation unit calculates the fault current value when the power grid fails based on the steady-state value of the virtual power angle; the acquisition module is used to obtain the fault current limit value corresponding to the power grid failure; The fault current comparison module is used to compare the fault current value with the fault current limit value; The virtual impedance iteration module increases the value of the initial value of the virtual impedance based on the fault current value being greater than the fault current limit value and jumps to the calculation module to recalculate until the fault current value is less than or equal to the fault current limit value, so as to obtain the virtual impedance corresponding to the droop control inverter and its corresponding fault current.

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