Fault measurement impedance calculation method and device for hybrid wind power system with grid

Abstract

The present application provides a method and device for calculating the fault impedance measurement of a grid-connected hybrid wind power system. The method includes obtaining the characteristics of grid-connected and grid-connected converters and the corresponding low voltage ride-through control strategy; establishing an equivalent circuit under the corresponding fault state; simplifying the analysis of the hybrid wind power system to obtain an equivalent network diagram; obtaining a subcircuit diagram of each power source excitation acting alone; obtaining the short-circuit current on both sides of the corresponding system and the grid connection point voltage when each power source excitation acts alone; superimposing to obtain the total short-circuit current on both sides of the hybrid wind power system and the total grid connection point voltage; and obtaining the measured impedance at the protection installation on the new energy side. This method can accurately calculate the measured impedance value at the protection installation after a hybrid wind power system fault, ensure the setting calculation of the distance protection, accurately reflect the fault characteristic information of the hybrid AC system, provide a basis for the study of the applicability of the distance protection of the hybrid wind power system, and help to ensure the safe and stable operation of the new energy system.

Description

Technical Field

[0001] The present application relates to the technical field of fault analysis of new energy systems, and specifically to a method and device for calculating impedance for fault measurement of a hybrid wind power system in a grid-connected wind farm. Background Art

[0002] At present, most new energy power systems adopt grid-following grid-connected control, especially in remote areas with abundant wind and light resources, and wind power stations, photovoltaic power stations and other scenes are gradually being built. However, due to the large volatility of new energy power systems such as wind power generation and photovoltaic power generation, problems such as insufficient system support capacity, voltage and frequency stability under weak power grids continue to emerge. Therefore, grid-building new energy with independent support capabilities is also gradually participating in the construction of the power grid, thus forming a grid-building hybrid wind power system that combines grid-following grid-connected control and grid-building grid-connected control.

[0003] Due to the differences in control structures between different wind turbines, the grid-connected hybrid wind power system has complex stability problems in time and space. At the same time, due to the different fault ride-through control strategies adopted by different types of units, the short-circuit current calculation methods of grid-connected converters and grid-connected converters in different operating environments after the fault are different. In addition, in different scenarios of new energy transmission systems, the highly power-electronic new energy power generation equipment interacts with the dynamic characteristics of the system in a complex manner. At the same time, affected by the short-circuit current tolerance capability of power electronic devices and the system strength, the weak feedback characteristics of new energy are further amplified, and the short-circuit currents on both sides of the system have great differences in amplitude and phase angle. With the increase in the capacity of grid-connected new energy and the complex actual working conditions, the topological structure of the transmission system has changed greatly, making it difficult to analyze the fault characteristics of the entire new energy system, and it is difficult for traditional distance protection calculation methods to accurately calculate the short-circuit current of the grid-connected hybrid wind power system. Therefore, there is an urgent need for a method that can analyze the measured impedance of a grid-connected hybrid wind power system that includes a grid-connected converter and a grid-connected converter, so as to accurately solve the measured impedance of the grid-connected hybrid wind power system and then correctly respond to short-circuit faults, in the hope of ensuring the safe and stable operation of the new energy system. Summary of the invention

[0004] In order to overcome the deficiencies of the above-mentioned prior art, the present application provides a method and device for calculating the fault measurement impedance of a hybrid wind power system in a grid-connected structure, which specifically adopts the following technical solutions:

[0005] The present application discloses a method for calculating the fault measurement impedance of a grid-connected hybrid wind power system, wherein the grid-connected hybrid wind power system comprises at least a grid-connected converter and a grid-connected converter, and the method comprises the following steps:

[0006] Obtain the characteristics of grid-following and grid-forming converters in the power grid and the corresponding low voltage ride-through control strategies under fault conditions;

[0007] According to the characteristics of grid-following and grid-forming converters and the low voltage ride-through control strategy, an equivalent circuit under the corresponding fault state is established;

[0008] According to the equivalent circuit, the hybrid wind power system with grid connection is simplified and analyzed to obtain the equivalent network diagram under the corresponding fault state;

[0009] Analyze the equivalent network diagram to obtain the sub-circuit diagram of each power source excitation acting alone; the power source excitation at least includes a grid-forming voltage source, a grid-following current source, and a system-side voltage source;

[0010] Analyze the subcircuit diagrams corresponding to each power source excitation, and obtain the short-circuit current and grid connection point voltage on the system side and the new energy side when each power source excitation acts alone;

[0011] The short-circuit currents on the system side and the renewable energy side corresponding to each power source excitation are respectively superimposed to obtain the total short-circuit currents on the system side and the renewable energy side in the hybrid wind power system of the grid; the grid connection point voltages corresponding to each power source excitation are superimposed to obtain the total grid connection point voltage of the hybrid wind power system of the grid;

[0012] The measured impedance at the protection installation point on the renewable energy side is obtained by calculating the total short-circuit current on the system side and the renewable energy side in the hybrid wind power system.

[0013] Optionally: the corresponding low voltage ride through control strategy under the fault state includes a low voltage ride through control strategy of a meshed converter; the specific steps of the low voltage ride through control strategy of the meshed converter include:

[0014] The active power reference value of the virtual synchronous machine is changed from Adjust to , the power angle of the virtual synchronous machine is kept stable during the fault, where the power angle of the virtual synchronous machine changes during the fault for:

[0015] ;

[0016] By switching the virtual internal potential amplitude reference value and reactive power reference value of the virtual synchronous machine, the short-circuit current is limited and reactive support is provided to the hybrid wind power system of the follow-up grid; the virtual internal potential reference value of the virtual synchronous machine switched during the fault is:

[0017] ;

[0018] Wherein a is the first calculated intermediate value, and a is obtained by the first intermediate value calculation formula, and the first intermediate value calculation formula is:

[0019] ;

[0020] Wherein b is the second calculated intermediate value, and b is obtained by the second intermediate value calculation formula, and the second intermediate value calculation formula is:

[0021] ;

[0022] in, is the rate of change of virtual rotation angular velocity of the virtual synchronous machine; P ref is the active power reference value of the virtual synchronous machine; P e is the output active power of the virtual synchronous machine; D is the virtual damping coefficient; J is the virtual moment of inertia; is the rated value of the grid frequency; E 0 is the potential inside the virtual synchronous machine before the fault; U g0 is the grid voltage before the fault; I 0 is the rated output current of the virtual synchronous machine; E ref It is the potential reference value of the virtual synchronous machine during the fault period; u gf is the voltage amplitude of the fault phase of the power grid; I gf is the virtual synchronous machine fault steady-state current limit threshold; Z eq is the equivalent total impedance of the virtual synchronous machine grid-connected system;

[0023] When a fault occurs, a virtual impedance link is put into operation to perform short-circuit current limiting control and increase the equivalent output impedance of the virtual synchronous machine to suppress the short-circuit instantaneous large current impact of the virtual synchronous machine during the fault period.

[0024] Optionally: the corresponding low voltage ride through control strategy under the fault state includes a low voltage ride through control strategy of a grid-following converter; the specific steps of the low voltage ride through control strategy of the grid-following converter are:

[0025] According to the voltage drop degree of the grid connection point after the fault, the direct axis of the current inner loop is set. d With cross axis q The current reference value is:

[0026] ;

[0027] in For straight axis d Current reference value in the coordinate system; Cross axis q Current reference value in the coordinate system; is the active power reference value; U 0 is the grid connection point voltage; I max It is the maximum limiting current of the grid-following converter; I n is the rated current; Is the per unit value.

[0028] According to the current carrying capacity of power electronic equipment in the hybrid wind power system of the grid, limiting control is set for the PI link of the current inner loop controller, and the maximum current limiting amplitude is 1.2~2 times of the rated current.

[0029] Optional: The grid-connected hybrid wind power system includes m Grid-connected converters and n When a fault occurs in a grid-type inverter, m The grid-type converter described above is equivalent to m Series equivalent output impedance Z s1 Voltage source E s node, n The grid-type converter described above is equivalent to n A current source controlled by the grid voltage I s1 node;

[0030] When the equivalent circuit process corresponding to the fault state is established, m A voltage source node is equivalent to a voltage source E s and equivalent output impedance Z 1 series branch, the equivalent voltage source model of the corresponding branch is:

[0031] ;

[0032] in E S Output potential for equivalent voltage source; E ref is the potential reference value of the virtual synchronous machine during the fault period; Z 1 is the equivalent output impedance value; Z s1 is the equivalent output impedance value corresponding to a single voltage source node;

[0033] n A current source node is equivalent to a current source I s branch, the equivalent current source model of the corresponding branch is:

[0034] ;

[0035] in is the amplitude of the short-circuit current output by the current source in the equivalent circuit; is the amplitude of the output short-circuit current corresponding to a single current source; is the phase angle of the short-circuit current output by the current source in the equivalent circuit; , They are d , q Current reference value in the coordinate system; U 0 is the grid connection point voltage.

[0036] Optionally: When the grid-type voltage source acts alone, the short-circuit current on the system side and the new energy side and the grid connection point voltage are:

[0037] ;

[0038] in, To measure the grid connection point voltage of the hybrid wind power system; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the internal potential of the network-type equivalent voltage source; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the power supply on the system side; for and The total resistance calculated after connecting in parallel.

[0039] Optional: When the grid-following current source acts alone, the short-circuit current on the system side and the new energy side and the grid connection point voltage are:

[0040] ;

[0041] in To measure the grid connection point voltage of the hybrid wind power system; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; It is the output current of the network-type equivalent current source; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the power supply on the system side; for and The total resistance calculated after connecting in parallel.

[0042] Optional: When the system-side voltage source acts alone, the short-circuit current and the grid-connected point voltage corresponding to the system side and the new energy side are:

[0043] ;

[0044] in To measure the grid connection point voltage of the hybrid wind power system; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the potential inside the power supply on the system side; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the power supply on the system side; for , and The total resistance calculated after connecting in parallel.

[0045] Optional: The total short-circuit current and the total voltage at the grid connection point in the hybrid wind power system of the grid are obtained by superimposing the excitations of each power source:

[0046] ;

[0047] in To measure the grid connection point voltage of the hybrid wind power system; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the internal potential of the network-type equivalent voltage source; It is the output current of the network-type equivalent current source; is the potential inside the power supply on the system side; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the power supply on the system side.

[0048] Optional: The step of calculating the measured impedance at the protection installation location on the new energy side according to the total short-circuit current on the system side and the new energy side in the hybrid wind power system of the grid includes:

[0049] ;

[0050] in The measured voltage at the protection installation location on the new energy side; The measured current at the protection installation of the new energy side; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the internal potential of the network-type equivalent voltage source; It is the output current of the network-type equivalent current source; is the potential inside the power supply on the system side; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; is the additional impedance; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the power supply on the system side.

[0051] The present invention also discloses a fault measurement impedance calculation device for a hybrid wind power system, the device comprising:

[0052] A state acquisition module is used to obtain the characteristics of the grid-following and grid-forming converters in the power grid and the corresponding low voltage ride-through control strategy under fault conditions;

[0053] An equivalent circuit construction module is used to establish an equivalent circuit under the corresponding fault state according to the characteristics of the grid-following and grid-forming converters and the low voltage ride-through control strategy;

[0054] The simplified analysis module is used to simplify the analysis of the hybrid wind power system based on the equivalent circuit to obtain the equivalent network diagram under the corresponding fault state;

[0055] An equivalent network analysis module analyzes the equivalent network diagram to obtain a sub-circuit diagram of each power source excitation acting alone; the power source excitation at least includes a grid-forming voltage source, a grid-following current source, and a system-side voltage source;

[0056] The power supply excitation analysis module is used to analyze the sub-circuit diagrams corresponding to each power supply excitation, and obtain the short-circuit current and grid connection point voltage on the system side and the new energy side when each power supply excitation acts alone;

[0057] The superposition analysis module is used to superimpose the short-circuit currents on the system side and the renewable energy side corresponding to each power source excitation to obtain the total short-circuit currents on the system side and the renewable energy side in the hybrid wind power system of the grid, and to superimpose the grid connection point voltages corresponding to each power source excitation to obtain the total grid connection point voltage of the hybrid wind power system of the grid;

[0058] The impedance output module is used to calculate the measured impedance of the protection installation on the new energy side according to the total short-circuit current on the system side and the new energy side in the hybrid wind power system of the grid.

[0059] Beneficial Effects

[0060] The technical solution of this application has the following beneficial effects:

[0061] The calculation method of the present application analyzes the characteristics of the grid-connected converter and the grid-connected converter in the grid-connected hybrid wind power system, establishes the equivalent circuits of the grid-connected converter and the grid-connected converter respectively, and establishes the equivalent network diagram of the grid-connected hybrid wind power system based on the analysis of each equivalent circuit, and solves the various state quantities on both sides of the system after the fault based on the superposition theorem, and then solves the measured impedance at the protection installation on the new energy side. This calculation method can accurately calculate the short-circuit current on both sides of the grid-connected hybrid wind power system after the fault, so as to further solve the measured impedance value at the protection installation. The result error between the calculated value and the actual measured value obtained by this method does not exceed 3%, which ensures the setting calculation of the distance protection, can accurately reflect the fault characteristic information of the grid-connected hybrid wind power system, and provide a basis for the study of the applicability of the distance protection of the grid-connected hybrid wind power system, and ensure the safe and stable operation of the new energy system. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Figure 1 It is a flow chart of a fault analysis method for a grid-connected hybrid wind power system in an embodiment of the present application.

[0063] Figure 2 This is a model diagram for analyzing the grid-connected transmission lines of a grid-connected hybrid wind power system in an embodiment of the present application.

[0064] Figure 3 This is a main circuit and control block diagram of a grid-type converter in an embodiment of the present application.

[0065] Figure 4 This is a main circuit and control block diagram of a grid-connected converter in an embodiment of the present application.

[0066] Figure 5 This is the steady-state equivalent circuit of the grid-type converter in the embodiment of the present application.

[0067] Figure 6 This is the steady-state equivalent circuit of the grid-type converter in the embodiment of the present application.

[0068] Figure 7 This is an equivalent network diagram when a three-phase fault occurs in the grid-connected hybrid wind power system in the embodiment of the present application.

[0069] Figure 8 This is an equivalent network diagram when the grid-type equivalent voltage source acts alone in the grid-connected hybrid wind power system in the embodiment of the present application.

[0070] Fig. 9 This is an equivalent network diagram when the system-side voltage source acts alone in the grid-connected hybrid wind power system in the embodiment of the present application.

[0071] Fig.10 This is an equivalent network diagram when the grid-type equivalent current source acts alone in the grid-connected hybrid wind power system in the embodiment of the present application.

[0072] Fig.11 Schematic diagram of the structure of the impedance calculation device for measuring the grid-connected hybrid wind power system in the embodiment of the present application.

[0073] Fig.12 This is a schematic diagram of the structure of an electronic device in an embodiment of the present application. DETAILED DESCRIPTION

[0074] The technical solutions in the embodiments of the present application will be clearly and completely described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are part of the embodiments of the present application, rather than all of the embodiments. Therefore, it should be appreciated by those of ordinary skill in the art that various changes and modifications may be made to the embodiments described herein without departing from the scope and spirit of the present disclosure. Similarly, for the sake of clarity and conciseness, the description of well-known functions and structures is omitted in the following description. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative work are within the scope of protection of the present application.

[0075] like Figure 2 As shown in (a), it is a schematic diagram of the transmission line of the grid-connected hybrid wind power system in the embodiment of the present application. In the embodiment of the present application, the grid-connected new energy station adopts a wind power station or a photovoltaic station, and the grid-connected new energy station adopts a virtual synchronous machine control. The grid-connected new energy station and the grid-connected new energy station are both connected to the transmission line through a transformer. The AC system on the right is an infinite power supply, and the AC system is a weak system. In the embodiment of the present application, a three-phase short circuit fault is set on the transmission line, and it is grounded through a transition resistor. In addition, in order to further simplify the calculation, the present application ignores the mutual interaction between the new energy units. Figure 2 After equivalent simplification of the transmission line model of the wind power system in the grid-connected hybrid power system (a), we can get Figure 2 The equivalent transmission system analysis model of the wind power system in the grid-connected hybrid power system in (b) is used in the embodiment of the present application for analysis and calculation.

[0076] Specifically, Figure 1 As shown, the embodiment of the present application discloses a method for calculating the fault measurement impedance of a grid-connected hybrid wind power system, wherein the grid-connected hybrid wind power system comprises at least a grid-connected converter and a grid-connected converter, and the method comprises the following steps:

[0077] Step 1: Obtain the characteristics of grid-following and grid-forming converters in the power grid and the corresponding low voltage ride-through control strategy under fault conditions.

[0078] Analyze the characteristics of grid-type converters and the low voltage ride-through control strategy:

[0079] In the embodiment of the present application, a grid-connected converter based on virtual synchronous machine control and grid-connected operation is analyzed. It adopts decoupling control in the dq coordinate system, and its control mainly includes three parts: virtual synchronous machine control, virtual impedance control, and voltage and current double closed-loop control. Correspondingly, the main circuit and control block diagram of the grid-connected converter are shown in FIG. Figure 3As shown. Among them, the virtual synchronous machine control provides damping and inertia for the system through active-frequency control and reactive-voltage control, and achieves synchronization through its own output power structure internal potential amplitude and phase angle. Virtual impedance control simulates stator impedance by introducing a virtual impedance link, which equivalently increases the system equivalent impedance and improves the current limiting capacity of the converter. Finally, the voltage and current dual closed-loop control tracks and controls the current through the voltage and current dual closed-loop of dual PI control, and controls the generated voltage signal through PWM modulation.

[0080] When a fault occurs, the low voltage ride-through control strategy of the grid-type converter is: the active power reference value of the virtual synchronous machine is changed from Adjust to , so that the power angle of the virtual synchronous machine is maintained stable during the fault; and by switching the virtual internal potential amplitude reference value and reactive power reference value of the virtual synchronous machine, it can provide a certain reactive power support for the system while limiting the short-circuit current. Specifically, the power angle change of the virtual synchronous machine during the fault is shown in formula (1), and the switched internal potential amplitude reference value is shown in formula (2):

[0081] (1);

[0082] (2);

[0083] Wherein a is the first calculated intermediate value, and a is obtained by the first intermediate value calculation formula, and the first intermediate value calculation formula is:

[0084] ;

[0085] Wherein b is the second calculated intermediate value, and b is obtained by the second intermediate value calculation formula, and the second intermediate value calculation formula is:

[0086] ;

[0087] in, is the rate of change of virtual rotation angular velocity of the virtual synchronous machine; P ref is the active power reference value of the virtual synchronous machine; P e is the output active power of the virtual synchronous machine; D is the virtual damping coefficient; J is the virtual moment of inertia; is the rated value of the grid frequency; E 0 is the potential inside the virtual synchronous machine before the fault; U g0 is the grid voltage before the fault; I 0 is the rated output current of the virtual synchronous machine; E refIt is the potential reference value of the virtual synchronous machine during the fault period; u gf is the voltage amplitude of the fault phase of the power grid; I gf is the virtual synchronous machine fault steady-state current limit threshold, which is generally set to 1.3 in this embodiment. I 0; Z eq is the equivalent total impedance of the virtual synchronous machine grid-connected system.

[0088] In addition, in the embodiment of the present application, a virtual impedance link is also used when a fault occurs to perform short-circuit current limiting control (generally taking I max =1.3 I 0), thereby increasing the equivalent output impedance of the virtual synchronous machine to suppress the short-circuit instantaneous impact large current of the virtual synchronous machine during the fault. During an asymmetric fault in the power grid, the embodiment of the present application constructs a 90° phase shift link through positive and negative sequence separation control, and uses a double generalized integrator to filter out the double frequency fluctuation components of the positive and negative sequence dq components of the voltage and current in the opposite rotating coordinate system, thereby obtaining the dq DC components in each rotating coordinate system. Based on the above foundation, the embodiment of the present application achieves control under different objectives by adjusting the negative sequence current reference value in the negative sequence loop of the virtual synchronous machine.

[0089] Analysis of the characteristics of grid-following converters and low voltage ride-through control strategies:

[0090] In the embodiment of the present application, the grid-connected converter is analyzed, which also adopts dq The decoupling control in the coordinate system mainly includes the outer loop control, the inner loop control and the phase-locked loop (PLL) and other components. Correspondingly, the main circuit and control block diagram of the grid-following converter are as follows: Figure 4 As shown in the figure, the phase-locked loop (PLL) tracks the AC voltage to obtain the AC voltage amplitude and phase angle of the power grid, and then synchronizes with the AC power grid. The outer loop controller adopts PQ decoupling control, and sets the reference value to and It realizes accurate tracking of active and reactive power control to achieve constant output power. Furthermore, the inner loop controller adopts the current inner loop decoupling control method, which tracks the current reference value through the PI link to achieve direct output current control.

[0091] When a fault occurs in the power grid, photovoltaic power stations and other new energy sources must have low voltage ride-through capabilities according to national standards. During the fault period, a certain amount of reactive power support must be provided to the power grid to maintain its stable operation. Therefore, the new energy station using a grid-following converter needs to set the current reference values ​​of the direct axis d and the quadrature axis q of the current inner loop according to the voltage drop degree of the grid connection point after the fault. The low voltage ride-through control strategy adopted at this time is shown in formula (3):

[0092] (3);

[0093] in, , Spindle d , cross axis q Current reference value in the coordinate system; is the active power reference value; U 0 is the grid connection point voltage; I max It is the maximum limiting current of the grid-following converter; I n is the rated current; Is the per unit value.

[0094] In addition, affected by the current carrying capacity of power electronic equipment, the PI link of the inner loop controller in the embodiment of the present application is set to limit control, and the maximum current limit amplitude is generally selected as 1.2 to 2 times of the rated current. In addition, when an asymmetric fault occurs in the power grid, it controls the negative sequence current component through the positive and negative sequence separation link to achieve different control objectives such as output current balance or suppressing power fluctuations.

[0095] Step 2: Establish an equivalent circuit under the corresponding fault state according to the characteristics of the grid-following and grid-forming converters and the low voltage ride-through control strategy.

[0096] First, the embodiment of the present application establishes an equivalent circuit under a fault state according to the characteristics of the grid-type converter and the corresponding low voltage ride-through control strategy.

[0097] Specifically, since the grid-type converter can independently construct the voltage amplitude and phase angle, and is affected by the low voltage ride-through control strategy of the virtual synchronous machine, the potential and power angle inside the virtual synchronous machine remain constant during the low voltage ride-through process. Therefore, the virtual synchronous machine can be equivalent to a voltage source during the fault. In normal operation, the grid-type converter calculates its corresponding equivalent output impedance based on the voltage relationship between the filter impedance and the inner loop controller, and its value remains constant. During the fault period, due to the introduction of virtual impedance in the control loop R v + JXj v , at this time, the positive and negative sequence equivalent system impedance of the grid-type converter is shown in formula (4):

[0098] (4);

[0099] in m is the current limiting coefficient; R f , L f They are the filter parasitic resistance and filter inductance respectively; k p1 , k i1 They are the proportional constant and integral constant of the voltage outer loop respectively; k p2 , k i2 are the proportional constant and integral constant of the inner current loop respectively; R V is a virtual resistor; L V is the virtual inductor.

[0100] According to formula (4), the equivalent impedance of the grid-type converter is related to the parameters of the PI controller, the limiting coefficient and the filter impedance, and its value remains constant during the steady state process. Therefore, during the fault period, the grid-type converter can be equivalent to a voltage source connected in series with the equivalent output impedance, such as Figure 5 As shown. Since the virtual internal potential of the virtual synchronous machine is determined by the control strategy of the reactive loop, the steady-state output short-circuit current can be expressed by equation (5):

[0101] (5);

[0102] in is the potential amplitude and phase angle within the network; is the voltage amplitude and phase angle at the grid connection point.

[0103] Therefore, when there are several inverters in the new energy station connected to the grid in parallel through transformers, the grid-connected hybrid wind power system may include m Grid-connected converters and n The grid-type converter. m The grid-type converter can be equivalent to m Series equivalent output impedance Z s1 Voltage source E s Therefore, when a short-circuit fault occurs on the system side, the interaction between renewable energy sources is ignored, and it is assumed that the same inverter is used in the grid-building and grid-following renewable energy stations. m A voltage source node is equivalent to a voltage source E s and equivalent output impedance Z 1( Z 1=Z s1 / m ) is a branch connected in series. Its equivalent voltage source model can be expressed as:

[0104] ;

[0105] in E S Output potential for equivalent voltage source; E ref is the potential reference value of the virtual synchronous machine during the fault period; Z 1 is the equivalent output impedance value; Z s1 is the equivalent output impedance value corresponding to a single voltage source node.

[0106] Secondly, the embodiment of the present application establishes an equivalent circuit under a fault state according to the characteristics of the grid-following converter and the corresponding low voltage ride-through control strategy.

[0107] Specifically, the AC terminal of the grid-following converter simulates the current source characteristics, keeps synchronization with the grid voltage according to the phase-locked loop, and controls the output power by directly controlling the output current. The control adopted in the embodiment of the present application is oriented to the grid connection point voltage, so the amplitude and phase angle of the short-circuit current output by the grid-following converter are affected by the grid connection point voltage. When a fault occurs, the grid-following converter can be equivalent to a current source controlled by the grid connection point voltage. I s ,like Figure 6 As shown, the amplitude and phase angle of the output short-circuit current are expressed as follows:

[0108] (6);

[0109] When a short circuit occurs on the system side, the interaction between renewable energy sources is ignored, and it is assumed that the same inverter is used in both the grid-building and grid-following renewable energy stations. n The station-grid type converter can be equivalent to n A current source controlled by the grid voltage I s1 Node, in the equivalent circuit n A current source node can be equivalent to a current source I s ( I s = nI s1 ) branch, its equivalent current source model can be expressed as:

[0110] ;

[0111] in is the amplitude of the short-circuit current output by the current source in the equivalent circuit; is the amplitude of the output short-circuit current corresponding to a single current source; is the phase angle of the short-circuit current output by the current source in the equivalent circuit; , They are d , q Current reference value in the coordinate system; U 0 is the grid connection point voltage.

[0112] Step 3: Simplify the analysis of the grid-connected hybrid wind power system based on the equivalent circuit to obtain the equivalent network diagram under the corresponding fault state.

[0113] Firstly based on Figure 2 The grid-connected hybrid wind power system converts all devices to the same voltage level. Figure 2 middle f A three-phase short circuit occurs at the point, and the transition resistor R g Grounding. According to the above analysis, under the influence of low voltage ride-through control strategy, the grid-following type new energy station can be equivalent to a current source controlled by the grid connection point voltage, and the grid-building type new energy station can be equivalent to a voltage source with series impedance. The system side uses an infinite power supply instead. For the convenience of calculation, both transformers use ideal transformer models. The corresponding grid-building type hybrid wind power system fault equivalent network diagram is established as follows Figure 7 It should be noted that Figure 7 middle E s is the internal potential of the network-type equivalent voltage source; I s It is the output current of the network-type equivalent current source; Z 1 is the equivalent output impedance of the network; I M and I N They are the fault short-circuit currents on both sides of the system; Z MN is the transmission line impedance; α is the fault location coefficient; R g is the transition resistance; U and Z s3 are the internal potential and internal resistance of the power supply on the system side respectively.

[0114] Step 4: Analyze the equivalent network diagram to obtain the sub-circuit diagram of each power source excitation acting alone; the power source excitation at least includes a grid-forming voltage source, a grid-following current source, and a system-side voltage source.

[0115] When a three-phase short-circuit fault occurs on the system side, the converter on the renewable energy side quickly enters the steady-state stage after the fault due to the short transient process of the converter on the renewable energy side. Therefore, the three-phase short-circuit current calculation on both the renewable energy side and the system side can be considered as the solution of the periodic component in the steady-state circuit. Due to the difference between the renewable energy control structure and the traditional synchronous machine, the amplitude and phase angle of the short-circuit current fed on both sides of the system are very different, resulting in changes in the measurement impedance characteristics of the protection installation in the renewable energy system, and the traditional measurement impedance calculation method is no longer applicable.

[0116] Depend on Figure 7 It can be seen that based on the superposition theorem, the short-circuit current on both sides of the grid-connected hybrid wind power system can be considered to be caused by the grid-connected voltage source E s , grid-following current source I s And the system side voltage source U The result of the combined action of the three power sources is that when the grid voltage source E s When acting alone, the system side voltage source U Short circuit, network-type current source I s Open circuit, the corresponding system equivalent circuit diagram is as follows Figure 8 As shown; when the system side voltage source U When short circuit acts alone, the network voltage source E s Short circuit, network-type current source I s Open circuit, the corresponding system equivalent circuit diagram is as follows Fig. 9 As shown; when the grid current source I s When acting alone, the grid voltage source E s and system side voltage source U Short circuit, the corresponding system equivalent circuit diagram is as follows Fig.10 Therefore, according to the sub-circuit diagram when the three power sources act alone, the short-circuit current fed from the system side and the new energy side after a three-phase short-circuit fault occurs on the corresponding line when the power source acts alone can be obtained, and then the superposition solution is performed.

[0117] Step 5: Analyze the sub-circuit diagrams corresponding to each power source excitation to obtain the short-circuit current and grid connection point voltage on the system side and the new energy side corresponding to each power source excitation when each power source excitation acts alone.

[0118] Specifically, the embodiments of the present application are respectively directed to a grid-type voltage source E s , grid-following current source I s And the system side voltage source UThe three power excitations are analyzed separately:

[0119] For grid voltage source E s Power excitation acts alone:

[0120] When only the grid-type equivalent voltage source acts alone in the power grid, the corresponding system equivalent circuit diagram is as follows: Figure 8 As shown. Since the AC system is a weak system, the system side impedance is large, and when the ground transition resistance is small, the network on the right R g and Z eq4 The parallel connection mainly shows the characteristics of transition resistance, while the network type equivalent output impedance Z eq1 Therefore, when the grid-type voltage source acts alone, the short-circuit current on the system side and the new energy side and the grid connection point voltage are as shown in formula (7):

[0121] (7);

[0122] in, To measure the grid connection point voltage of the hybrid wind power system; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the internal potential of the network-type equivalent voltage source; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the system side power supply. It should be noted that It indicates and When a parallel connection is used, the result is calculated using resistors in parallel.

[0123] It can be seen from the above formula (7) that when the grid-type voltage source acts alone, the grid-connected point voltage and the short-circuit current on the renewable energy side are mainly affected by the grid-type internal potential, the grid-type equivalent output impedance and the fault location, and the system side short-circuit current is mainly affected by the grid-type internal potential, the grid-type equivalent output impedance, the system side equivalent impedance and the transition resistance.

[0124] For system side voltage source UPower excitation acts alone:

[0125] When there is only system-side voltage source in the power grid U When acting alone, the system equivalent circuit diagram is as follows Fig. 9 As shown. Since the grounding transition resistance is small and the equivalent output impedance of the network is Z eq1 The parallel circuit on the left side mainly shows the transition resistance characteristics. At this time, when the grid-following current source acts alone, the short-circuit current on the system side and the new energy side and the grid connection point voltage are as shown in formula (8):

[0126] (8);

[0127] in To measure the grid connection point voltage of the hybrid wind power system; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the potential inside the power supply on the system side; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the system side power supply. It should be noted that It indicates , and When a parallel connection is used, the result is calculated using resistors in parallel.

[0128] It can be seen from the above formula (8) that when the system-side voltage source acts alone, the grid-connected point voltage and the output short-circuit current of the renewable energy side are mainly affected by the system-side voltage, the system-side equivalent impedance, the grid-type equivalent output impedance and the transition resistance, and the system-side output short-circuit current is mainly affected by the system-side voltage and the system-side equivalent impedance.

[0129] For grid-following current source I s Power excitation acts alone:

[0130] When there is only a grid-type equivalent current source in the power grid I s When acting alone, the system equivalent circuit diagram is as follows Fig.10 As shown. Due to the equivalent impedance of the system sideZ eq4 The parallel circuit on the right side mainly shows the transition resistance characteristics. At this time, when the grid-following current source acts alone, the short-circuit current on the system side and the new energy side and the grid connection point voltage are as shown in formula (9):

[0131] (9);

[0132] in To measure the grid connection point voltage of the hybrid wind power system; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; It is the output current of the network-type equivalent current source; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the system side power supply. It should be noted that It indicates and When a parallel connection is used, the result is calculated using resistors in parallel.

[0133] It can be seen from the above formula that at this time, the grid-connected point voltage and the output short-circuit current on the renewable energy side are mainly affected by the grid-connected equivalent output impedance, the fault location and the photovoltaic output current. The system-side output short-circuit current is mainly affected by the grid-connected equivalent output impedance, the system-side equivalent impedance, the grid-connected output current and the transition resistance.

[0134] Step 6: Superimpose the short-circuit currents on the system side and the renewable energy side and the grid connection point voltage corresponding to each power source excitation to obtain the total short-circuit current on the system side and the renewable energy side and the grid connection point total voltage in the hybrid wind power system.

[0135] Specifically, according to the superposition theorem, the total short-circuit current on the system side and the renewable energy side and the total voltage at the grid connection point in the wind power system with a grid-connected hybrid connection can be obtained by superimposing the related quantities of the three excitations acting separately. Therefore, in the embodiment of the present application, the total short-circuit current on the system side and the renewable energy side and the total voltage at the grid connection point in the wind power system with a grid-connected hybrid connection can be obtained by superimposing the excitations of each power source, as shown in formula (10):

[0136] (10);

[0137] in To measure the grid connection point voltage of the hybrid wind power system; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the internal potential of the network-type equivalent voltage source; It is the output current of the network-type equivalent current source; is the potential inside the power supply on the system side; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the power supply on the system side.

[0138] Step 7: Calculate the measured impedance of the protection installation on the new energy side based on the short-circuit current on the system side and the new energy side in the hybrid wind power system.

[0139] Specifically, the measured impedance at the new energy side protection installation location in the embodiment of the present application can be expressed as shown in formula (11):

[0140] (11);

[0141] in The measured voltage at the protection installation location on the new energy side; The measured current at the protection installation of the new energy side; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the internal potential of the network-type equivalent voltage source; It is the output current of the network-type equivalent current source; is the potential inside the power supply on the system side; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; is the additional impedance; α is the fault location coefficient; R gis the transition resistance; Z s3 is the internal resistance of the power supply on the system side.

[0142] It should be understood that the measured impedance solved in the embodiment of the present application mainly includes the fault impedance Z eq3 With additional impedance Due to the different types of new energy access systems, the additional impedance characteristics change greatly. From the above formula (11), it can be seen that the additional impedance It is mainly related to the equivalent internal potential of the grid-forming type, the equivalent output impedance of the grid-forming type, the equivalent output current of the grid-following type, the internal potential and equivalent impedance of the system side, and the fault location. The system is affected by different low-voltage ride-through control strategies, which may cause the additional impedance to show resistive-capacitive or resistive-inductive characteristics, making it impossible for the protection to correctly measure the actual fault impedance, thereby causing the distance protection to refuse to operate or malfunction. Therefore, the embodiment of the present application can analyze the characteristics of the measured impedance after the fault of the hybrid new energy system according to the above-mentioned impedance calculation process, and accurately calculate the measured impedance value at the protection installation point on the new energy side after the fault, so as to reasonably design the fault strategy and parameter ratio of the grid-following type and grid-forming type new energy, provide a basis for the study of the applicability of distance protection of the grid-following hybrid wind power system, and ensure the safe and stable operation of the new energy system.

[0143] Furthermore, in order to verify the effectiveness of the above-mentioned fault measurement impedance calculation method for the hybrid wind power system, a simulation model is built in the PSCAD simulation platform. The overall structure diagram of the simulation model is shown in FIG. Figure 2 As shown in the figure, the grid-following station adopts wind farm station or photovoltaic power station, and the grid-building new energy adopts virtual synchronous machine control. The two stations are connected to the transmission line through a transformer and connected to the grid through a 220KV line. The capacity of the two new energy stations is 75MW, the total length of the transmission line is 80km, and the line impedance parameter is (0.076+0.338j)Ω / km. The main parameters of the two types of inverters are shown in Table 1 and Table 2:

[0144] Table 1 Parameters of grid-connected inverter

[0145]

[0146] Table 2 Parameters of grid-connected inverter

[0147]

[0148] All inverters in the new energy station of the embodiment of the present application adopt the fault ride-through strategy adopted by the above-mentioned grid-following type and grid-building type converters, and f Point setting three-phase short circuit fault, through the transition resistor R gGrounding. Since the influence of transition resistance on additional impedance may cause incorrect operation of distance protection, the embodiment of the present application simulates the fault conditions of different grounding transition resistances to obtain the output short-circuit current and measurement impedance on the system side and the new energy side, and calculates the output short-circuit current and measurement impedance on the system side and the new energy side based on the above calculation method. Taking the fault at the midpoint of the line as an example, the embodiment of the present application simulates and calculates the output short-circuit current and measurement impedance on the system side and the new energy side under the fault conditions of different grounding transition resistances, and the calculated values ​​and simulation values ​​are shown in Table 3. By comparing the results in Table 3, it can be seen that the calculation method proposed in the embodiment of the present application can accurately and effectively calculate the output short-circuit current value and measurement impedance value on the system side and the new energy side of the hybrid wind power system with a grid, and in the case of grounding through a small transition resistance, the amplitude error of the measured impedance obtained by calculation and the simulation result does not exceed 3%, and the angle error does not exceed 1 degree.

[0149] Table 3 Comparison of simulation and theoretical values ​​of short-circuit current and measured impedance on both sides under different transition resistances

[0150]

[0151] Furthermore, the embodiment of the present application performs simulation and calculation for different fault locations while keeping the ground transition resistance Rg=3Ω unchanged, and obtains the output short-circuit current and measurement impedance on the system side and the new energy side, and the calculated values ​​and simulation values ​​are shown in Table 4. By comparing the results, it can be seen that the calculation method proposed in the embodiment of the present application can accurately and effectively calculate the output short-circuit current value and measurement impedance value on the system side and the new energy side of the wind power system with a grid-connected hybrid system, and the amplitude error of the calculated measurement impedance does not exceed 3%, and the angle error does not exceed 1 degree.

[0152] Table 4 Comparison of simulation and theoretical values ​​of short-circuit current and measured impedance on both sides at different fault locations

[0153]

[0154] Based on the above results, it can be seen that the fault measurement impedance calculation method for the hybrid wind power system in the hybrid grid proposed in the embodiment of the present application can accurately calculate the short-circuit current on the system side and the new energy side after the hybrid wind power system fails, and can further solve the measured impedance value at the protection installation. The result can accurately reflect the actual measured impedance value at the protection installation on the new energy side, and has a very small result error with the actual result, which ensures the setting calculation of the protection, can accurately reflect the fault characteristic information of the hybrid AC system, and provide a basis for the study of the applicability of distance protection of the hybrid wind power system in the hybrid grid, which is of great significance for ensuring the safe and stable operation of the new energy system.

[0155] In addition, Fig.11As shown, in addition, the embodiment of the present application also discloses a fault measurement impedance calculation device for a hybrid wind power system with a grid, the device comprising:

[0156] A state acquisition module is used to obtain the characteristics of the grid-following and grid-forming converters in the power grid and the corresponding low voltage ride-through control strategy under fault conditions;

[0157] An equivalent circuit construction module is used to establish an equivalent circuit under the corresponding fault state according to the characteristics of the grid-following and grid-forming converters and the low voltage ride-through control strategy;

[0158] The simplified analysis module is used to simplify the analysis of the hybrid wind power system based on the equivalent circuit to obtain the equivalent network diagram under the corresponding fault state;

[0159] An equivalent network analysis module analyzes the equivalent network diagram to obtain a sub-circuit diagram of each power source excitation acting alone; the power source excitation at least includes a grid-forming voltage source, a grid-following current source, and a system-side voltage source;

[0160] The power supply excitation analysis module is used to analyze the sub-circuit diagrams corresponding to each power supply excitation, and obtain the short-circuit current and grid connection point voltage on the system side and the new energy side when each power supply excitation acts alone;

[0161] The superposition analysis module is used to superimpose the short-circuit currents on the system side and the renewable energy side corresponding to each power source excitation to obtain the total short-circuit currents on the system side and the renewable energy side in the hybrid wind power system of the grid, and to superimpose the grid connection point voltages corresponding to each power source excitation to obtain the total grid connection point voltage of the hybrid wind power system of the grid;

[0162] The impedance output module is used to calculate the measured impedance of the protection installation on the new energy side according to the total short-circuit current on the system side and the new energy side in the hybrid wind power system of the grid.

[0163] The device provided in the embodiment of the present application can achieve Figure 1 To avoid repetition, the various processes implemented by the method embodiment will not be described again here.

[0164] like Fig.12 As shown, the embodiment of the present application also provides an electronic device, including a processor and a memory, a program or instruction stored in the memory and executable on the processor, and when the program or instruction is executed by the processor, the following is achieved: Figure 1 The various processes of the method embodiment shown in the figure can achieve the same technical effect. To avoid repetition, they will not be described again here.

[0165] The embodiment of the present application also provides a readable storage medium on which a program or instruction is stored. When the program or instruction is executed by a processor, the above Figure 1The various processes of the method embodiments described above can achieve the same technical effect, and will not be described again here to avoid repetition.

[0166] The present application also provides a computer program product, including computer instructions, which, when executed by a processor, implement the above Figure 1 The various processes of the method embodiments described above can achieve the same technical effect, and will not be described again here to avoid repetition.

[0167] It should be understood that "one embodiment" or "an embodiment" mentioned throughout the specification means that specific features, structures or characteristics related to the embodiment are included in at least one embodiment of the present application. Therefore, "in one embodiment" or "in an embodiment" appearing throughout the specification does not necessarily refer to the same embodiment. In addition, these specific features, structures or characteristics can be combined in one or more embodiments in any suitable manner. It should be understood that in various embodiments of the present application, the size of the sequence number of the above-mentioned processes does not mean the order of execution, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application. The above-mentioned sequence numbers of the embodiments of the present application are only for description and do not represent the advantages and disadvantages of the embodiments.

[0168] It should be noted that, in this article, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprises a ..." does not exclude the existence of other identical elements in the process, method, article or device including the element.

[0169] In the several embodiments provided in the present application, it should be understood that the disclosed devices and methods can be implemented in other ways. The device embodiments described above are only schematic. For example, the division of the units is only a logical function division. There may be other division methods in actual implementation, such as: multiple units or components can be combined, or can be integrated into another device, or some features can be ignored or not executed. In addition, the coupling, direct coupling, or communication connection between the components shown or discussed can be through some interfaces, and the indirect coupling or communication connection of the devices or units can be electrical, mechanical or other forms.

[0170] The units described above as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units; they may be located in one place or distributed on multiple network units; some or all of the units may be selected according to actual needs to achieve the purpose of the present embodiment.

[0171] In addition, all functional units in the embodiments of the present application may be integrated into one processing unit, or each unit may be separately configured as a unit, or two or more units may be integrated into one unit; the above-mentioned integrated units may be implemented in the form of hardware or in the form of hardware plus software functional units.

[0172] A person skilled in the art can understand that all or part of the steps of implementing the above method embodiment can be completed by hardware related to program instructions, and the aforementioned program can be stored in a computer-readable storage medium. When the program is executed, it executes the steps of the above method embodiment; and the aforementioned storage medium includes: mobile storage devices, read-only memories (ROM), magnetic disks or optical disks, etc., various media that can store program codes.

[0173] Alternatively, if the above-mentioned integrated unit of the present application is implemented in the form of a software function module and sold or used as an independent product, it can also be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the embodiment of the present application can essentially or in other words, the part that contributes to the prior art can be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for a device (which can be a terminal or a platform, etc.) to execute all or part of the methods described in each embodiment of the present application. The aforementioned storage medium includes: various media that can store program codes, such as mobile storage devices, ROMs, magnetic disks, or optical disks.

[0174] The above is only a preferred implementation of the present application. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present application. These improvements and modifications should also be regarded as the scope of protection of the present application.

Claims

1. A method for calculating fault impedance of a hybrid wind power system connected to a grid, characterized in that: The grid-connected hybrid wind power system comprises at least a grid-connected converter and a grid-connected converter, and the method comprises the following steps: Obtain the characteristics of grid-following and grid-forming converters in the power grid and the corresponding low voltage ride-through control strategies under fault conditions; According to the characteristics of grid-following and grid-forming converters and the low voltage ride-through control strategy, an equivalent circuit under the corresponding fault state is established; According to the equivalent circuit, the hybrid wind power system with grid connection is simplified and analyzed to obtain the equivalent network diagram under the corresponding fault state; Analyze the equivalent network diagram to obtain the sub-circuit diagram of each power source excitation acting alone; the power source excitation at least includes a grid-forming voltage source, a grid-following current source, and a system-side voltage source; Analyze the subcircuit diagrams corresponding to each power source excitation, and obtain the short-circuit current and grid connection point voltage on the system side and the new energy side when each power source excitation acts alone; The short-circuit currents on the system side and the renewable energy side corresponding to each power source excitation are respectively superimposed to obtain the total short-circuit currents on the system side and the renewable energy side in the hybrid wind power system of the grid; the grid connection point voltages corresponding to each power source excitation are superimposed to obtain the total grid connection point voltage of the hybrid wind power system of the grid; The measured impedance at the protection installation point on the renewable energy side is obtained by calculating the total short-circuit current on the system side and renewable energy side in the hybrid wind power system: ； in It is the measured voltage at the protection installation of the new energy side; The measured current at the protection installation of the new energy side; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the internal potential of the network-type equivalent voltage source; It is the output current of the network-type equivalent current source; is the potential inside the power supply on the system side; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; is the additional impedance; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the power supply on the system side.

2. The method for calculating fault measurement impedance of a wind power system in a parallel-connected grid according to claim 1 is characterized in that: The corresponding low voltage ride through control strategy under the fault state includes a low voltage ride through control strategy of a grid-type converter; The specific steps of the low voltage ride through control strategy of the grid-type converter include: The active power reference value of the virtual synchronous machine is changed from Adjust to , the power angle of the virtual synchronous machine is kept stable during the fault, where the power angle of the virtual synchronous machine changes during the fault for: ； By switching the virtual internal potential amplitude reference value and reactive power reference value of the virtual synchronous machine, the short-circuit current is limited and reactive support is provided to the hybrid wind power system of the follow-up grid; the virtual internal potential reference value of the virtual synchronous machine switched during the fault is: ； Wherein a is the first calculated intermediate value, and a is obtained by the first intermediate value calculation formula, and the first intermediate value calculation formula is: ； Wherein b is the second calculated intermediate value, and b is obtained by the second intermediate value calculation formula, and the second intermediate value calculation formula is: ; in, is the rate of change of virtual rotation angular velocity of the virtual synchronous machine; P ref is the active power reference value of the virtual synchronous machine; P e is the output active power of the virtual synchronous machine; D is the virtual damping coefficient; J is the virtual moment of inertia; is the rated value of the grid frequency; E 0 is the potential inside the virtual synchronous machine before the fault; U g0 is the grid voltage before the fault; I 0 is the rated output current of the virtual synchronous machine; E ref It is the potential reference value of the virtual synchronous machine during the fault period; u gf is the voltage amplitude of the fault phase of the power grid; I gf is the virtual synchronous machine fault steady-state current limit threshold; Z eq is the equivalent total impedance of the virtual synchronous machine grid-connected system; When a fault occurs, a virtual impedance link is put into operation to perform short-circuit current limiting control and increase the equivalent output impedance of the virtual synchronous machine to suppress the short-circuit instantaneous large current impact of the virtual synchronous machine during the fault period.

3. The method for calculating fault impedance of a wind power system connected to a grid according to claim 1 is characterized in that: The corresponding low voltage ride through control strategy under the fault state includes a low voltage ride through control strategy of a grid-following converter; The specific steps of the low voltage ride-through control strategy of the grid-following converter include: According to the voltage drop degree of the grid connection point after the fault, the direct axis of the current inner loop is set. d With cross axis q The current reference value is: ； in For straight axis d Current reference value in the coordinate system; Cross axis q Current reference value in the coordinate system; is the active power reference value; U 0 is the grid connection point voltage; I max It is the maximum limiting current of the grid-following converter; I n is the rated current; is the per unit value; According to the current carrying capacity of power electronic equipment in the hybrid wind power system of the grid, limiting control is set for the PI link of the current inner loop controller, and the maximum current limiting amplitude is 1.2~2 times of the rated current.

4. The method for calculating fault measurement impedance of a wind power system in a parallel-connected grid according to claim 1 is characterized in that: The grid-connected hybrid wind power system comprises m Grid-connected converters and n When a fault occurs in a grid-type inverter, m The grid-type converter described above is equivalent to m Series equivalent output impedance Z s1 Voltage source E s node, n The grid-type converter described above is equivalent to n A current source controlled by the grid voltage I s1 node; When the equivalent circuit process corresponding to the fault state is established, m A voltage source node is equivalent to a voltage source E s and equivalent output impedance Z 1 series branch, the equivalent voltage source model of the corresponding branch is: ； in E S Output potential for equivalent voltage source; E ref is the potential reference value of the virtual synchronous machine during the fault period; Z 1 is the equivalent output impedance value; Z s1 is the equivalent output impedance value corresponding to a single voltage source node; n A current source node is equivalent to a current source I s branch, the equivalent current source model of the corresponding branch is: ； in is the amplitude of the short-circuit current output by the current source in the equivalent circuit; is the amplitude of the output short-circuit current corresponding to a single current source; is the phase angle of the short-circuit current output by the current source in the equivalent circuit; , Straight Axis d , cross axis q Current reference value in the coordinate system; U 0 is the grid connection point voltage.

5. The method for calculating fault measurement impedance of a wind power system in a parallel-connected grid according to claim 1 is characterized in that: When the grid-type voltage source acts alone, the short-circuit current on the system side and the new energy side and the grid connection point voltage are: ; in, To measure the grid connection point voltage of the hybrid wind power system; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the internal potential of the network-type equivalent voltage source; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the power supply on the system side; for and The total resistance calculated after connecting in parallel.

6. The method for calculating fault impedance of a parallel-connected wind power system according to claim 5 is characterized in that: When the grid-following current source acts alone, the short-circuit current on the system side and the new energy side and the grid connection point voltage are: ； in To measure the grid connection point voltage of the hybrid wind power system; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; It is the output current of the network-type equivalent current source; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the power supply on the system side; for and The total resistance calculated after connecting in parallel.

7. The method for calculating fault impedance of a parallel-connected wind power system according to claim 6 is characterized in that: When the system-side voltage source acts alone, the short-circuit current on the system side and the new energy side and the grid connection point voltage are: ； in To measure the grid connection point voltage of the hybrid wind power system; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the potential inside the power supply on the system side; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the power supply on the system side; for , and The total resistance calculated after connecting in parallel.

8. The method for calculating fault impedance of a parallel-connected wind power system according to claim 7 is characterized in that: The total short-circuit current on the system side and the new energy side and the total voltage at the grid connection point in the hybrid wind power system obtained by superimposing the excitation of each power source are: ； in To measure the grid connection point voltage of the hybrid wind power system; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the internal potential of the network-type equivalent voltage source; It is the output current of the network-type equivalent current source; is the potential inside the power supply on the system side; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the power supply on the system side.

9. A fault measurement impedance calculation device for a hybrid wind power system with a grid, characterized in that: The device comprises: A state acquisition module is used to obtain the characteristics of the grid-following and grid-forming converters in the power grid and the corresponding low voltage ride-through control strategy under fault conditions; An equivalent circuit construction module is used to establish an equivalent circuit under the corresponding fault state according to the characteristics of the grid-following and grid-forming converters and the low voltage ride-through control strategy; The simplified analysis module is used to simplify the analysis of the hybrid wind power system based on the equivalent circuit to obtain the equivalent network diagram under the corresponding fault state; An equivalent network analysis module analyzes the equivalent network diagram to obtain a sub-circuit diagram of each power source excitation acting alone; the power source excitation at least includes a grid-forming voltage source, a grid-following current source, and a system-side voltage source; The power supply excitation analysis module is used to analyze the sub-circuit diagrams corresponding to each power supply excitation, and obtain the short-circuit current and grid connection point voltage on the system side and the new energy side when each power supply excitation acts alone; The superposition analysis module is used to superimpose the short-circuit currents on the system side and the renewable energy side corresponding to each power source excitation to obtain the total short-circuit currents on the system side and the renewable energy side in the hybrid wind power system of the grid, and to superimpose the grid connection point voltages corresponding to each power source excitation to obtain the total grid connection point voltage of the hybrid wind power system of the grid; The impedance output module is used to calculate the measured impedance of the protection installation on the renewable energy side according to the total short-circuit current on the system side and renewable energy side in the hybrid wind power system: ； in It is the measured voltage at the protection installation of the new energy side; The measured current at the protection installation of the new energy side; It is the short-circuit current of the fault on the new energy side; is the short-circuit current of the system side fault; is the internal potential of the network-type equivalent voltage source; It is the output current of the network-type equivalent current source; is the potential inside the power supply on the system side; , , ; Z 1 is the equivalent output impedance of the network; Z L Transmit line impedance for network-type stations; Z MN It is the impedance of the high voltage side transmission line; is the additional impedance; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the power supply on the system side.

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