Fault measurement impedance calculation method and device for hybrid grid-following / grid-forming wind power system

Abstract

The present disclosure provides a fault measurement impedance calculation method and device for a hybrid grid-following / grid-forming wind power system. The method comprises: acquiring characteristics of a grid-following converter and a grid-forming converter, and corresponding low-voltage ride-through control strategies; establishing an equivalent circuit corresponding to a fault state; performing simplified analysis on a hybrid wind power system to obtain an equivalent network diagram; obtaining a branch circuit diagram under individual action of each power excitation; obtaining short-circuit currents and grid-connected point voltages on two sides of the system corresponding to the individual action of each power excitation; performing superimposition to obtain a total short-circuit current and a total grid-connected point voltage at the two sides of the hybrid wind power system; and obtaining a measurement impedance at a protection installation point on a new energy side. In the method, a measurement impedance value at a protection installation point of a hybrid wind power system after a fault occurs can be accurately calculated, so that the setting calculation of distance protection is ensured, so as to accurately reflect fault characteristic information of a hybrid AC system, thereby providing a basis for the distance protection applicability research of the hybrid wind power system, and thus ensuring the safe and stable operation of new energy systems.

Description

Fault measurement impedance calculation method and device for grid-connected hybrid wind power system TECHNICAL FIELD

[0001] The present disclosure relates to the technical field of new energy system fault analysis, in particular to a fault measurement impedance calculation method and device for grid-connected hybrid wind power system. BACKGROUND

[0002] At present, most new energy power systems adopt grid-connected control, especially in remote areas where wind and light resources are rich, and wind power stations, photovoltaic power stations and other scenes are gradually constructed. However, due to the large fluctuation of wind power and photovoltaic power systems, the system support capability is insufficient under weak power grid, and the problems of voltage and frequency stability are constantly emerging. Therefore, grid-connected new energy with independent support capability also gradually participates in the construction of power grid, thereby forming a grid-connected hybrid wind power system combining grid-connected control and grid-connected control. SUMMARY

[0003] In order to overcome the shortcomings of the prior art, the present disclosure provides a fault measurement impedance calculation method and device for grid-connected hybrid wind power system, which specifically adopts the following technical solutions:

[0004] The fault measurement impedance calculation method for grid-connected hybrid wind power system disclosed by the present disclosure comprises a grid-connected converter and a grid-connected converter, and the method comprises the following steps:

[0005] Obtain the characteristics of the grid-connected converter and the grid-connected converter in the power grid and the corresponding low voltage ride through control strategy under fault state;

[0006] Establish the equivalent circuit under the corresponding fault state according to the characteristics of the grid-connected converter and the grid-connected converter and the low voltage ride through control strategy;

[0007] Simplify the analysis of the grid-connected hybrid wind power system according to the equivalent circuit to obtain the equivalent network diagram under the corresponding fault state;

[0008] Analyze the equivalent network diagram to obtain the subcircuit diagram of the individual action of each power source excitation; the power source excitation at least includes a grid-connected voltage source, a grid-connected current source and a system-side voltage source;

[0009] Analyze the subcircuit diagram corresponding to each power source excitation to obtain the short-circuit current and the grid-connected point voltage corresponding to the system side and the new energy side when each power source excitation acts alone, respectively;

[0010] The short-circuit currents of the corresponding system side and new energy side of each power source are superimposed to obtain the total short-circuit currents of the system side and the new energy side of the grid-connected hybrid wind power system; the grid point voltages corresponding to each power source are superimposed to obtain the total grid point voltage of the grid-connected hybrid wind power system.

[0011] The measured impedance at the installation position of the new energy side protection is calculated according to the total short-circuit currents of the system side and the new energy side in the grid-connected hybrid wind power system.

[0012] Optionally, the low-voltage ride-through control strategy in the fault state includes a low-voltage ride-through control strategy of the grid-connected converter; and the low-voltage ride-through control strategy of the grid-connected converter includes the following steps:

[0013] The active power reference value of the virtual synchronous machine is adjusted from P ref to P e , so as to maintain the power angle stability of the virtual synchronous machine during the fault, wherein the power angle change Δδ of the virtual synchronous machine during the fault is:

[0014] The short-circuit current is limited and the reactive power support is provided for the grid-connected hybrid wind power system by switching the virtual internal voltage amplitude reference value and the reactive power reference value of the virtual synchronous machine; wherein the switched virtual internal voltage reference value of the virtual synchronous machine during the fault is:

[0015] wherein a is a first calculation intermediate value, and a is obtained from a first intermediate value calculation formula, and the first intermediate value calculation formula is:

[0016] wherein b is a second calculation intermediate value, and b is obtained from a second intermediate value calculation formula, and the second intermediate value calculation formula is:

[0017] wherein Δω is the virtual rotational speed change rate 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; ω n is the rated value of the grid frequency; E0 is the internal voltage of the virtual synchronous machine before the fault; U g0 is the grid voltage before the fault; I0 is the rated output current of the virtual synchronous machine; E ref is the internal voltage reference value of the virtual synchronous machine during the fault; u gf is the voltage amplitude of the fault phase of the 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.

[0018] A virtual impedance element is put in when a fault occurs to perform short-circuit current limiting control and increase the equivalent output impedance of the virtual synchronous machine to suppress the short-circuit transient impact current of the virtual synchronous machine during the fault.

[0019] Optionally, the low-voltage ride-through control strategy in the fault state comprises a low-voltage ride-through control strategy of the grid-following converter, and the specific steps of the low-voltage ride-through control strategy of the grid-following converter are as follows:

[0020] According to the voltage drop of the grid-connected point after the fault, the current reference values of the direct axis d and the quadrature axis q of the current inner loop are set as:

[0021] wherein I dref is the current reference value in the direct axis d coordinate system; I qref is the current reference value in the quadrature axis q coordinate system; P ref is the active power reference value; U0 is the grid-connected point voltage; I max is the maximum limited current of the grid-following converter; I n is the rated current; and p.u. is the unit value.

[0022] According to the current resistance of the power electronic equipment in the grid-constructing and grid-following hybrid wind power system, the PI elements of the current inner loop controller are all set to have limiting control, and the maximum current limiting amplitude is 1.2-2 times the rated current.

[0023] Optionally, the grid-constructing and grid-following hybrid wind power system comprises m grid-constructing converters and n grid-following converters, and when a fault occurs, the m grid-constructing converters are equivalent to m voltage sources E s1 nodes in series with equivalent output impedances Z s , and the n grid-following converters are equivalent to n current sources I s1 nodes controlled by the grid-connected point voltage.

[0024] When the equivalent circuit in the corresponding fault state is established, the m voltage source nodes are equivalent to a branch of a voltage source E s in series with an equivalent output impedance Z1, and the equivalent voltage source model of the corresponding branch is:

[0025] wherein E S is the output potential of the equivalent voltage source; E ref is the internal potential reference value of the virtual synchronous machine during the fault; Z1 is the equivalent output impedance value; and Z s1 is the equivalent output impedance value corresponding to a single voltage source node.

[0026] The n current source nodes are equivalent to a current source I s branch, and the equivalent current source model of the corresponding branch is:

[0027] wherein I S is the amplitude of the short-circuit current output by the current source in the equivalent circuit; I S1 is the amplitude of the short-circuit current output by a single current source; δ S is the phase angle of the short-circuit current output by the current source in the equivalent circuit; Id re f, I qre f are the current reference values in the d, q coordinate system respectively; U0 is the grid-connection point voltage.

[0028] Optionally, the short-circuit currents corresponding to the system side and the new energy side and the grid-connection point voltage of the grid-connection type voltage source acting alone are:

[0029] wherein, is the grid-connection point voltage of the grid-connection and hybrid wind power system; is the new energy side fault short-circuit current; is the system side fault short-circuit current; is the internal potential of the grid-connection type equivalent voltage source; Z eq1 = Z1+ Z L , Z eq3 = αZ MN , Z eq4 = (1-α)Z MN + Z s3 ; Z1 is the grid-connection type equivalent output impedance; Z L is the grid-connection type substation outgoing line impedance; Z MN is the high-voltage side outgoing line impedance; α is the fault position coefficient; R g is the transition resistance; Z s3 is the internal resistance of the system side power supply; Z eq4 / / R g is the total resistance value calculated after Z eq4 and R g are connected in parallel.

[0030] Optionally, the short-circuit currents corresponding to the system side and the new energy side and the grid-connection point voltage of the grid-connection type voltage source acting alone are:

[0031] wherein is the grid-connection point voltage of the grid-connection and hybrid wind power system; is the new energy side fault short-circuit current; is the system side fault short-circuit current; is the output current of the grid-connection type equivalent current source; Z eq1 = Z1+ Z L , Z eq3 = αZ MNZ eq4 =(1-α)Z MN +Z s3 Z1 is the network-type equivalent output impedance; Z L For network-type substations, the output line impedance is Z. MN The high-voltage side transmission line impedance is α; the fault location coefficient is R. g For transition resistance; Z s3 Z is the internal resistance of the system-side power supply; eq4 / / R g For Z eq4 With R g The total resistance calculated after parallel connection.

[0032] Optionally: When the system-side voltage source operates alone, the short-circuit current on the system side and the new energy source side, as well as the grid connection point voltage, are as follows:

[0033] in To match the grid connection voltage of the hybrid wind power system; This refers to the fault short-circuit current on the new energy side. This refers to the system-side fault short-circuit current; Z represents the internal potential of the system-side power supply. eq1 =Z1+Z L Z eq3 =αZ MN Z eq4 =(1-α)Z MN +Z s3 Z1 is the network-type equivalent output impedance; Z L The impedance of the transmission lines for grid-type substations; Z MN The high-voltage side transmission line impedance is α; the fault location coefficient is R. g For transition resistance; Z s3 The internal resistance of the system-side power supply; (Z) eq1 +Z eq3 ) / / R g For Z eq1 Z eq3 With R g The total resistance calculated after parallel connection.

[0034] Optional: The total short-circuit current and total grid connection voltage of the system side and the renewable energy side in the grid-connected hybrid wind power system obtained by superimposing the excitations of various power sources are:

[0035] in To match the grid connection voltage of the hybrid wind power system; This refers to the fault short-circuit current on the new energy side. This refers to the system-side fault short-circuit current; The internal potential of the grid-constructing equivalent voltage source is: The output current of the grid-following equivalent current source is: The internal potential of the system-side power supply is: eq1 = Z1+ Z L , Z eq3 = αZ MN , Z eq4 = (1- α)Z MN + Z s3 ; Z1 is the equivalent output impedance of the grid-constructing type; Z L is the impedance of the outgoing line of the grid-constructing type station; Z MN is the impedance of the outgoing line of the high-voltage side; α is a fault position coefficient; R g is a transition resistance; Z s3 is the internal resistance of the system-side power supply.

[0036] Optionally, the step of obtaining the measured impedance at the installation position of the new energy side protection according to the total short-circuit current of the system side and the new energy side in the grid-constructing and grid-following hybrid wind power system comprises:

[0037] wherein is the measured voltage at the installation position of the new energy side protection; is the measured current at the installation position of the new energy side protection; is the fault short-circuit current of the new energy side; is the fault short-circuit current of the system side; The internal potential of the grid-constructing equivalent voltage source is: The output current of the grid-following equivalent current source is: The internal potential of the system-side power supply is: eq1 = Z1+ Z L , Z eq3 = αZ MN , Z eq4 = (1- α)Z MN + Z s3 ; Z1 is the equivalent output impedance of the grid-constructing type; Z L is the impedance of the outgoing line of the grid-constructing type station; Z MN is the impedance of the outgoing line of the high-voltage side; ΔZ is an additional impedance; α is a fault position coefficient; R g is a transition resistance; Z s3 is the internal resistance of the system-side power supply.

[0038] The application further discloses a device for calculating the fault measured impedance of a grid-constructing and grid-following hybrid wind power system, and the device comprises:

[0039] a state acquisition module, configured to acquire the characteristics of the grid-following type and the grid-constructing type converters in the power grid and the corresponding low-voltage ride-through control strategy under the fault state;

[0040] An equivalent circuit construction module is configured to construct an equivalent circuit under a corresponding fault state according to characteristics of the grid-following converter and the grid-forming converter and a low-voltage ride-through control strategy;

[0041] A simplified analysis module is configured to perform simplified analysis on the grid-following and grid-forming hybrid wind power system according to the equivalent circuit to obtain an equivalent network diagram under the corresponding fault state;

[0042] An equivalent network analysis module is configured to analyze the equivalent network diagram to obtain a sub-circuit diagram under the action of each power source excitation; the power source excitation at least includes a grid-forming voltage source, a grid-following current source, and a system-side voltage source;

[0043] A power source excitation analysis module is configured to analyze the sub-circuit diagram corresponding to each power source excitation to respectively obtain short-circuit currents of the system side and the new energy side and a grid-connected point voltage under the action of each power source excitation;

[0044] A superposition analysis module is configured to superimpose the short-circuit currents of the system side and the new energy side corresponding to each power source excitation to obtain total short-circuit currents of the system side and the new energy side in the grid-following and grid-forming hybrid wind power system, and superimpose the grid-connected point voltages corresponding to each power source excitation to obtain a total grid-connected point voltage of the grid-following and grid-forming hybrid wind power system;

[0045] An impedance output module is configured to calculate a measurement impedance at a protection installation position of the new energy side according to the total short-circuit currents of the system side and the new energy side in the grid-following and grid-forming hybrid wind power system. BRIEF DESCRIPTION OF DRAWINGS

[0046] FIG. 1 is a flowchart of a fault analysis method of a grid-following and grid-forming hybrid wind power system according to an embodiment of the present disclosure.

[0047] FIG. 2 is an analysis model diagram of a grid-connected sending line of the grid-following and grid-forming hybrid wind power system according to an embodiment of the present disclosure.

[0048] FIG. 3 is a main circuit and control block diagram of a grid-forming converter according to an embodiment of the present disclosure.

[0049] FIG. 4 is a main circuit and control block diagram of a grid-following converter according to an embodiment of the present disclosure.

[0050] FIG. 5 is a steady-state equivalent circuit of the grid-forming converter according to an embodiment of the present disclosure.

[0051] FIG. 6 is a steady-state equivalent circuit of the grid-following converter according to an embodiment of the present disclosure.

[0052] FIG. 7 is an equivalent network diagram of the grid-following and grid-forming hybrid wind power system under a three-phase fault according to an embodiment of the present disclosure.

[0053] FIG. 8 is an equivalent network diagram of the grid-forming converter under the action of an equivalent voltage source according to an embodiment of the present disclosure.

[0054] Fig. 9 is an equivalent network diagram when the system-side voltage source of the grid-constructing hybrid wind power system in the embodiment of the present disclosure acts alone.

[0055] Fig. 10 is an equivalent network diagram when the grid-connection type equivalent current source of the grid-constructing hybrid wind power system in the embodiment of the present disclosure acts alone.

[0056] Fig. 11 is a structural schematic diagram of a measuring impedance calculation device of the grid-constructing hybrid wind power system in the embodiment of the present disclosure.

[0057] Fig. 12 is a structural schematic diagram of an electronic device in the embodiment of the present disclosure. DETAILED DESCRIPTION

[0058] The technical solutions in the embodiments of the present disclosure will be described clearly and completely below with reference to the drawings in the embodiments of the present disclosure. Obviously, the described embodiments are only part of the embodiments of the present disclosure, rather than all the embodiments. Therefore, those skilled in the art should recognize that various changes and modifications can be made to the described embodiments without departing from the scope and spirit of the present disclosure. Also, in order to be clear and concise, the description of the well-known functions and structures is omitted in the following description. Based on the embodiments in the present disclosure, all other embodiments obtained by those skilled in the art without making creative efforts fall within the scope of protection of the present disclosure.

[0059] Due to the differences in the control structures between different wind turbine generators, the grid-constructing hybrid wind power system has complex stability problems in time and space scales. Meanwhile, due to the differences in the fault ride-through control strategies adopted by different types of units, the short-circuit current calculation methods of the grid-connection type converter and the grid-constructing type converter under different operating environments after a fault exist differences. In addition, in different new energy sending system scenarios, the dynamic characteristics of the highly power electronic new energy generation equipment and system interact complexly, and are affected by the short-circuit current tolerance of the power electronic devices and the system strength, so that the weak feeding characteristics of the new energy are further amplified, and there is a big difference in the amplitude and phase angle of the short-circuit current on both sides of the system. With the increase of the grid-connected new energy capacity and the complex actual working conditions, the topology structure of the sending system changes greatly, making it difficult to analyze the fault characteristics of the entire new energy system, and the traditional distance protection calculation method is difficult to accurately calculate the short-circuit current of the grid-constructing hybrid wind power system. Therefore, it is urgent to develop a method for analyzing the measured impedance of the grid-constructing hybrid wind power system containing the grid-connection type converter and the grid-constructing type converter, so as to accurately solve the measured impedance of the grid-constructing hybrid wind power system, and then correctly reflect the short-circuit fault, so as to ensure the safe and stable operation of the new energy system.

[0060] The technical solutions of the present disclosure have the following beneficial effects:

[0061] The calculation method of the present disclosure analyzes the characteristics of the grid-following converter and the grid-forming converter in the grid-following and grid-forming hybrid wind power system, respectively establishes the equivalent circuit of the grid-following converter and the grid-forming converter, analyzes and establishes the equivalent network diagram of the grid-following and grid-forming hybrid wind power system for each equivalent circuit, respectively solves each state quantity on both sides of the system after the fault based on the superposition theorem, and further solves the measured impedance at the installation place of the new energy side protection. The calculation method can accurately calculate the short-circuit current on both sides of the grid-following and grid-forming hybrid wind power system after the fault, so as to further solve the measured impedance value at the installation place of the protection. The result error between the calculation value obtained by the method and the actual measured value is not more than 3%, which ensures the setting calculation of the distance protection, can accurately reflect the fault characteristic information of the grid-following and grid-forming hybrid wind power system, provides a basis for the distance protection applicability research of the grid-following and grid-forming hybrid wind power system, and ensures the safe and stable operation of the new energy system.

[0062] As shown in (a) of FIG. 2, it is a schematic diagram of the sending-out line of the grid-following and grid-forming hybrid wind power system in the embodiment of the present disclosure. In the embodiment of the present disclosure, the grid-following new energy station adopts a wind power station or a photovoltaic station, and the grid-forming new energy station adopts a virtual synchronous machine control. The grid-following new energy station and the grid-forming new energy station are connected through a transformer and a sending-out line. The AC system on the right side is an infinite power source, and the AC system is a weak system. In the embodiment of the present disclosure, a three-phase short-circuit fault occurs on the sending-out line, and the fault is grounded through a transition resistance. In addition, in order to further simplify the calculation, the present disclosure ignores the mutual interaction between the new energy units. After the model of the sending-out line of the grid-following and grid-forming hybrid wind power system in (a) of FIG. 2 is simplified, the equivalent sending-out system analysis model of the grid-following and grid-forming hybrid wind power system in (b) of FIG. 2 is obtained. The embodiment of the present disclosure analyzes and calculates based on the equivalent sending-out system analysis model of the grid-following and grid-forming hybrid wind power system.

[0063] Specifically, as shown in FIG. 1, the embodiment of the present disclosure discloses a calculation method of a fault measured impedance of a grid-following and grid-forming hybrid wind power system. The grid-following and grid-forming hybrid wind power system at least includes a grid-following converter and a grid-forming converter. The method includes the following steps:

[0064] Step one, obtaining the characteristics of the grid-following converter and the grid-forming converter in the power grid and the corresponding low voltage ride through control strategy under the fault state.

[0065] For the analysis of the characteristics of the grid-forming converter and the low voltage ride through control strategy:

[0066] The disclosure embodiment analyzes the grid-connected converter based on the virtual synchronous machine control, which adopts the decoupling control in the dq coordinate system, and the control mainly includes three parts of the virtual synchronous machine control, the virtual impedance control and the voltage and current double closed loop control. Correspondingly, the main circuit and the control block diagram of the grid-connected converter are as shown in FIG. 3. The virtual synchronous machine control provides damping and inertia for the system through the active-power-frequency control and the reactive-power-voltage control, and realizes synchronization through the internal potential amplitude and phase angle constructed by the output power itself. The virtual impedance control simulates the stator impedance by introducing the virtual impedance link, equivalently increases the equivalent impedance of the system and improves the current limiting capability of the converter. Finally, the voltage and current double closed loop control tracks and controls the current through the voltage and current double closed loop of the double PI control, and realizes the control through the PWM modulation of the generated voltage signal.

[0067] When a fault occurs, the low voltage ride through control strategy of the grid-connected converter is to adjust the active power reference value of the virtual synchronous machine from P ref to P e , so as to maintain the power angle stability of the virtual synchronous machine during the fault; and by switching the virtual internal potential amplitude reference value and the reactive power reference value of the virtual synchronous machine, the system can be provided with certain reactive power support while limiting the short circuit current. Specifically, the power angle change of the virtual synchronous machine during the fault is as shown in formula (1), and the switched internal potential amplitude reference value is as shown in formula (2):

[0068] Wherein a is a first calculation intermediate value, a is obtained from a first intermediate value calculation formula, and the first intermediate value calculation formula is:

[0069] Wherein b is a second calculation intermediate value, b is obtained from a second intermediate value calculation formula, and the second intermediate value calculation formula is:

[0070] Wherein, Δω is the virtual rotational speed change rate 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 rotational inertia; ω n is the rated value of the grid frequency; E0 is the internal potential of the virtual synchronous machine before the fault; U g0 is the grid voltage before the fault; I0 is the rated output current of the virtual synchronous machine; E ref is the internal potential reference value of the virtual synchronous machine during the fault; u gf is the voltage amplitude of the fault phase of the grid; I gf is the virtual synchronous machine fault steady-state current limit threshold, which is generally taken as 1.3I0; Z eq is the equivalent total impedance of the virtual synchronous machine grid-connected system.

[0071] In addition, in the embodiment of the present disclosure, a virtual impedance element is put into operation when a fault occurs, so as to perform short-circuit current limiting control (generally, I max =1.3I0), and further increase the equivalent output impedance of the virtual synchronous machine, so as to suppress the short-circuit transient current that occurs during the fault of the virtual synchronous machine. In the embodiment of the present disclosure, during the asymmetric fault of the power grid, through positive and negative sequence separation control, a 90° phase shift element is constructed, and a double generalized integrator is used to filter out the double-frequency fluctuation components of the voltage and current positive and negative sequence dq components in the opposite rotating coordinate system, and then the dq direct current components in the respective rotating coordinate systems are obtained. Based on the above, in the embodiment of the present disclosure, the negative sequence current reference value in the negative sequence loop of the virtual synchronous machine is adjusted to realize control under different targets.

[0072] The characteristics of the grid-following converter and the low-voltage ride-through control strategy are analyzed.

[0073] In the embodiment of the present disclosure, the grid-following converter in grid-connected operation is analyzed, which also uses decoupling control in the dq coordinate system. The control mainly includes outer loop control, inner loop control, and phase-locked loop (PLL) components. Correspondingly, the main circuit and control block diagram of the grid-following converter are shown in FIG. 4. Among them, the phase-locked loop PLL synchronizes with the AC power grid by tracking the AC voltage to obtain the amplitude and phase angle of the AC voltage of the power grid. The outer loop controller adopts PQ decoupling control, and sets the reference value as P ref and Q ref to realize accurate tracking of active and reactive power control, so as to realize constant output power. Further, the inner loop controller adopts current inner loop decoupling control, which tracks the current reference value through a PI element to realize direct output current control.

[0074] When a fault occurs in the power grid, the new energy such as photovoltaic power station needs to have low-voltage ride-through capability according to the national standard. At this time, the power grid needs to be provided with certain reactive power support to maintain stable operation during the fault. Therefore, according to the voltage drop of the grid-connected point after the fault, the 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. At this time, the low-voltage ride-through control strategy adopted is shown in formula (3):

[0075] Among them, I dref and I qref are the current reference values in the d-axis and q-axis coordinate systems; P ref is the active power reference value; U0 is the grid-connected point voltage; I max is the maximum limit current of the grid-following converter; I n is the rated current; and p.u. is the unit value.

[0076] In addition, affected by the current-carrying capacity of the power electronic device, the PI link of the inner loop controller in the embodiment of the present disclosure is provided with a limiting control, and the maximum current limiting amplitude is generally selected as 1.2-2 times of the rated current. In addition, when the power grid is subjected to asymmetric faults, the negative sequence current component is controlled through the positive and negative sequence separation link to achieve different control targets such as balanced output current or power fluctuation suppression.

[0077] Step two, according to the characteristics of grid-connected converters and network-constructed converters and the low-voltage ride-through control strategy, the equivalent circuit under the corresponding fault state is established.

[0078] Firstly, according to the characteristics of the network-constructed converter and the corresponding low-voltage ride-through control strategy, the equivalent circuit under the fault state is established.

[0079] Specifically, the network-constructed converter can be equivalent to a voltage source during the fault period because it can autonomously construct the voltage amplitude and phase angle, and the virtual synchronous machine inner potential and power angle remain constant during the low-voltage ride-through process under the influence of the virtual synchronous machine low-voltage ride-through control strategy. In normal operation, the network-constructed converter calculates its corresponding equivalent output impedance according to the voltage relationship between the filter impedance and the inner loop controller, and the value remains constant. During the fault period, the virtual impedance R v +jX v is introduced in the control loop, and the positive and negative sequence equivalent system impedance of the network-constructed converter is shown in formula (4):

[0080] Wherein m is the current limiting coefficient; R f , L f are the filter parasitic resistance and filter inductance respectively; k p1 , k i1 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 current inner loop respectively; R V is the virtual resistance; L V is the virtual inductance.

[0081] According to formula (4), the equivalent impedance of the network-constructed converter is related to the parameters of the PI controller, the limiting coefficient and the filter impedance, and the value remains constant during the steady state process. Therefore, the network-constructed converter during the fault period can be equivalent to a voltage source in series with the equivalent output impedance, as shown in FIG. 5. Since the virtual inner potential of the virtual synchronous machine is determined by the control strategy of the reactive power loop, the steady-state output short-circuit current can be represented by formula (5) as:

[0082] Wherein E∠δ1 is the network-constructed inner potential amplitude and phase angle; U0∠δ0 is the grid-connected point voltage amplitude and phase angle.

[0083] Therefore, when several inverters in a new energy station are connected in parallel to the power grid through a transformer, the grid-following wind power system can include m grid-constructing converters and n grid-following converters. The m grid-constructing converters can be equivalent to m voltage sources E s1 in series with equivalent output impedance Z s , and therefore, when a short-circuit fault occurs at the system side, the interaction between new energies is ignored, and it is assumed that the same inverters are used in the grid-constructing and grid-following new energy stations. The m voltage source nodes are equivalent to a voltage source E s and a branch in series with equivalent output impedance Z1(Z1=Z s1 / m). The equivalent voltage source model can be represented as:

[0084] where E S is an equivalent voltage source output potential, E ref is a potential reference value in a virtual synchronous machine during a fault, Z1 is an equivalent output impedance value, and Z s1 is an equivalent output impedance value corresponding to a single voltage source node.

[0085] Secondly, according to the characteristics of the grid-following converter and the corresponding low-voltage ride-through control strategy, an equivalent circuit in a fault state is established.

[0086] Specifically, the AC terminal of the grid-following converter is simulated by a current source characteristic, the grid voltage is followed according to a phase-locked loop to keep synchronization, and the output power is controlled by directly controlling the size of the output current. The control adopted in the embodiment of the present disclosure is directed to the grid-connected point voltage, and therefore the amplitude and phase angle of the short-circuit current output by the grid-following converter are affected by the grid-connected point voltage. When a fault occurs, the grid-following converter can be equivalent to a current source I s , which is controlled by the grid-connected point voltage. As shown in FIG. 6, the amplitude and phase angle of the output short-circuit current are represented by formula (6):

[0087] When a short-circuit fault occurs at the system side, the interaction between new energies is ignored, and it is assumed that the same inverters are used in the grid-constructing and grid-following new energy stations. The n grid-following converters can be equivalent to n current sources I s1 controlled by the grid-connected point voltage, and the n current source nodes in the equivalent circuit can be equivalent to a current source I s (I s =nI s1 ) branch, and the equivalent current source model can be represented as:

[0088] where I Sis the amplitude of the short-circuit current output by the current source in the equivalent circuit; I S1 is the amplitude of the short-circuit current output by a single current source; δ S is the phase angle of the short-circuit current output by the current source in the equivalent circuit; I dref , I qref are current reference values in d and q coordinate systems, respectively; U0 is the grid-connected point voltage.

[0089] Step three, the equivalent network diagram under the fault state is obtained by simplifying the analysis of the grid-connected hybrid wind power system according to the equivalent circuit.

[0090] Firstly, based on the grid-connected hybrid wind power system in FIG. 2, all devices are reduced to the same voltage level. When a three-phase short-circuit fault occurs at point f in FIG. 2, the transition resistance R g is grounded. According to the above analysis, it can be known that, under the influence of the low-voltage ride-through control strategy, the grid-connected new energy station can be equivalent to a current source controlled by the grid-connected point voltage, the grid-forming new energy station can be equivalent to a voltage source with series impedance, and the system side adopts an infinite power source instead. In order to facilitate calculation, ideal transformer models are adopted for both two-stage transformers, and the corresponding fault equivalent network diagram of the grid-connected hybrid wind power system is established as shown in FIG. 7. It should be noted that, in FIG. 7, E s is the internal potential of the grid-forming equivalent voltage source; I s is the output current of the grid-connected equivalent current source; Z1 is the output impedance of the grid-forming equivalent; I M and I N are fault short-circuit currents on the system side; Z MN is the sending-out line impedance; α is a fault position coefficient; R g is the transition resistance; U and Z s3 are the internal potential and internal resistance of the system side power source, respectively.

[0091] Step four, the equivalent network diagram is analyzed to obtain a subcircuit diagram of the excitation of each power source. The excitation of the power source at least includes the grid-forming voltage source, the grid-connected current source and the system side voltage source.

[0092] When a three-phase short-circuit fault occurs on the system side, since the transient process of the new energy side converter is short, the new energy side converter quickly enters the steady state stage after the fault, and therefore the three-phase short-circuit currents on the new energy side and the system side can be considered as the solution of the periodic component in the steady state circuit. Since the new energy control structure is different from the traditional synchronous machine, there is a great difference between the amplitudes and phase angles of the short-circuit currents fed out on the system side, which causes the change of the measured impedance characteristics at the installation position of the protection in the new energy system, and the traditional calculation method of the measured impedance is no longer applicable.

[0093] As shown in FIG. 7, based on the superposition theorem, the short-circuit currents on the two sides of the grid-connected hybrid wind power system can be considered as the sum of the short-circuit currents of the grid-forming voltage source Es , the grid-following current source I s and the system-side voltage source U three power sources excitation result, when the grid-forming voltage source E s alone, the system-side voltage source U short circuit, the grid-following current source I s open circuit, its corresponding system equivalent circuit diagram as shown in Figure 8; when the system-side voltage source U short circuit alone, the grid-forming voltage source E s short circuit, the grid-following current source I s open circuit, its corresponding system equivalent circuit diagram as shown in Figure 9; when the grid-following current source I s alone, the grid-forming voltage source E s and the system-side voltage source U short circuit, its corresponding system equivalent circuit diagram as shown in Figure 10. Therefore, according to the subcircuit diagram of the three power sources alone, the short circuit current of the system side and the new energy side after the three-phase short circuit fault occurs on the corresponding line can be obtained, and then superposition solution is carried out.

[0094] Step five, analyze the subcircuit diagram corresponding to each power source excitation, and obtain the short circuit current of the system side and the new energy side and the grid-connected point voltage when each power source excitation alone.

[0095] Specifically, the embodiments of the present disclosure analyze the three power sources excitation alone, namely the grid-forming voltage source E s , the grid-following current source I s and the system-side voltage source U:

[0096] For the grid-forming voltage source E s excitation alone:

[0097] When only the grid-forming equivalent voltage source alone in the grid, its corresponding system equivalent circuit diagram as shown in Figure 8. Because the AC system is a weak system, the system-side impedance is large, and when the grounding transition resistance is small, the right side network R g and Z eq4 parallel mainly shows the characteristics of the transition resistance, and the grid-forming equivalent output impedance Z eq1 is large, so the short circuit current of the system side and the new energy side and the grid-connected point voltage when the grid-forming voltage source alone are as shown in formula (7):

[0098] Wherein, is the grid-connected point voltage of the grid-forming and grid-following hybrid wind power system; is the fault short circuit current of the new energy side; is the fault short circuit current of the system side; is the internal potential of the grid-forming voltage source; Z eq1 =Z1+Z L , Zeq3 = aZ MN , Z eq4 = (1-a)Z MN + Z s3 ; Z1is the equivalent output impedance of the grid-forming type; Z L is the grid-forming type substation outgoing line impedance; Z MN is the high-voltage side outgoing line impedance; a 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 Z eq4 / / R g indicates that Z eq4 and R g are connected in parallel, and the result is calculated in parallel resistance.

[0099] From the above formula (7), it can be seen that when the grid-forming type voltage source acts alone, the grid-connected point voltage and the new energy side short-circuit current are mainly affected by the grid-forming type internal potential, the grid-forming type equivalent output impedance and the fault location, and the system side short-circuit current is mainly affected by the grid-forming type internal potential, the grid-forming type equivalent output impedance, the system side equivalent impedance and the transition resistance.

[0100] For the system side voltage source U power excitation alone:

[0101] When only the system side voltage source U acts alone in the power grid, the system equivalent circuit diagram is shown in FIG. 9. Due to the small grounding transition resistance and the large grid-forming type equivalent output impedance Z eq1 , the left parallel circuit mainly shows the transition resistance characteristic. At this time, the short-circuit currents on the system side and the new energy side and the grid-connected point voltage corresponding to the grid-forming type current source acting alone are shown in formula (8):

[0102] wherein is the grid-connected point voltage of the grid-forming and grid-mixing wind power system; is the new energy side fault short-circuit current; is the system side fault short-circuit current; is the system side power internal potential; Z eq1 = Z1+ Z L , Z eq3 = aZ MN , Z eq4 = (1-a)Z MN + Z s3 ; Z1is the equivalent output impedance of the grid-forming type; Z L is the grid-forming type substation outgoing line impedance; Z MN is the high-voltage side outgoing line impedance; a is the fault location coefficient; R g is the transition resistance; Z s3is the internal resistance of the system-side power supply. It should be noted that Z eq1 +Z eq3 / / R g is to indicate that Z eq1 and R eq3 are connected in parallel, and the result is calculated as resistance in parallel. g s is connected in parallel, and the result is calculated as resistance in parallel.

[0103] From the above formula (8), it can be seen that when the system-side voltage source acts alone, the grid-connected point voltage and the new energy side output short-circuit current are mainly affected by the system-side voltage, the system-side equivalent impedance, the grid-forming type equivalent output impedance, and the transition resistance. The system-side output short-circuit current is mainly affected by the system-side voltage and the system-side equivalent impedance.

[0104] For the grid-following type current source I s power excitation alone:

[0105] When only the grid-following type equivalent current source I s acts alone, the system equivalent circuit diagram is shown in FIG. 10. Since the system-side equivalent impedance Z eq4 is large, the right side parallel circuit mainly shows the transition resistance characteristic. At this time, the short-circuit current corresponding to the system side and the new energy side and the grid-connected point voltage of the grid-following type current source acting alone are shown in formula (9):

[0106] wherein V is the grid-connected point voltage of the grid-forming and grid-connecting mixed wind power system; is the fault short-circuit current of the new energy side; is the fault short-circuit current of the system side; is the output current of the grid-following type equivalent current source; Z eq1 = Z1+ Z L , Z eq3 = αZ MN , Z eq4 = (1-α)Z MN +Z s3 ; Z1 is the grid-forming type equivalent output impedance; Z L is the grid-forming type station sending out line impedance; Z MN is the high-voltage side sending out line impedance; α is the fault position coefficient; R g is the transition resistance; Z s3 is the internal resistance of the system-side power supply. It should be noted that Z eq4 / / R g is to indicate that Z eq4 and R g are connected in parallel, and the result is calculated as resistance in parallel.

[0107] From the above formula, it can be seen that the grid-connected point voltage and the short-circuit current output on the new energy side are mainly affected by the equivalent output impedance of the network type, the fault position and the output current of the photovoltaic, and the system side output short-circuit current is mainly affected by the equivalent output impedance of the network type, the equivalent impedance of the system side, the output current of the network type and the transition resistance.

[0108] Step six, superimpose the short-circuit current of the corresponding system side and the new energy side and the grid-connected point voltage of each power excitation to obtain the total short-circuit current of the system side and the new energy side and the total voltage of the grid-connected point in the wind power system with network and grid mixed connection.

[0109] Specifically, according to the superposition theorem, the total short-circuit current of the system side and the new energy side and the total voltage of the grid-connected point in the wind power system with network and grid mixed connection can be obtained by superimposing the relevant quantities of three excitation single actions. Therefore, the total short-circuit current of the system side and the new energy side and the total voltage of the grid-connected point in the wind power system with network and grid mixed connection obtained by superimposing each power excitation in the embodiment of the present disclosure is shown in formula (10):

[0110] Among them V is the grid-connected point voltage of the wind power system with network and grid mixed connection; I is the fault short-circuit current on the new energy side; I is the fault short-circuit current on the system side; V is the internal potential of the network type equivalent voltage source; I is the output current of the network type equivalent current source; V is the internal potential of the system side power supply; Z eq1 =Z1+Z L , Z eq3 =αZ MN , Z eq4 =(1-α)Z MN +Z s3 ; Z1 is the equivalent output impedance of the network type; Z L is the impedance of the transmission line of the network type station; Z MN is the impedance of the transmission line of the high-voltage side; α is the fault position coefficient; R g is the transition resistance; Z s3 is the internal resistance of the system side power supply.

[0111] Step seven, calculate the measurement impedance at the installation position of the new energy side protection according to the short-circuit current of the system side and the new energy side in the wind power system with network and grid mixed connection.

[0112] Specifically, the measurement impedance at the installation position of the new energy side protection in the embodiment of the present disclosure can be represented by formula (11):

[0113] Among them V is the measurement voltage at the installation position of the new energy side protection; the measured current at the installation site of the new energy side protection; the fault short-circuit current at the new energy side; the fault short-circuit current at the system side; the internal potential of the grid-constructing equivalent voltage source; the output current of the grid-following equivalent current source; the internal potential of the system side power supply; eq1 = Z1+ Z L , Z eq3 = αZ MN , Z eq4 = (1- α)Z MN + Z s3 ; Z1 is the grid-constructing equivalent output impedance; Z L is the grid-constructing equivalent station outgoing line impedance; Z MN is the high-voltage side outgoing line impedance; ΔZ is the additional impedance; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the system side power supply.

[0114] It should be understood that the measured impedance solved in the embodiments of the present disclosure mainly includes two parts of the fault impedance Z eq3 and the additional impedance ΔZ. Due to the access of different types of new energy to the system, the additional impedance characteristics change greatly. As can be seen from the above formula (11), the additional impedance ΔZ is mainly related to the grid-constructing equivalent internal potential, the grid-constructing equivalent output impedance, the grid-following equivalent output current, the system side internal potential and equivalent impedance, and the fault location. The system is affected by different low-voltage ride-through control strategies, which may cause the additional impedance to present resistive-capacitive or resistive-inductive characteristics, so that the protection cannot correctly measure the actual fault impedance, thereby causing the distance protection to refuse to act or malfunction. Therefore, according to the above impedance calculation process, the embodiments of the present disclosure can analyze the characteristics of the measured impedance after the fault of the hybrid new energy system, accurately calculate the measured impedance value at the installation site of the new energy side protection after the fault, reasonably design the fault strategy and parameter matching of the grid-following and grid-constructing new energy, provide a basis for the distance protection applicability research of the grid-following and grid-constructing hybrid wind power system, and ensure the safe and stable operation of the new energy system.

[0115] Further, the embodiment of the present disclosure is to verify the effectiveness of the proposed fault measurement impedance calculation method of the hybrid wind power system with grid-connected and grid-constructed, and therefore a simulation model is built in the PSCAD simulation platform. The overall structure diagram of the simulation model is shown in FIG. 2, the grid-connected type station adopts a wind power station or a photovoltaic power station, the grid-constructed type new energy adopts a virtual synchronous machine control, the two stations are collected to a sending-out line through a transformer, and the sending-out line is connected to a power grid through a 220KV line. The capacity of the two new energy stations is 75MW, the total length of the sending-out line is 80km, the line impedance parameter is (0.076+0.338j)Ω / km, and the main parameters of the two types of inverters are shown in Table 1 and Table 2:

[0116] Table 1: Grid-constructed inverter parameters

[0117] Table 2: Grid-connected inverter parameters

[0118] The fault ride-through strategies of the above-mentioned grid-connected and grid-constructed converters are adopted in all inverters in the new energy station of the embodiment of the present disclosure, and a three-phase short-circuit fault is set at the f point on the sending-out line through a transition resistance R g Grounding. Since the influence of the transition resistance on the additional impedance may cause incorrect action of the distance protection, the embodiment of the present disclosure respectively simulates the output short-circuit current and the measurement impedance of the system side and the new energy side under the fault conditions of different grounding transition resistances, and calculates the output short-circuit current and the measurement impedance of the system side and the new energy side based on the above-mentioned calculation method. The embodiment of the present disclosure takes the fault of the line midpoint as an example, respectively simulates and calculates the output short-circuit current and the measurement impedance of the system side and the new energy side under the fault conditions of different grounding transition resistances, and the calculated values and the simulation values are shown in Table 3. It can be known from the comparison of Table 3 that the calculation method proposed in the embodiment of the present disclosure can accurately and effectively calculate the output short-circuit current values and the measurement impedance values of the system side and the new energy side of the hybrid wind power system with grid-connected and grid-constructed, and in the case of grounding through a small transition resistance, the amplitude error of the calculated measurement impedance and the simulation result is not more than 3%, and the angle error is not more than 1 degree.

[0119] Table 3: Comparison of simulation values and theoretical values of short-circuit currents and measurement impedances on both sides under different transition resistances

[0120] Further, the disclosure embodiments keep the ground transition resistance Rg unchanged at 3Ω, and simulate and calculate the output short-circuit current and the measured impedance on the system side and the new energy side for different fault positions, respectively. The calculated values and the simulation values are shown in Table 4. It can be known from the comparison results that the calculation method disclosed by the disclosure embodiments can accurately and effectively calculate the output short-circuit current values and the measured impedance values on the system side and the new energy side of the grid-connected hybrid wind power system, and the amplitude error of the calculated measured impedance is not more than 3%, and the angle error is not more than 1 degree.

[0121] Table 4 Comparison of simulation values and theoretical values of short-circuit currents and measured impedances on two sides under different fault positions

[0122] It can be known from the above results that the fault measured impedance calculation method for the grid-connected hybrid wind power system disclosed by the disclosure embodiments can accurately calculate the short-circuit currents on the system side and the new energy side after the fault of the hybrid wind power system, and can further solve the measured impedance values at the protection installation positions. The results can accurately reflect the actual measured impedance values at the protection installation positions on the new energy side, have very small result errors with the true results, ensure the setting calculation of the protection, can accurately reflect the fault characteristic information of the hybrid AC system, provide a basis for the distance protection applicability research of the grid-connected hybrid wind power system, and have important significance for ensuring the safe and stable operation of the new energy system.

[0123] In addition, as shown in FIG. 11, the disclosure embodiments also disclose a fault measured impedance calculation device for a grid-connected hybrid wind power system, which comprises:

[0124] a state acquisition module, configured to acquire the characteristics of the grid-connected converter and the grid-forming converter in the power grid and the corresponding low-voltage ride-through control strategy under the fault state;

[0125] an equivalent circuit construction module, configured to establish an equivalent circuit under the corresponding fault state according to the characteristics of the grid-connected converter and the grid-forming converter and the low-voltage ride-through control strategy;

[0126] a simplified analysis module, configured to perform simplified analysis on the grid-connected hybrid wind power system according to the equivalent circuit, and obtain an equivalent network diagram under the corresponding fault state;

[0127] an equivalent network analysis module, configured to analyze the equivalent network diagram and obtain a subcircuit diagram of the action of each power source excitation alone; the power source excitation at least includes a grid-forming voltage source, a grid-connected current source and a system-side voltage source;

[0128] a power source excitation analysis module, configured to analyze the subcircuit diagram corresponding to each power source excitation, and respectively obtain the short-circuit currents on the system side and the new energy side and the grid-connected point voltage when each power source excitation acts alone;

[0129] The superimposition analysis module is configured to superimpose the short-circuit currents of the system side and the new energy side corresponding to each power source excitation to obtain total short-circuit currents of the system side and the new energy side in the grid-connected hybrid wind power system, and superimpose the grid-connected point voltages corresponding to each power source excitation to obtain total grid-connected point voltage of the grid-connected hybrid wind power system.

[0130] The impedance output module is configured to calculate a measurement impedance at a new energy side protection installation position according to the total short-circuit currents of the system side and the new energy side in the grid-connected hybrid wind power system.

[0131] The apparatus provided by the embodiments of the present disclosure can realize each process of the method embodiments described in FIG. 1, and thus details are not repeated here.

[0132] As shown in FIG. 12, the embodiments of the present disclosure further provide an electronic device including a processor and a memory, a program or instructions stored on the memory and executable on the processor, which, when executed by the processor, realize each process of the method embodiments shown in FIG. 1 and achieve the same technical effects, and thus details are not repeated here.

[0133] The embodiments of the present disclosure further provide a readable storage medium having a program or instructions stored thereon, which, when executed by a processor, realize each process of the method embodiments described in FIG. 1 and achieve the same technical effects, and thus details are not repeated here.

[0134] The embodiments of the present disclosure further provide a computer program product including computer instructions, which, when executed by a processor, realize each process of the method embodiments described in FIG. 1 and achieve the same technical effects, and thus details are not repeated here.

[0135] It should be understood that the "one embodiment" or "an embodiment" mentioned throughout the specification means that the specific features, structures or characteristics related to the embodiment are included in at least one embodiment of the present disclosure. 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 disclosure, the size of the serial number of each process does not mean the execution order, and the execution order of each process should be determined according to its function and inherent logic, and should not constitute any limitation on the implementation process of the embodiments of the present disclosure. The serial number of the above embodiments of the present disclosure is only for description, not representing the advantages or disadvantages of the embodiments.

[0136] It should be noted that, in the present document, the terms "comprising", "containing" or any other similar term are intended to encompass non-exclusive inclusion, such that processes, methods, articles, or apparatuses that comprise a list of elements are not limited to those elements, but can also include other elements not expressly listed, or also include elements inherent in such processes, methods, articles, or apparatuses. Without further limitation, an element defined by the phrase "comprising a" does not exclude the presence of additional identical elements in the process, method, article, or apparatus that includes the element.

[0137] In several embodiments provided by the present disclosure, it should be understood that the disclosed devices and methods can be implemented in other ways. The device embodiments described above are merely illustrative, for example, the division of the units is only a logical function division, and actual implementation can have another division manner, 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 or direct coupling or communication connection between the various components shown or discussed can be through some interface, indirect coupling or communication connection between devices or units, which can be electrical, mechanical or other forms.

[0138] The units described above as separate components can or can not be physically separate, and the components shown as units can or can not be physical units; they can be located in one place or distributed on multiple network units; and part or all of the units can be selected according to actual needs to achieve the purpose of the embodiment.

[0139] In addition, each functional unit in each embodiment of the present disclosure can be integrated into one processing unit, or each unit can be a separate unit, or two or more units can be integrated into one unit; the integrated unit can be realized in the form of hardware or in the form of hardware plus software functional unit.

[0140] Those of ordinary skill in the art can understand that all or part of the steps of the above method embodiments can be completed by program instruction-related hardware, and the aforementioned program can be stored in a computer-readable storage medium, and the program executes the steps of the above method embodiments when executed; and the aforementioned storage medium includes: mobile storage equipment, read-only memory (ROM), magnetic disc or optical disc, and various storage program codes.

[0141] Alternatively, the above-mentioned integrated units of the present disclosure, if implemented in the form of software function modules and sold or used as independent products, can also be stored in a computer-readable storage medium. Based on such understanding, the technical solutions of the embodiments of the present disclosure can be embodied in the form of a software product, which is stored in a storage medium and includes several instructions for causing an apparatus (which can be a terminal or a platform, etc.) to perform all or part of the methods described in the various embodiments of the present disclosure. The aforementioned storage medium includes: mobile storage devices, ROM, magnetic disks or optical disks, and various other media that can store program codes.

[0142] The above only describes the preferred embodiments of the present disclosure, and it should be pointed out that for those skilled in the art, without departing from the technical principles of the present disclosure, a number of improvements and modifications can be made, and these improvements and modifications should also be considered as the protection scope of the present disclosure.

Claims

1. A method for calculating fault measurement impedance of a hybrid wind power system with grid-forming, characterized in that, The follow-network and grid-constructing hybrid wind power system at least comprises a follow-network converter and a grid-constructing converter, and the method comprises the following steps: characteristics of the follow-network and grid-constructing converters in the power grid and corresponding low-voltage ride-through control strategies under fault states are acquired; an equivalent circuit under the corresponding fault states is established according to the characteristics of the follow-network and grid-constructing converters and the low-voltage ride-through control strategies; the follow-network and grid-constructing hybrid wind power system is simplified and analyzed according to the equivalent circuit, and an equivalent network diagram under the corresponding fault states is obtained; the equivalent network diagram is analyzed, and a sub-circuit diagram of each power source excitation alone is obtained; the power source excitation at least comprises a grid-constructing voltage source, a follow-network current source and a system-side voltage source; the sub-circuit diagram corresponding to each power source excitation is analyzed, and short-circuit currents of the system side and the new energy side and a grid-connection point voltage corresponding to each power source excitation alone are obtained respectively; the short-circuit currents of the system side and the new energy side corresponding to each power source excitation are superimposed respectively to obtain total short-circuit currents of the system side and the new energy side in the follow-network and grid-constructing hybrid wind power system; and the grid-connection point voltages corresponding to each power source excitation are superimposed to obtain a total grid-connection point voltage of the follow-network and grid-constructing hybrid wind power system; a measured impedance at a new energy side protection installation position is calculated according to the total short-circuit currents of the system side and the new energy side in the follow-network and grid-constructing hybrid wind power system. 2.The method of claim 1, wherein, The corresponding low-voltage ride-through control strategy under the fault states comprises a low-voltage ride-through control strategy of the grid-constructing converter; specific steps of the low-voltage ride-through control strategy of the grid-constructing converter comprise: The active power reference value of the virtual synchronous machine is adjusted from P ref to P e , so that the power angle of the virtual synchronous machine is maintained during a fault, wherein the change in power angle Δδ of the virtual synchronous machine during the fault is: By switching the virtual internal voltage amplitude reference value and the reactive power reference value of the virtual synchronous machine, the short-circuit current is limited and reactive power support is provided for the grid-connected wind power system; wherein the virtual internal voltage reference value of the virtual synchronous machine switched during the fault is: where a is a first calculation intermediate value, a is obtained from a first intermediate value calculation formula, and the first intermediate value calculation formula is: where b is a second calculation intermediate value, b is obtained from a second intermediate value calculation formula, and the second intermediate value calculation formula is: wherein, Δω is the virtual synchronous machine virtual rotating angular velocity change rate; 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; ω n is the rated value of the grid frequency; E0 is the pre-fault virtual synchronous machine internal potential; U g0 is the pre-fault grid voltage; I0 is the rated output current of the virtual synchronous machine; E ref is the virtual synchronous machine internal potential reference value during the fault; u gf is the voltage amplitude of the faulted phase of the 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 the fault occurs, a virtual impedance link is put into operation to perform short-circuit current limiting control, and an equivalent output impedance of a virtual synchronous machine is increased to suppress a short-circuit transient impact large current appearing in the virtual synchronous machine during the fault.

3. The method of claim 1 or 2, wherein the method further comprises: The corresponding low-voltage ride-through control strategy under the fault states comprises a low-voltage ride-through control strategy of the follow-network converter; specific steps of the low-voltage ride-through control strategy of the follow-network converter comprise: According to the drop degree of the grid-connected point voltage after the fault, the current reference values of the direct axis d and the quadrature axis q of the current inner loop are set as: where I dref is the current reference value in the direct axis d coordinate system; I qref is the current reference value in the quadrature axis q coordinate system; P ref is the active power reference value; U0is the grid point voltage; I max is the maximum limit current of the grid- following converter; I n is the rated current; p.u. is the per unit value; according to a current-carrying capacity of power electronic equipment in the follow-network and grid-constructing hybrid wind power system, PI links of current inner loop controllers are all provided with limiting control, and a maximum current limiting amplitude is 1.2-2 times of a rated current.

4. The method of claim 1-3, wherein, The grid-connection-following hybrid wind power system comprises m grid-connection type converters and n grid-following type converters, when a fault occurs, the m grid-connection type converters are equivalent to m series equivalent output impedance Z s1 voltage sources E s nodes, and the n grid-following type converters are equivalent to n grid-connected voltage-controlled current sources I s1 nodes; When the process of establishing the equivalent circuit corresponding to the fault state is established, m voltage source nodes are equivalent to one voltage source E s And the branch in series with the equivalent output impedance Z1, then the equivalent voltage source model corresponding to the branch is: where E S is the equivalent voltage source output potential; E ref is the potential reference value within the virtual synchronous machine during a fault; Z1 is the equivalent output impedance value; Z s1 is the equivalent output impedance value corresponding to a single voltage source node; n current source nodes are equivalent to one current source I s If the branch is a current source, then the equivalent current source model for the branch is: where I S is the amplitude of the short-circuit current output by the current source in the equivalent circuit; I S1 is the amplitude of the short-circuit current output by a single current source; δ S is the phase angle of the short-circuit current output by the current source in the equivalent circuit; I dref , I qref are the current reference values in the direct-axis d and quadrature-axis q coordinate systems, respectively; U0is the grid-connected point voltage.

5. The method of claim 1-4, wherein, The short-circuit current of the system side and the new energy side and the grid connection point voltage of the network-constructed voltage source when acting alone are: wherein, To follow the network mixed wind power system grid point voltage; For new energy side fault short-circuit current; for system side fault short circuit current; Z represents the internal potential of the network-type equivalent voltage source. eq1 =Z1+Z L Z eq3 =αZ MN Z eq4 =(1-α)Z MN +Z s3 Z1 is the network-type equivalent output impedance; Z L The impedance of the transmission lines for grid-type substations; Z MN The high-voltage side transmission line impedance is α; the fault location coefficient is R. g For transition resistance; Z s3 Z is the internal resistance of the system-side power supply; eq4 / / R g For Z eq4 With R g The total resistance calculated after parallel connection.

6. The method according to claim 5, wherein, The short-circuit currents corresponding to the system side and the new energy side when the follow-up network type current source acts alone and the grid-connected point voltage are: wherein To follow the network mixed wind power system grid point voltage; For new energy side fault short-circuit current; for system side fault short circuit current; For the equivalent current source output current; Z eq1 = Z1+ Z L , Z eq3 = αZ MN , Z eq4 = (1-α)Z MN + Z s3 ; Z1 is the network type equivalent output impedance; Z L is the network type field station outgoing line impedance; Z MN is the high voltage side outgoing line impedance; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the system side power supply; Z eq4 / / R g is the total resistance value calculated after Z eq4 and R g are connected in parallel.

7. The method of claim 1-6, wherein, The short-circuit currents of the system side and the new energy side and the grid connection point voltage when the system side voltage source acts alone are: wherein To follow the network mixed wind power system grid point voltage; For new energy side fault short-circuit current; For system side fault short circuit current; Z represents the internal potential of the system-side power supply. eq1 =Z1+Z L Z eq3 =αZ MN Z eq4 =(1-α)Z MN +Z s3 Z1 is the network-type equivalent output impedance; Z L The impedance of the transmission lines for grid-type substations; Z MN The high-voltage side transmission line impedance is α; the fault location coefficient is R. g For transition resistance; Z s3 The internal resistance of the system-side power supply; (Z) eq1 +Z eq3 ) / / R g For Z eq1 Z eq3 With R g The total resistance calculated after parallel connection.

8. The method of claim 1-7, wherein, The total short-circuit current of the system side and the new energy side and the total voltage of the grid connection point in the grid-connected wind power system are obtained by superimposing the excitation of each power supply: wherein To follow the network mixed wind power system grid point voltage; For new energy side fault short-circuit current; For system side fault short circuit current; for constructing the network-type equivalent voltage source internal potential; to follow the network type equivalent current source output current; Z eq1 = Z1+ Z L , Z eq3 = aZ MN , Z eq4 = (1 - a)Z MN + Z s3 ; Z1is the network- forming equivalent output impedance; Z L is the network- forming field station outgoing line impedance; Z MN is the high voltage side outgoing line impedance; a is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the system side power source.

9. The method of claim 1-8, wherein, The step of obtaining the measured impedance at the installation position of the new energy side protection according to the total short-circuit current calculation of the system side and the new energy side in the hybrid wind power system includes: wherein measuring voltage for installation of new energy side protection; To measure the current installed at the new energy side protection; For new energy side fault short-circuit current; for system side fault short circuit current; for constructing the network-type equivalent voltage source internal potential; to follow the network type equivalent current source output current; Z eq1 = Z1+ Z L , Z eq3 = αZ MN , Z eq4 = (1 - α)Z MN + Z s3 ; Z1 is the network-configuration type equivalent output impedance; Z L is the network-configuration type field station outgoing line impedance; Z MN is the high-voltage side outgoing line impedance; ΔZ is the additional impedance; α is the fault location coefficient; R g is the transition resistance; Z s3 is the internal resistance of the system side power supply.

10. A device for calculating fault measurement impedance of a hybrid wind power system connected to a power grid, characterized in that, The device comprises: a state acquisition module configured to acquire characteristics of follow-network and grid-constructing converters in a power grid and corresponding low-voltage ride-through control strategies under fault states; an equivalent circuit construction module configured to establish an equivalent circuit under the corresponding fault states according to the characteristics of the follow-network and grid-constructing converters and the low-voltage ride-through control strategies; a simplification analysis module configured to perform simplification analysis on the follow-network and grid-constructing hybrid wind power system according to the equivalent circuit, and obtain an equivalent network diagram under the corresponding fault states; an equivalent network analysis module configured to analyze the equivalent network diagram, and obtain a sub-circuit diagram of each power source excitation alone; the power source excitation at least comprises a grid-constructing voltage source, a follow-network current source and a system-side voltage source; a power source excitation analysis module configured to analyze the sub-circuit diagram corresponding to each power source excitation, and obtain short-circuit currents of the system side and the new energy side and a grid-connection point voltage corresponding to each power source excitation alone respectively; The superposition analysis module is configured to superimpose the short-circuit currents of the system side and the new energy side corresponding to each power excitation to obtain total short-circuit currents of the system side and the new energy side in the grid-connected hybrid wind power system, and superimpose the grid-connected point voltages corresponding to each power excitation to obtain total grid-connected point voltages of the grid-connected hybrid wind power system. The impedance output module is configured to calculate and obtain a measurement impedance at a new energy side protection installation position according to the total short-circuit currents of the system side and the new energy side in the grid-connected hybrid wind power system.

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