New energy grid-connected short-circuit current and peak value calculation method considering voltage drop transient state

Abstract

The invention provides a new energy grid-connected short-circuit current and peak value calculation method considering a voltage drop transient state, and belongs to the field of new energy grid-connected system short-circuit current calculation. According to the method, a topological structure and electrical parameters of the doubly-fed wind power generation system are imported, an electromagnetic differential equation is established, a fault voltage initial value expression is deduced, a matrix equation is constructed to solve a node voltage initial value and a loop current derivative initial value, an equivalent attenuation time constant after a fault is calculated, and an approximate analytical expression of a node voltage drop process is established. And calculating a fault point short-circuit current peak value and a double-fed fan short-circuit current peak value. According to the new energy grid-connected short-circuit current and peak value calculation method considering the voltage drop transient state, full-time-domain iterative calculation is avoided, calculation precision and convergence are both considered, the method is suitable for a doubly-fed wind power generation system, and important reference can be provided for equipment type selection and protection setting calculation.

Description

Technical Field

[0001] The present invention relates to the field of short-circuit current calculation of a new energy grid-connected system, and in particular to a method for calculating the short-circuit current and peak value of a new energy grid-connected system taking into account voltage drop transients. Background Art

[0002] As a vital component of the renewable energy sector, wind power generation has seen a sustained growth in grid-connected capacity in recent years. Doubly-fed wind turbines are a mainstream model in my country's wind power industry and are widely used in large and medium-sized wind farms. Due to their ability to connect to the grid at partial power, doubly-fed wind turbines can deliver peak short-circuit currents far exceeding their rated values ​​in the event of a fault. This characteristic makes them a significant source of fault current that must be considered during equipment selection and protection setting calculations, profoundly impacting the selection of related equipment and the formulation of protective measures.

[0003] However, the line and equipment parameters of different doubly-fed wind farms vary significantly. Furthermore, actual turbine operating conditions and system impedance also vary. To account for the impact of these factors on voltage transients, detailed full-time-domain modeling is often relied upon. However, full-time-domain iterative calculations of the system suffer from efficiency and convergence issues, necessitating a method for calculating short-circuit current and peak values ​​suitable for doubly-fed wind power generation systems. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for calculating the short-circuit current and peak value of a new energy grid-connected system taking into account the transient state of voltage drop, thereby achieving the goal of taking into account the transient process of voltage drop of the new energy grid-connected system while avoiding full time domain iteration, and proposing a method for calculating the short-circuit current and peak value that takes into account both calculation accuracy and convergence.

[0005] To achieve the above objectives, the present invention provides a method for calculating the short-circuit current and peak value of a new energy grid-connected system taking into account voltage sag transients, comprising the following steps: Step 1: Import the topology of the doubly-fed wind power generation system and the electrical parameters of each component; Step 2: Establish the electromagnetic differential equation in the synchronous rotating coordinate system of the doubly fed wind turbine. Based on the voltage drop ratio at the generator end after the fault and the crowbar input state, derive the initial value expression of the generator end fault voltage. Step 3: construct and solve the matrix equations of the node voltage initial values ​​and loop current derivative initial values ​​of the doubly-fed wind power generation system; Step 4: Calculate the equivalent decay time constant of the doubly-fed wind power generation system after a fault; Step 5: Obtain the fault steady-state voltage and current through iterative calculation, and establish an approximate analytical expression for the node voltage drop process; Step 6: Calculate the short-circuit current peak value at the fault point based on the voltage expression in step 5; Step 7: Calculate the short-circuit current component on the stator side of the doubly-fed wind turbine under voltage step conditions; Step 8: Calculate the instantaneous speed of the composite vector of the stator power frequency component and the rotor slip frequency component in the short-circuit current; Step 9: Determine the linear range of the synthetic vector speed and the corresponding average speed; Step 10: Calculate the peak time and peak value of the short-circuit current under the voltage step using the average speed; Step 11: Establish the current response of the voltage transient component excitation, take the average of the voltage step response peak time and the transient response peak time, substitute it into the total current expression to obtain the final peak value of the doubly fed wind turbine short-circuit current.

[0006] Preferably, in step 2, first establish the electromagnetic differential equation of the doubly fed wind turbine: ; in, 、 、 They represent the stator voltage, current and flux in complex form in the synchronous rotating coordinate system, 、 、 They represent the rotor voltage, current and flux in complex form in the synchronous rotating coordinate system, 、 They represent the stator and rotor resistances, represents the synchronous angular frequency, represents the slip angular frequency, represents the rotor angular frequency, represents the mutual inductance between stator and rotor, represents the stator inductance, represents the rotor inductance, 、 They represent the stator and rotor leakage inductance, represents an imaginary unit; The terminal voltage after the fault drops to the voltage before the fault. times, the crowbar is immediately activated and put into the uncontrolled stage. The flux equation in the synchronous rotating coordinate system is: ; in, represents the stator coupling coefficient, represents the rotor coupling coefficient, represents the stator subtransient time constant, represents the rotor subtransient time constant, represents the rotor equivalent resistance, Indicates the crowbar resistance and has been converted to the stator side. represents the rotor transient inductance, represents the stator transient inductance, represents the stator voltage before the fault in the synchronous rotating coordinate system; The stator voltage expression of the doubly fed wind turbine is rewritten as: ; After the fault At this moment, the stator current and stator voltage are substituted into the stator and rotor flux to obtain: ; Convert to the A phase coordinate system: ; in, 、 They represent the instantaneous initial values ​​of the voltage and current derivatives of phase A after the fault, Indicates the instantaneous value of the voltage of phase A before the fault, represents the initial value of the stator voltage after the fault in the synchronous rotating coordinate system, Indicates the transformer ratio of the doubly fed wind turbine unit, Indicates the initial phase of the orientation angle of the synchronous rotating coordinate system at the time of the fault, represents a natural constant, represents the operation of taking the real part, It represents the initial value of the stator current before the fault in the form of a complex number in the synchronous rotating coordinate system.

[0007] Preferably, in step 3, the matrix equation is: ; in, represents the return-branch incidence matrix, represents transpose, 、 Both represent diagonal matrices with the number of branches as their dimension, represents the initial value of the loop current derivative after the fault, , It represents a one-dimensional matrix composed of the initial transient value of the equivalent post-fault voltage provided by the doubly fed wind turbine. If there is a doubly fed wind turbine in this branch, the corresponding element is , otherwise it is 0, represents a diagonal matrix with resistors as elements, Represents the one-dimensional matrix formed by the original voltage source of the branch, A one-dimensional matrix representing the instantaneous values ​​of the three-phase currents of each branch before the fault.

[0008] Preferably, in step 4, the decay time constant is as follows: ; in, represents the system decay time constant, The Thevenin impedance seen from the fault point, represents the transition resistance, Indicates the operation of taking the imaginary part.

[0009] Preferably, in step 5, the approximate analytical expression of the node voltage drop process is as follows: ; in, Indicates the The voltage of a phase of a node changes with time The instantaneous value of Indicates the initial value of the node phase voltage fault, represents the amplitude of the steady-state phase voltage after the fault, Indicates the phase of the steady-state phase voltage after the fault.

[0010] Preferably, in step 6, the peak value of the short-circuit current at the fault point is as follows: ; in, Indicates the peak value of short-circuit current at the fault point, Indicates the amplitude of the steady-state phase voltage at the fault point after the fault.

[0011] Preferably, in step 7, the stator side short-circuit current component is as follows: ; in, represents the stator current in the two-phase stationary coordinate system, 、 and They represent the initial values ​​of the stator frequency component, DC component and rotor frequency component in the two-phase stationary coordinate system respectively.

[0012] Preferably, in step 8, the instantaneous speed of the synthetic vector is as follows: ; in, represents the instantaneous speed of the resultant vector, express and The angle of Represents the synthesized rotor frequency and stator frequency components.

[0013] Preferably, in step 9, the average rotation speed is as follows: ; in, represents the average speed, express Take the value at the time of fault occurrence, Indicates the operation of taking the complex phase.

[0014] Preferably, in step 10, the approximate peak moment is taken as the time when the double power frequency component coincides with the sum of the initial values ​​of the two DC components: ; in, express The approximate peak moment of Indicates the short-circuit current over time under the voltage transient component Response.

[0015] Therefore, the present invention adopts the above-mentioned method for calculating the short-circuit current and peak value of renewable energy grid-connected short-circuit current taking into account the transient voltage drop, and the beneficial technical effects are as follows: (1) It can quickly evaluate and quantify the transient process of system voltage sag, providing theoretical support for mechanism analysis; (2) The proposed method can support the study of the short-circuit current peak time regularity at the fault point and the doubly fed wind turbine terminal.

[0016] (3) The proposed method has high computational efficiency and good computational convergence. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a flow chart of the method for calculating short-circuit current and peak value of renewable energy grid-connected power generation taking into account voltage sag transients according to the present invention; Figure 2 It is the grid-connected and electromagnetic transient equivalent topology diagram of the doubly-fed wind turbine; Figure 3 is the relationship between the stator frequency and rotor frequency components and time; Figure 4 This is the topology diagram of the grid-connected system of the doubly-fed wind farm; Figure 5 Comparison of transient calculation results of three-phase voltage of doubly fed wind turbine. DETAILED DESCRIPTION

[0018] The technical solution of the present invention is further described below with reference to the accompanying drawings and embodiments.

[0019] Unless otherwise defined, technical or scientific terms used in the present invention shall have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs.

[0020] Example 1

[0021] See also Figure 1 The calculation method of short-circuit current and peak value of renewable energy grid-connected with transient voltage drop is based on the example of doubly-fed wind power generation system, which includes the following steps: Step 1: First, determine the doubly-fed wind power generation system scenario to be calculated, obtain the voltage level, rated capacity and connection topology, electrical parameters of the line, transformer and doubly-fed wind turbine, fault location and transition resistance.

[0022] Step 2: Establish the electromagnetic differential equation of the doubly fed wind turbine in the synchronous rotating coordinate system. Based on the voltage drop ratio at the generator end after the fault and the crowbar input state, derive the initial value expression of the generator end fault voltage.

[0023] See also Figure 2 , where the wind turbine is connected to the generator via a gearbox, the stator side is connected to the grid via a transformer, and the rotor side is connected to the grid via a converter during normal operation. In the event of a fault, the crowbar is equivalent to cutting off the converter protection device. Based on this, the electromagnetic differential equation of the doubly fed wind turbine is established: ; in, 、 、 They represent the stator voltage, current and flux in complex form in the synchronous rotating coordinate system, 、 、 They represent the rotor voltage, current and flux in complex form in the synchronous rotating coordinate system, 、 They represent the stator and rotor resistances, represents the synchronous angular frequency, represents the slip angular frequency, represents the rotor angular frequency, represents the mutual inductance between stator and rotor, represents the stator inductance, represents the rotor inductance, 、 They represent the stator and rotor leakage inductance, represents an imaginary unit; Assume that the terminal voltage drops instantaneously to the voltage before the fault after the fault. times, the crowbar is immediately activated and put into the uncontrolled stage. After eliminating the current term in the above equation, the flux equation in the synchronous rotating coordinate system is: ; in, represents the stator coupling coefficient, represents the rotor coupling coefficient, represents the stator subtransient time constant, represents the rotor subtransient time constant, represents the rotor equivalent resistance, Indicates the crowbar resistance and has been converted to the stator side. represents the rotor transient inductance, represents the stator transient inductance, represents the stator voltage before the fault in the synchronous rotating coordinate system; Then the stator voltage expression of the doubly fed wind turbine is rewritten as: ; After the fault At this moment, the stator current and stator voltage are substituted into the stator and rotor flux, and after simplification, we have: ; in, Indicates the slip rate, 、 They represent the initial values ​​of the stator voltage before and after the fault in the form of complex numbers in the synchronous rotating coordinate system, It represents the initial value of the stator current derivative after the fault in the complex form in the synchronous rotating coordinate system.

[0024] Convert to the A phase coordinate system: ; in, 、 They represent the instantaneous initial values ​​of the voltage and current derivatives of phase A after the fault, Indicates the instantaneous value of the voltage of phase A before the fault, Indicates the transformer ratio of the doubly fed wind turbine unit, Indicates the initial phase of the orientation angle of the synchronous rotating coordinate system at the time of the fault, represents a natural constant, represents the operation of taking the real part, It represents the initial value of the stator current before the fault in the form of a complex number in the synchronous rotating coordinate system.

[0025] Step 3: Construct and solve the matrix equations of the initial values ​​of node voltages and loop current derivatives of the doubly-fed wind power generation system.

[0026] For the initial transient value of voltage after a fault, we can solve it by analogy with the loop current method, and write the branch characteristics as: ; in, 、 Respectively represent the one-dimensional matrix formed by the initial values ​​of the voltage branch and current branch derivatives after the fault, The diagonal matrix with the number of branches as the dimension, the elements are the inductances of each branch, represents a diagonal matrix with resistors as elements, Represents the one-dimensional matrix formed by the original voltage source of the branch, A one-dimensional matrix representing the initial values ​​of the current branch derivatives before the fault.

[0027] Then the transient initial values ​​of each component of the system and the initial values ​​of the current derivatives can be equivalently expressed in a matrix equation similar to the loop current method: ; Among them, in addition to It is a one-dimensional matrix composed of the instantaneous values ​​of the three-phase currents of each branch before the fault, and the others are matrices composed of the transient initial values ​​after the fault. represents the initial value of the loop current derivative after the fault, The diagonal matrix representing the number of branches as the dimension, the branch element where the doubly fed wind turbine is located is , otherwise it is 0, , It represents a one-dimensional matrix composed of the initial transient value of the equivalent post-fault voltage provided by the doubly fed wind turbine. If there is a doubly fed wind turbine in this branch, the corresponding element is , otherwise it is 0, represents a diagonal matrix with resistors as elements, Represents the one-dimensional matrix formed by the original voltage source of the branch.

[0028] Further organized into the following formula: ; After matrix inversion, the initial value of the loop current derivative is obtained, and the initial value of the voltage at each node after the corresponding fault can also be obtained.

[0029] Step 4: Calculate the equivalent decay time constant of the doubly-fed wind power generation system after a fault.

[0030] Ignore transition resistance The fault point is injected with unit current and other power sources are set to 0, and then the Thevenin impedance seen from the fault point is obtained according to the node impedance matrix. , which is the self-impedance of the fault point. Then calculate the equivalent inductance, and add the real part The equivalent resistance is obtained, and the system decay time constant is : ; in, Indicates the operation of taking the real part.

[0031] Step 5: Obtain the fault steady-state voltage and current through iterative calculation, and establish an approximate analytical expression for the node voltage drop process.

[0032] Regarding the fault-prone steady-state voltage and current of a doubly-fed wind turbine system, after constructing the node voltage matrix equation, the injected current includes both the synchronous generator power supply and the doubly-fed wind turbine power supply. For the synchronous generator power supply, the injected current is constant and is the current source current after the subtransient electromotive force and subtransient reactance are processed by the Norton equivalent. For the doubly-fed wind turbine power supply, the injected current is the actual output current under a given state, the value of which is determined by the PCC point voltage. This calculation is used to obtain the voltage at each node of the network. The calculated node voltage is used to calculate the output current of the full-power inverter power supply and update the injected current matrix. This method is iterated until the voltage at each node of the system converges.

[0033] At this point, the transient process of the three-phase voltage drop can be represented. The fault node voltage is decomposed into a steady-state power frequency component, a decaying DC component, and an oscillating component that is difficult to analyze. Ignoring the oscillating component and assuming that each node only has an additional DC component with the same decay rate, the node voltage transient expression is approximately analyzed as follows: ; in, Indicates the The voltage of a phase of a node changes with time The instantaneous value of Indicates the initial value of the node phase voltage fault, represents the amplitude of the steady-state phase voltage after the fault, Indicates the phase of the steady-state phase voltage after the fault.

[0034] Furthermore, we can transform to the synchronously rotating coordinate system dq axis and ignore the superscript: ; in, represents the d-axis component corresponding to the steady-state sinusoidal component, It represents the initial phase of Park transform orientation angle under d-axis voltage orientation, represents the d-axis voltage over time The instantaneous value of represents the q-axis voltage over time The instantaneous value of .

[0035] 、 are all intermediate quantities, and their calculation methods are as follows: First, the initial values ​​of the attenuation terms in the three-phase voltage expression are recorded as 、 、 , as a matrix transformation to the synchronous rotating coordinate system: ; in, is the Park transformation matrix, satisfying: ; therefore 、 satisfy: ; Step 6: Calculate the short-circuit current peak value at the fault point based on the voltage expression in step 5.

[0036] According to the transient analytical formula of the system three-phase voltage drop, the transition resistance The fault point to ground current is: ; in, Indicates the current of a phase at the fault point over time The instantaneous value of Indicates the current of a phase at the fault point over time The instantaneous value of Indicates the initial fault value of the phase voltage at the fault point, It represents the initial phase of Park transform orientation angle under fault point d-axis voltage orientation.

[0037] Under symmetrical fault, the fault point is grounded instantaneously through the resistor, so the initial value of the voltage at the fault point is 0, that is, =0, then the fault point current is: ; The above formula is almost the same as the three-phase short-circuit current form of the traditional synchronous machine, so the peak current at the fault point can be calculated from the peak value of the sinusoidal component. Taking phase A as an example, Should be / 2, the peak moment is half of the power frequency cycle, then the short-circuit current peak value at the fault point is for: ; in, Indicates the amplitude of the steady-state phase voltage at the fault point after the fault.

[0038] Step 7: Calculate the short-circuit current component on the stator side of the doubly fed wind turbine under voltage step conditions.

[0039] Secondly, the short-circuit current of the doubly fed wind turbine is calculated taking into account the transient process of voltage drop. The first step is to calculate the short-circuit current and peak value of the doubly fed wind turbine under the voltage step. According to the flux equation in the synchronous rotating coordinate system, since the stator resistance is small, the stator DC quantity decays slowly, and it can be considered that it does not decay in the first half cycle after the fault; the stator and rotor resistances are very small compared to the crowbar resistance, so they can be ignored and satisfy = 1. Since the stator and rotor leakage inductance is very small compared to the mutual inductance, we can get ≈ , ≈ ≈1; let the initial values ​​of stator frequency component, DC component and rotor frequency component be 、 and Finally, the stator short-circuit current in the two-phase stationary coordinate system is simplified and expressed as: ; in, represents the stator current in the two-phase stationary coordinate system.

[0040] Step 8: Calculate the instantaneous speed of the composite vector of the stator power frequency component and the rotor slip frequency component in the short-circuit current.

[0041] The synthesized rotor frequency and stator frequency components are recorded as , the next moment is ; The angle between the rotor frequency and stator frequency components is , the next moment is ; Its rate of change is the slip angular frequency ,remember and The angle is The next moment is ; The synthesized rotor frequency and stator frequency components are recorded as The next moment is And use red arrows to indicate, and use dotted lines to indicate each quantity at the next moment, so we can get Figure 3 The relationship between the stator frequency and rotor frequency components and time is shown.

[0042] According to the vector relationship, the resulting rotation component Speed satisfy: ; In a vector triangle, the sine theorem gives satisfy: ; According to the above formula and the cosine theorem, we can get derivative, and let have: ; The ratio of the initial value amplitude The value of represents the comprehensive impact of different situations and different parameters.

[0043] The corresponding speed characteristics always have good linear conditions, and the increase and decrease are determined by and cos The physical explanation is that the small time span and the large initial amplitude difference and small speed difference of the two components lead to a small change in the speed of the resultant component.

[0044] Step 9: Determine the linear range of the synthetic vector speed and the corresponding average speed; In the desired peak time scale, Can be approximated as the angle or time The linear function of time integral is the phase of the synthetic component rotation. The endpoint values ​​are and The phase difference and the phase difference when the rotor frequency coincides with the DC component are 0, The linear speed characteristic is , In the linear range average speed.

[0045] ; in, represents the average speed, express Take the value at the time of fault occurrence, Indicates the operation of taking the complex phase.

[0046] Step 10: Calculate the peak time and peak value of the short-circuit current under the voltage step using the average speed.

[0047] remember and The initial phase difference is ; The peak moment coincides with the DC component and is recorded as Then we have: ; The linear interval already contains the peak moment and the difference between its average speed and the average speed of the integral interval is small, so it can be replaced by the average speed of the linear interval. The quadratic equation for the quantity to be solved can be simplified to a linear form: ; in, Indicates the approximate peak time after average speed substitution.

[0048] Step 11: Establish the current response of the voltage transient component excitation, take the average of the voltage step response peak time and the transient response peak time, substitute it into the total current expression to obtain the final peak value of the doubly fed wind turbine short-circuit current.

[0049] The second step is to calculate the short-circuit current peak after superimposing the voltage transient component. According to the transient analytical formula of the system dq axis voltage drop, since the d axis component is the dominant factor and the peak time is very short and the most serious situation is considered, the q axis component is ignored and simplified to: ; From the above formula, we can see that there is an additional oscillation attenuation component in the dq axis voltage, which may cause a much more severe impact than the assumed instantaneous voltage drop in the early stage of the fault.

[0050] Substitute the above equation into the voltage flux equation as the stator voltage excitation and solve it using Laplace transform. and If the gap is huge, the zero-pole cancellation method can be used. Finally, after the Laplace inverse transformation and simplification, the two-phase stationary coordinate system is obtained with time. The total short-circuit current expression is : ; in, 、 、 satisfy: ; The total short-circuit current is the superposition of the current response of the voltage step component and the DC decay component, which are respectively expressed as 、 After: ; It represents the response of short-circuit current with time t under voltage step change in two-phase stationary coordinate system. The peak value calculation method is step 10. The voltage drop degree Obtained from the steady-state fault calculation in step 5. The short-circuit current under the transient component of the voltage in the two-phase stationary coordinate system changes with time The response consists of a DC component, a twice stator frequency component, and an attenuated DC component, and its initial value satisfies =0, it is obvious that the approximate peak moment can be taken as the time when the double power frequency component and the sum of the initial values ​​of the two DC components coincide with each other: ; in, express The approximate peak moment of Indicates the short-circuit current over time under the voltage transient component Response.

[0051] remember is the synchronization angular frequency The value is / 4 and has nothing to do with system parameters. Substituting it into the total short-circuit current expression yields Since the total short-circuit current is the difference between the two vector currents, it is considered that the peak value of the total short-circuit current is obtained when each of the two vector currents reaches its peak and forms an obtuse angle.

[0052] Response to voltage dips The peak time is obtained from step 10, because Generally less than , the average speed is near the rotor angular frequency, so and If the difference is not large, the average value can be taken as the peak time of the total current response. Substituting it into the total short-circuit current expression, the short-circuit current peak time and magnitude of the doubly fed wind turbine are obtained.

[0053] When the new energy units and network parameters cause the voltage transient to decay rapidly, the node voltage can be regarded as a step change. At this time, there is almost no peak current at the fault point, and the calculation of the short-circuit current and peak value of each unit is degenerated into the method under voltage step. Specifically, the current response peak time of the voltage transient component is / 4, considering a certain margin, it is believed that < When the voltage transient effect is small at / 6, the calculation condition of voltage step change can be adopted.

[0054] Calculation example 1

[0055] This example is based on PSCAD / EMTDC. Figure 4 The double-fed wind farm grid-connected system model is is the system voltage; is the system impedance, ~ is the collector line impedance, The impedance of the station transmission line is 10 km long. The grid frequency is 60 Hz. There are four doubly fed wind turbines after equalization, which were operating at nearly full capacity before the fault. The specific parameters of the units and the network are shown in Tables 1 and 2.

[0056] Table 1 DFIG parameters ;

[0057] Table 2 Parameters of each part of the doubly-fed wind farm grid-connected system ;

[0058] According to the steady-state calculation results of the proposed system fault, the steady-state voltage drops to about 0.45pu, and it is judged that all units will be put into crowbar. =4.27ms. According to the proposed method, the voltage transient process at this time will significantly affect the short-circuit current and peak calculation results. The proposed voltage transient approximate analytical method is used to obtain the terminal voltage expression in the synchronous rotating coordinate system. After transformation to three phases, the calculated value is compared with the simulation value. Figure 5 As shown in the figure, in this case, the voltage transient lasted for more than half a power frequency cycle, and the maximum difference between the instantaneous voltage and the steady-state voltage after the fault was about 0.45 pu, which will have a significant impact on the unit's fault response.

[0059] Different transition resistors and crowbar resistance values ​​were set to calculate the short-circuit current peak value and time at the fault point. Table 3 shows the comparison results of the instantaneous value of phase A current using the method proposed in this invention, the IEC standard, and the simulation value.

[0060] Table 3 Different transition resistances Comparison of the calculation results of the peak short-circuit current at the fault point under the crowbar ;

[0061] Table 3 shows that both methods can calculate the peak value of the fault point under metallic fault conditions, and the proposed method has a smaller error. When the transition resistance and parameters vary, the proposed method still has a smaller error. This also demonstrates the rationality of taking the short-circuit current peak time at the fault point as half a power frequency cycle when the voltage transient process is significant.

[0062] The following verification method for calculating the peak short-circuit current at the machine end is still assumed to occur in the same position with a three-phase symmetrical fault. The simulation value of the complex modulus of the current of wind turbine No. 3 in the two-phase stationary coordinate system is taken. Table 4 shows the comparison results of the method proposed in the present invention, the calculated value of the assumed instantaneous drop, and the simulation value.

[0063] Table 4 Different transition resistances Comparison of calculation results of peak short-circuit current of doubly fed wind turbine under crowbar ;

[0064] These results demonstrate the necessity of accounting for voltage sag transients. Compared to the peak calculation using the assumption of voltage sag, which has a maximum error of 21.8%, the proposed method exhibits an error of less than 6%. While the peak time of a voltage sag is typically around one-third of the power frequency cycle, accounting for voltage transients shifts it significantly earlier.

[0065] The main reason for the overall larger peak time is the neglect of the effects of attenuation and the approximation of the rotational speed. However, the main reason for the smaller effect on the peak magnitude error is that the effects of vector rotational speed and attenuation on the peak magnitude cancel each other out near the peak time, resulting in a relatively flat current profile. The main reason for the increased error in the transient drop calculation when the fault steady-state voltage and crowbar resistance increase is that the crowbar resistance is positively correlated with the magnitude of the voltage transient response current. Increasing the fault steady-state voltage further increases the proportion of the transient voltage response current, and ignoring this will increase the calculation error.

[0066] The proposed approximate voltage transient process method can characterize the impact of fault conditions, network topology parameters, and wind turbine grid connection conditions. It also has certain reference value for calculating single-unit and distributed doubly fed wind turbine grid-connected systems with severe voltage transient processes. The proposed calculation method can support the study of peak time patterns. By quantifying the transient changes in voltage sag, it is mechanistically determined that the peak time of the fault point current can be taken as half of the power frequency cycle, and that the peak time of the short-circuit current at the doubly fed wind turbine end will shift forward compared to the assumed transient voltage sag. The proposed method has high computational efficiency and good computational convergence. By comparing the proposed short-circuit current and peak value calculation method with IEC standards, simulation examples, and measured waveforms, it is verified that the peak value error calculated by the proposed method is within 6%.

[0067] It is worth noting that the contents not elaborated in detail in the present invention are all prior art and are well known to those skilled in the art.

[0068] Therefore, the present invention adopts the above-mentioned new energy grid-connected short-circuit current and peak value calculation method taking into account voltage drop transients, avoiding full time domain iterative calculations, taking into account both calculation accuracy and convergence, and is suitable for doubly fed wind power generation systems. It can provide an important reference for equipment selection and protection setting calculations.

[0069] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit the same. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solutions of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for calculating short-circuit current and peak value of renewable energy grid-connected power generation taking into account voltage drop transients, characterized in that: The following steps are involved: Step 1: Import the topology of the doubly-fed wind power generation system and the electrical parameters of each component; Step 2: Establish the electromagnetic differential equation in the synchronous rotating coordinate system of the doubly fed wind turbine. Based on the voltage drop ratio at the generator end after the fault and the crowbar input state, derive the initial value expression of the generator end fault voltage. Step 3: construct and solve the matrix equations of the node voltage initial values ​​and loop current derivative initial values ​​of the doubly-fed wind power generation system; Step 4: Calculate the equivalent decay time constant of the doubly-fed wind power generation system after a fault; Step 5: Obtain the fault steady-state voltage and current through iterative calculation, and establish an approximate analytical expression for the node voltage drop process; Step 6: Calculate the short-circuit current peak value at the fault point based on the voltage expression in step 5; Step 7: Calculate the short-circuit current component on the stator side of the doubly-fed wind turbine under voltage step conditions; Step 8: Calculate the instantaneous speed of the composite vector of the stator power frequency component and the rotor slip frequency component in the short-circuit current; Step 9: Determine the linear range of the synthetic vector speed and the corresponding average speed; Step 10: Calculate the peak time and peak value of the short-circuit current under the voltage step using the average speed; Step 11: Establish the current response of the voltage transient component excitation, take the average of the voltage step response peak time and the transient response peak time, substitute it into the total current expression to obtain the final peak value of the doubly fed wind turbine short-circuit current.

2. The method for calculating short-circuit current and peak value of new energy grid-connected power taking into account voltage drop transients according to claim 1 is characterized in that: In step 2, first establish the electromagnetic differential equation of the doubly fed wind turbine: ； in, 、 、 They represent the stator voltage, current and flux in complex form in the synchronous rotating coordinate system, 、 、 They represent the rotor voltage, current and flux in complex form in the synchronous rotating coordinate system, 、 They represent the stator and rotor resistances, represents the synchronous angular frequency, represents the slip angular frequency, represents the rotor angular frequency, represents the mutual inductance between stator and rotor, represents the stator inductance, represents the rotor inductance, 、 They represent the stator and rotor leakage inductance, represents an imaginary unit; The terminal voltage after the fault drops to the voltage before the fault. times, the crowbar is immediately activated and put into the uncontrolled stage. The flux equation in the synchronous rotating coordinate system is: ； in, represents the stator coupling coefficient, represents the rotor coupling coefficient, represents the stator subtransient time constant, represents the rotor subtransient time constant, represents the rotor equivalent resistance, Indicates the crowbar resistance and has been converted to the stator side. represents the rotor transient inductance, represents the stator transient inductance, represents the stator voltage before the fault in the synchronous rotating coordinate system; The stator voltage expression of the doubly fed wind turbine is rewritten as: ； After the fault At this moment, the stator current and stator voltage are substituted into the stator and rotor flux to obtain: ； Convert to the A phase coordinate system: ； in, Indicates the slip rate, represents the initial value of the stator current derivative after the fault in the form of a complex number in the synchronous rotating coordinate system, 、 They represent the instantaneous initial values ​​of the voltage and current derivatives of phase A after the fault, Indicates the instantaneous value of the voltage of phase A before the fault, represents the initial value of the stator voltage after the fault in the synchronous rotating coordinate system, Indicates the transformer ratio of the doubly fed wind turbine unit, Indicates the initial phase of the orientation angle of the synchronous rotating coordinate system at the time of the fault, represents a natural constant, represents the operation of taking the real part, It represents the initial value of the stator current before the fault in the form of a complex number in the synchronous rotating coordinate system.

3. The method for calculating short-circuit current and peak value of new energy grid-connected power taking into account voltage drop transients according to claim 2 is characterized in that: In step 3, the matrix equation is: ； in, represents the return-branch incidence matrix, represents transpose, 、 Both represent diagonal matrices with the number of branches as their dimension, represents the initial value of the loop current derivative after the fault, , It represents a one-dimensional matrix composed of the initial transient value of the equivalent post-fault voltage provided by the doubly fed wind turbine. If there is a doubly fed wind turbine in this branch, the corresponding element is , otherwise it is 0, represents a diagonal matrix with resistors as elements, Represents the one-dimensional matrix formed by the original voltage source of the branch, A one-dimensional matrix representing the instantaneous values ​​of the three-phase currents of each branch before the fault.

4. The method for calculating short-circuit current and peak value of new energy grid-connected power taking into account voltage drop transients according to claim 3 is characterized in that: In step 4, the decay time constant is as follows: ； in, represents the system decay time constant, The Thevenin impedance seen from the fault point, represents the transition resistance, Indicates the operation of taking the imaginary part.

5. The method for calculating short-circuit current and peak value of new energy grid-connected power taking into account voltage drop transients according to claim 4 is characterized in that: In step 5, the approximate analytical expression of the node voltage drop process is as follows: ； in, Indicates the The voltage of a phase of a node changes with time The instantaneous value of Indicates the initial value of the node phase voltage fault, represents the amplitude of the steady-state phase voltage after the fault, Indicates the phase of the steady-state phase voltage after the fault.

6. The method for calculating short-circuit current and peak value of new energy grid-connected power taking into account voltage drop transients according to claim 5, characterized in that: In step 6, the short-circuit current peak value at the fault point is as follows: ； in, Indicates the peak value of short-circuit current at the fault point, Indicates the amplitude of the steady-state phase voltage at the fault point after the fault.

7. The method for calculating short-circuit current and peak value of new energy grid-connected power taking into account voltage drop transients according to claim 6, characterized in that: In step 7, the stator side short-circuit current components are as follows: ； in, represents the stator current in the two-phase stationary coordinate system, 、 and They represent the initial values ​​of the stator frequency component, DC component and rotor frequency component in the two-phase stationary coordinate system respectively.

8. The method for calculating short-circuit current and peak value of new energy grid-connected power taking into account voltage drop transients according to claim 7, characterized in that: In step 8, the instantaneous speed of the resultant vector is as follows: ； in, represents the instantaneous speed of the resultant vector, express and The angle of Represents the synthesized rotor frequency and stator frequency components.

9. The method for calculating short-circuit current and peak value of new energy grid-connected power taking into account voltage drop transients according to claim 8, characterized in that: In step 9, the average speed is as follows: ； in, represents the average speed, express Take the value at the time of fault occurrence, Indicates the operation of taking the complex phase.

10. The method for calculating short-circuit current and peak value of new energy grid-connected power taking into account voltage drop transients according to claim 9, characterized in that: In step 10, the approximate peak moment is taken as the time when the double power frequency component coincides with the sum of the initial values ​​of the two DC components: ； in, express The approximate peak moment of Indicates the short-circuit current over time under the voltage transient component Response.

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