New energy grid-connected short-circuit current and peak calculation method considering voltage sag transient

Abstract

The application provides a new energy grid-connected short-circuit current and peak value calculation method considering voltage drop transient, and belongs to the field of new energy grid-connected system short-circuit current calculation. The method introduces the topological structure and electrical parameters of a doubly-fed wind power generation system, establishes electromagnetic differential equations, deduces a fault voltage initial value expression, constructs a matrix equation to solve node voltage initial value and loop current derivative initial value, calculates an equivalent decay time constant after a fault, establishes an approximate analytical expression of the node voltage drop process, and further calculates the fault point short-circuit current peak value and the doubly-fed wind turbine short-circuit current peak value. The application adopts the above-mentioned new energy grid-connected short-circuit current and peak value calculation method considering voltage drop transient, avoids full-time domain iterative calculation, takes into account calculation accuracy and convergence, is suitable for a doubly-fed wind power generation system, and can provide an important reference for equipment selection and protection setting calculation.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of short-circuit current calculation of new energy grid-connected system, and particularly to a new energy grid-connected short-circuit current and peak value calculation method considering voltage sag transient. BACKGROUND

[0002] As an important part of new energy field, the grid-connected scale of wind power generation has been growing continuously in recent years. Doubly-fed wind turbine is one of the mainstream models in China's wind power industry and has been widely used in large and medium-sized wind farms. Due to the partial power grid-connected characteristics of doubly-fed wind turbine, it can provide a peak short-circuit current much higher than its rated value when a fault occurs. This characteristic makes it an important fault current source that must be focused on in the process of equipment selection and protection setting calculation, and has a profound influence on the selection of related equipment and the formulation of protection measures.

[0003] However, the line and equipment parameters of different doubly-fed wind farms differ greatly, and in addition, the actual unit operating conditions and system impedance also change. To take into account the influence of these factors on voltage transient, it is often necessary to rely on full-time domain fine modeling. However, the full-time domain iterative calculation of the system has efficiency and convergence problems, and it is urgent to propose a short-circuit current and peak value calculation method suitable for doubly-fed wind power generation system. SUMMARY

[0004] The purpose of the present application is to provide a new energy grid-connected short-circuit current and peak value calculation method considering voltage sag transient, which realizes the consideration of voltage sag transient process of new energy grid-connected system while avoiding full-time domain iteration, and proposes a short-circuit current and peak value calculation method that takes into account the calculation accuracy and convergence.

[0005] To achieve the above purpose, the present application provides a new energy grid-connected short-circuit current and peak value calculation method considering voltage sag transient, comprising the following steps:

[0006] Step 1, importing the topological structure and electrical parameters of each element of the doubly-fed wind power generation system;

[0007] Step 2, establishing electromagnetic differential equations in the synchronous rotating coordinate system of doubly-fed wind turbine, and based on the post-fault voltage sag ratio and crowbar injection state, deducing the initial value expression of fault voltage at the machine end;

[0008] Step 3, constructing matrix equations of node voltage initial value and loop current derivative initial value of the doubly-fed wind power generation system and solving them;

[0009] Step 4, calculating the equivalent decay time constant of the doubly-fed wind power generation system after fault;

[0010] Step 5, obtaining fault steady-state voltage and current through iterative calculation, and establishing an approximate analytical expression of the node voltage sag process;

[0011] Step 6, calculate the peak short-circuit current based on the voltage expression of step 5;

[0012] Step 7, solve the stator side short-circuit current component under the voltage step condition of the doubly-fed wind turbine;

[0013] Step 8, calculate the instantaneous speed of the resultant vector of the stator power frequency component and the rotor slip frequency component in the short-circuit current;

[0014] Step 9, determine the linear interval of the resultant vector speed and the corresponding average speed;

[0015] Step 10, calculate the peak time and peak value of the short-circuit current under the voltage step using the average speed;

[0016] Step 11, establish the current response excited by the voltage transient component, take the average of the peak time of the voltage step response and the peak time of the transient response, and substitute it into the total current expression to obtain the final peak value of the short-circuit current of the doubly-fed wind turbine.

[0017] Preferably, in step 2, first establish the electromagnetic differential equation of the doubly-fed wind turbine:

[0018] ;

[0019] wherein, 、 、 represents the stator voltage, current, and flux linkage in complex form in the synchronous rotating coordinate system, 、 、 represents the rotor voltage, current, and flux linkage in complex form in the synchronous rotating coordinate system, 、 represents the stator and rotor resistances, represents the synchronous angular frequency, represents the slip angular frequency, represents the rotor angular frequency, represents the stator and rotor mutual inductance, represents the stator inductance, represents the rotor inductance, 、 represents the stator and rotor leakage inductance, represents the imaginary unit;

[0020] Let the instantaneous voltage drop at the machine terminal after the fault be times the voltage before the fault, the crowbar is immediately activated to put the unit into the uncontrolled stage, and the flux linkage equation in the synchronous rotating coordinate system is:

[0021] ;

[0022] where, is the stator coupling coefficient, is the rotor coupling coefficient, is the stator sub-transient time constant, is the rotor sub-transient time constant, is the rotor equivalent resistance, is the crowbar resistance and has been scaled to the stator side, is the rotor transient inductance, is the stator transient inductance, is the pre-fault stator voltage in the synchronous rotating reference frame;

[0023] The stator voltage expression of the doubly-fed wind turbine is rewritten as:

[0024] ;

[0025] At the fault time, the stator current and stator voltage are substituted into the expressions of the stator and rotor fluxes to obtain:

[0026] ;

[0027] Transformed to the A-phase reference frame:

[0028] ;

[0029] where, , are the post-fault instantaneous initial values of the voltage and current derivatives of the A-phase, respectively, is the pre-fault instantaneous value of the voltage of the A-phase, is the post-fault initial value of the stator voltage in the synchronous rotating reference frame, is the connection transformer ratio of the doubly-fed wind turbine unit, is the initial phase of the directional angle of the synchronous rotating reference frame at the fault time, is the natural constant, is the operation of taking the real part, is the initial value of the pre-fault stator current in the complex form in the synchronous rotating reference frame.

[0030] Preferably, in step 3, the matrix equation is:

[0031] ;

[0032] where, is the back-branch association matrix, is the transpose, , are diagonal matrices with the branch number as the dimension, The instantaneous initial value of the derivative of the loop current after the fault is represented. , This represents a one-dimensional matrix consisting of the equivalent post-fault voltage transient initial values ​​provided by the doubly-fed induction generator (DFIG). If the branch has a DFIG, the corresponding elements are... Otherwise, take 0. A diagonal matrix representing resistances as elements. This represents a one-dimensional matrix formed by the original voltage sources in the branch. This represents a one-dimensional matrix composed of the instantaneous values ​​of the three-phase currents of each branch before the fault.

[0033] Preferably, in step 4, the decay time constant is as follows:

[0034] ;

[0035] in, Represents the system decay time constant. The Thevenin impedance is shown as a view past the fault point. Indicates the transition resistance. This indicates the operation of taking the imaginary part.

[0036] Preferably, in step 5, the approximate analytical expression for the node voltage drop process is as follows:

[0037] ;

[0038] in, Indicates the first The phase voltage of a node changes with time. The instantaneous value, This indicates the initial value of the phase voltage fault at that node. This indicates the magnitude of the steady-state phase voltage after the fault. This indicates the phase of the steady-state phase voltage after a fault.

[0039] Preferably, in step 6, the peak value of the short-circuit current at the fault point is as follows:

[0040] ;

[0041] in, This indicates the peak value of the short-circuit current at the fault point. It represents the amplitude of the steady-state phase voltage at the fault point after the fault occurs.

[0042] Preferably, in step 7, the stator-side short-circuit current components are as follows:

[0043] ;

[0044] in, This represents the stator current in a two-phase stationary coordinate system. , and These represent the initial values ​​of the stator frequency component, DC component, and rotor frequency component in the two-phase stationary coordinate system, respectively.

[0045] Preferably, in step 8, the instantaneous rotational speed of the synthesized vector is as follows:

[0046] ;

[0047] in, Indicates the instantaneous rotational speed of the composite vector. express and The included angle, This represents the combined rotor and stator frequency components.

[0048] Preferably, in step 9, the average rotational speed is as follows:

[0049] ;

[0050] in, Indicates the average rotational speed. express Take the value at the time the fault occurred. This represents the operation of taking the complex phase.

[0051] Preferably, in step 10, the moment when the sum of the initial values ​​of the two DC components and the double power frequency component coincides is taken as the approximate peak time:

[0052] ;

[0053] in, express Approximate peak time, Indicates the short-circuit current as a function of time under the voltage transient component. The response.

[0054] Therefore, the present invention adopts the above-mentioned method for calculating the short-circuit current and peak value of new energy grid connection considering voltage drop transients, and the beneficial technical effects are as follows:

[0055] (1) It can quickly assess and quantify the transient process of system voltage drop, providing theoretical support for mechanism analysis;

[0056] (2) The proposed method can support the study of the peak time law of short-circuit current at the fault point and the terminal of the doubly fed wind turbine.

[0057] (3) The proposed method has high computational efficiency and good computational convergence. Attached Figure Description

[0058] Figure 1The flowchart shows the calculation method of short-circuit current and peak value of new energy grid-connected short circuit current considering voltage dip transients in this invention.

[0059] Figure 2 A topological diagram of grid connection and electromagnetic transients for a doubly fed wind turbine.

[0060] Figure 3 The graph shows the relationship between stator frequency and rotor frequency components and time.

[0061] Figure 4 This is a topology diagram of a doubly-fed wind farm grid-connected system.

[0062] Figure 5 Comparison of transient calculation results for three-phase voltage of doubly-fed wind turbines. Detailed Implementation

[0063] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0064] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0065] Example 1

[0066] See Figure 1 The calculation method for the short-circuit current and peak value of new energy grid-connected systems, taking voltage dip transients as an example, includes the following steps:

[0067] Step 1: First, determine the scenario of the doubly fed wind power generation system to be calculated, and obtain the voltage level, rated capacity and connection topology, electrical parameters of the lines, transformers and doubly fed wind turbines, fault location and transition resistance.

[0068] Step 2: Establish the electromagnetic differential equations in the synchronous rotating coordinate system of the doubly fed wind turbine. Based on the voltage drop ratio at the turbine terminals after the fault and the crowbar engagement status, derive the expression for the initial value of the fault voltage at the turbine terminals.

[0069] See Figure 2 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 case of a fault, engaging the crowbar is equivalent to disconnecting the converter's protection equipment. Based on this, the electromagnetic differential equation of the doubly-fed wind turbine is established:

[0070] ;

[0071] in, , , These represent the stator voltage, current, and flux linkage in complex form in a synchronous rotating coordinate system, respectively. , , Represent the rotor voltage, current, and flux linkage in complex form in a synchronous rotating coordinate system, respectively. , These represent the stator and rotor resistances, respectively. Indicates the synchronization angular frequency. Indicates the slip angular frequency. Indicates the rotor angular frequency. Indicates the mutual inductance between the stator and rotor. Indicates stator inductance, Indicates rotor inductance, , These represent the stator and rotor leakage inductance, respectively. Represents the imaginary unit;

[0072] Assuming the terminal voltage drops instantaneously to the level before the fault after the fault. The crowbar is immediately activated, putting the unit into an uncontrolled state. After eliminating the current term from the above equation, the flux linkage equation in the synchronous rotating coordinate system is now:

[0073] ;

[0074] in, Represents the stator coupling coefficient. Indicates the rotor coupling coefficient. Represents the stator subtransient time constant. Represents the rotor's subtransient time constant. Indicates the rotor equivalent resistance. This indicates the resistance of the crowbar, which has been factored into the stator side. Indicates the rotor transient inductance. Indicates the stator transient inductance. This represents the stator voltage before the fault in a synchronous rotating coordinate system.

[0075] Therefore, the stator voltage expression of the doubly-fed wind turbine can be rewritten as:

[0076] ;

[0077] After troubleshooting Substituting the stator current and stator voltage into the expressions representing the stator and rotor flux linkages, we can simplify and rearrange the equations as follows:

[0078] ;

[0079] in, Indicates the slip ratio. , Let these represent the initial values ​​of the stator voltage in complex form before and after the fault, respectively, in a synchronous rotating coordinate system. This represents the initial value of the stator current derivative in complex form in the synchronous rotating coordinate system after a fault.

[0080] Transform to phase A coordinate system:

[0081] ;

[0082] in, , Let represent the initial values ​​of the voltage and current derivatives of phase A immediately after the fault. This represents the instantaneous voltage value of phase A before the fault. This indicates the transformer turns ratio of the doubly-fed induction generator unit. This indicates the initial phase of the orientation angle of the synchronous rotating coordinate system at the moment of the fault. Represents the natural constant. This indicates the operation of taking the real part. This represents the initial value of the stator current in complex form before a fault, in a synchronous rotating coordinate system.

[0083] Step 3: Construct and solve the matrix equations for the initial values ​​of the node voltage and the initial values ​​of the derivative of the loop current in the doubly fed wind power generation system.

[0084] For the initial transient voltage value after a fault, it can be solved by analogy with the loop current method, and the branch characteristics can be written as:

[0085] ;

[0086] in, , Let represent the one-dimensional matrices formed by the initial values ​​of the derivatives of the voltage branch and the current branch after the fault, respectively. The diagonal matrix representing the number of branches has dimensions, and its elements are the inductances of each branch. A diagonal matrix representing resistances as elements. This represents a one-dimensional matrix formed by the original voltage sources in the branch. This represents a one-dimensional matrix formed by the initial values ​​of the current branch derivatives before the fault.

[0087] The transient initial values ​​and initial values ​​of current derivatives of each component in the system can then be equivalently expressed in matrix equations similar to the loop current method:

[0088] ;

[0089] Among them, except The first matrix is ​​a one-dimensional matrix composed of the instantaneous values ​​of the three-phase currents of each branch before the fault, while the others are matrices composed of the transient initial values ​​after the fault. The instantaneous initial value of the derivative of the loop current after the fault is represented. The diagonal matrix representing the number of branches has the following dimensions. The elements of the branch containing the doubly fed wind turbine are... Otherwise, take 0. , This represents a one-dimensional matrix consisting of the equivalent post-fault voltage transient initial values ​​provided by the doubly-fed induction generator (DFIG). If the branch has a DFIG, the corresponding elements are... Otherwise, take 0. A diagonal matrix representing resistances as elements. This represents a one-dimensional matrix formed by the original voltage sources of the branch.

[0090] Further rearranged into the following formula:

[0091] ;

[0092] After inverting the matrix, we obtain the initial value of the derivative of the loop current, and thus we can obtain the initial value of the voltage of each node after the fault.

[0093] Step 4: Calculate the equivalent decay time constant after a fault in the doubly fed wind power generation system.

[0094] Ignoring transition resistance Then, inject a unit current into the fault point while setting other power sources to zero, and then obtain the Thevenin impedance as seen from the fault point based on the node impedance matrix. This is the self-impedance at the fault point. Then, the equivalent inductance is calculated, and the real parts are superimposed. Once the equivalent resistance is obtained, the system decay time constant is: :

[0095] ;

[0096] in, This indicates the operation of taking the real part.

[0097] 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.

[0098] Regarding the fault steady-state voltage and current of a doubly-fed induction generator (DFIG) wind power system, after constructing the node voltage matrix equations, the injected current simultaneously includes the synchronous generator power supply and the DFIG power supply. For the synchronous generator power supply, the injected current is constant, representing the current source current after Norton equivalent evaluation of the subtransient electromotive force and subtransient reactance; for the DFIG power supply, it is the actual output current under a certain state, the value of which is determined by the PCC point voltage. The voltages of each node in the network are calculated from this, and the output current of the full-power inverter power supply is obtained using the calculated node voltages, thus updating the injected current matrix. This iterative calculation is performed until the voltages of all nodes in the system converge.

[0099] This can be used to represent the transient process of three-phase voltage drop. The fault node voltage can be 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 transient expression for the node voltage is approximately analytically derived as follows:

[0100] ;

[0101] in, Indicates the first The phase voltage of a node changes with time. The instantaneous value, This indicates the initial value of the phase voltage fault at that node. This indicates the magnitude of the steady-state phase voltage after the fault. This indicates the phase of the steady-state phase voltage after a fault.

[0102] Furthermore, it can be transformed to a synchronously rotating coordinate system with axes dq and the superscripts ignored:

[0103] ;

[0104] in, This represents the d-axis component corresponding to the steady-state sinusoidal component. This represents the initial phase of the Park transform orientation angle under d-axis voltage orientation. Indicates the d-axis voltage as a function of time The instantaneous value, Indicates the q-axis voltage as a function of time The instantaneous value.

[0105] , These are all intermediate quantities, and their calculation method is as follows: First, the initial values ​​of the attenuation term in the three-phase voltage expression are denoted as follows: , , As a matrix transformation to a synchronously rotating coordinate system, we have:

[0106] ;

[0107] in, Let be the Park transformation matrix, satisfying:

[0108] ;

[0109] therefore , satisfy:

[0110] ;

[0111] Step 6: Calculate the peak short-circuit current at the fault point based on the voltage expression in Step 5.

[0112] Based on the approximate transient analytical formula for the three-phase voltage drop of the system, through the transition resistor The fault point ground current is:

[0113] ;

[0114] in, This represents the phase current at the fault point as a function of time. The instantaneous value, This represents the phase current at the fault point as a function of time. The instantaneous value, This represents the initial value of the phase voltage at the fault point. This represents the initial phase of the Park transform orientation angle under the d-axis voltage orientation at the fault point.

[0115] Under a symmetrical fault, the fault point is instantaneously grounded through a resistor, therefore the initial voltage at the fault point is 0, i.e. =0, then the fault point current is:

[0116] ;

[0117] The above formula is almost identical to the form of the three-phase short-circuit current in a traditional synchronous machine, therefore 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, taking half a power frequency cycle as the peak time, then the peak short-circuit current at the fault point is... for:

[0118] ;

[0119] in, It represents the amplitude of the steady-state phase voltage at the fault point after the fault occurs.

[0120] Step 7: Solve for the stator-side short-circuit current component of the doubly fed wind turbine under voltage step conditions.

[0121] Next, the short-circuit current of the doubly-fed induction generator (DFIG) is calculated, taking into account the voltage drop transient process. The first step is to calculate the short-circuit current and peak value of the DFIG under voltage step. According to the flux linkage equation in the synchronous rotating coordinate system, since the stator resistance is small, the stator DC current decays slowly and can be considered to have approximately no decay in the first half of the cycle after the fault. The stator and rotor resistances are very small compared to the crowbar resistance, so they can be ignored and satisfy the condition. = 1; Since the leakage inductance between the stator and rotor is much smaller than their mutual inductance, it can be obtained that... ≈ , ≈ ≈1; set the initial values ​​of the stator frequency component, DC component, and rotor frequency component to ≈1 respectively. , and Finally, the stator short-circuit current in the two-phase stationary coordinate system, after simplification, can be expressed as:

[0122] ;

[0123] in, This represents the stator current in a two-phase stationary coordinate system.

[0124] 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.

[0125] Let the synthesized rotor frequency and stator frequency components be denoted as . The next moment is Let the angle between the rotor frequency and stator frequency components be denoted as . The next moment is Its rate of change is the slip angular frequency. ,remember and The included angle is The next moment is The synthesized rotor frequency and stator frequency components are denoted as... The next moment is And using red arrows to indicate the quantities at the next moment, and using dashed lines to represent the quantities in the graph, we get the following: Figure 3 The relationship between the stator frequency and rotor frequency components and time is shown.

[0126] Based on vector relationships, the synthesized rotational components rotational speed satisfy:

[0127] ;

[0128] In a vector triangle, we can obtain the following from the sine theorem: satisfy:

[0129] ;

[0130] Based on the above formula and the Law of Cosines, we can obtain... Derivative, and let have:

[0131] ;

[0132] The ratio of the initial value magnitude The value of represents the combined effect of different situations and different parameters.

[0133] The corresponding speed characteristics always exhibit good linearity, while the increase / decrease is determined by... With cos The relative magnitudes determine this. Physically, this is due to the small time interval and the large initial value amplitude difference and small rotational speed difference between the two components, resulting in a smaller change in the rotational speed of the resultant component.

[0134] Step 9: Determine the linear range of the composite vector rotational speed and the corresponding average rotational speed;

[0135] Within the desired peak time scale, It can be approximated as the included angle. or time The linear function, when integrated over time, yields the phase rotation of the composite component. (Linear interval) The endpoint values ​​are taken respectively and The phase difference and the phase difference when the rotor frequency coincides with the DC component correspond to the times 0, 1, and 2 respectively. Let the linearized speed characteristic be... , Within the linear interval The average rotational speed.

[0136] ;

[0137] in, Indicates the average rotational speed. express Take the value at the time the fault occurred. This represents the operation of taking the complex phase.

[0138] Step 10: Calculate the peak time and peak value of the short-circuit current under voltage step using the average rotational speed.

[0139] remember and The initial phase difference is ; The moment when the peak value coincides with the DC component is denoted as . Then we have:

[0140] ;

[0141] Since the linear interval already includes the peak time and its average speed differs little from the average speed of the integral interval, the average speed of the linear interval can be used as a substitute. The quadratic equation for the unknown quantity can be simplified to a linear form:

[0142] ;

[0143] in, This represents the approximate peak time after replacing the average rotational speed.

[0144] Step 11: Establish the current response of the voltage transient component excitation, take the average of the peak time of the voltage step response and the peak time of the transient response, and substitute it into the total current expression to obtain the final peak value of the short-circuit current of the doubly-fed wind turbine.

[0145] The second step is to calculate the peak short-circuit current after superimposing the transient voltage components. Based on the approximate transient analytical expression for the dq-axis voltage drop, since the d-axis component is the dominant factor, and considering the very short peak time and the most severe case, the q-axis component is ignored, simplifying the process to:

[0146] ;

[0147] The above formula shows that the dq axis voltage has an additional oscillation attenuation component, which may cause a much more severe impact than the assumed instantaneous voltage drop in the early stage of a fault.

[0148] Substituting the above equation as the stator voltage excitation into the voltage flux linkage equation and solving it using the Laplace transform, since... and For significant differences, a reduction-order method using pole-zero cancellation can be employed. Finally, after inverse Laplace transform and simplification, the time-dependent equations in the two-phase stationary coordinate system are obtained. Total short-circuit current expression :

[0149] ;

[0150] in, , , satisfy:

[0151] ;

[0152] The total short-circuit current is the superposition of the current response of the voltage step component and the DC decay component, denoted as […]. , Later:

[0153] ;

[0154] This represents the response of the short-circuit current with time t under a voltage step change in a two-phase stationary coordinate system. The peak value is calculated using step 10, which describes the voltage drop. It is obtained from the steady-state fault calculation in step 5. This represents the short-circuit current versus time under the transient voltage component in a two-phase stationary coordinate system. The response consists of a DC component, a twice-stator frequency component, and a damped DC component, and its initial value satisfies =0, so the moment when the sum of the initial values ​​of the two DC components and the double power frequency component coincides can be taken as the approximate peak time:

[0155] ;

[0156] in, express Approximate peak time, Indicates the short-circuit current as a function of time under the voltage transient component. The response.

[0157] remember Synchronous angular frequency The period below, this value is / 4 and is independent of system parameters; substituting this into the total short-circuit current expression yields the result. The peak value of the total short-circuit current. Since the total short-circuit current is the difference between the two vector currents, it is assumed that the peak value of the total short-circuit current is obtained when each vector current reaches its peak value and forms an obtuse angle.

[0158] Response to transient voltage drops The peak time is obtained from step 10, because its Generally less than The average speed is near the rotor angular frequency, therefore and If the differences are not significant, the average value can be taken as the peak time of the total current response. Substituting this into the total short-circuit current expression yields the peak time and magnitude of the short-circuit current of the doubly-fed induction generator (DFIG).

[0159] When the parameters of new energy generating units and the network cause rapid voltage transient decay, the node voltage can be considered as a step change. At this time, the fault point current has almost no peak phenomenon, and the calculation of the short-circuit current and peak value at the generator terminals degenerates into a method based on voltage step changes. Specifically, the peak time of the current response of the voltage transient component is... / 4, after considering a certain margin, it is believed that < At / 6, the voltage transient effect is relatively small, and the calculation conditions for voltage step change can be used.

[0160] Calculation example 1

[0161] This example is based on PSCAD / EMTDC, and a system was built in PSCAD / EMTDC as follows: Figure 4 A doubly fed wind farm grid-connected system model, in which System voltage; For system impedance, ~ For the collector line impedance, The power station outgoing line impedance is 10km in length; the grid frequency is 60Hz; after equivalence, there are four doubly fed wind turbines, which were close to full capacity before the fault. Specific parameters of the units and the network are shown in Tables 1 to 2.

[0162] Table 1. Parameters of Doubly Fed Fans

[0163] ;

[0164] Table 2 Parameters of various components of the doubly-fed wind farm grid-connected system

[0165] ;

[0166] Based on the proposed steady-state calculation results for system faults, if the steady-state voltage drops to around 0.45 pu, it is determined that the unit will engage the crowbar. Further calculations then yield... =4.27ms. According to the proposed method, the voltage transient process at this point will significantly affect the short-circuit current and peak value calculation results. The proposed approximate analytical method for voltage transients yields the expression for the generator terminal voltage in a synchronous rotating coordinate system. The calculated values ​​are compared with the simulated values ​​after transformation to three phases, as shown below. Figure 5 As shown in the figure, the voltage transient lasted for more than half a power frequency cycle in this case, 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.

[0167] By setting different transition resistors and crowbar resistance values, the peak value and time of the short-circuit current at the fault point are calculated. Table 3 shows the comparison results of the instantaneous value of the A-phase current of the method proposed in this invention, IEC standards, and simulation values.

[0168] Table 3 Different transition resistances Comparison of calculated peak short-circuit current at fault point under crowbar

[0169] ;

[0170] As shown in Table 3, both methods can calculate the peak value of the fault point under metallic faults, and the proposed method has a smaller error. Furthermore, when the transition resistance and parameters change, the proposed method still has a smaller calculation error, demonstrating the rationality of taking half a power frequency cycle as the peak time of the short-circuit current at the fault point when the voltage transient process is significant.

[0171] The following verification method for calculating the peak short-circuit current at the generator terminal is still assumed to be a three-phase symmetrical fault occurring at the same location. The simulated value of the complex modulus of the current of the No. 3 fan in the two-phase stationary coordinate system is taken. Table 4 shows the comparison results of the method proposed in this invention, the calculated value under the assumption of instantaneous drop, and the simulated value.

[0172] Table 4 Different Transition Resistances Comparison of calculation results of peak short-circuit current at the terminal of the doubly fed wind turbine under the crowbar

[0173] ;

[0174] The above results demonstrate the necessity of considering the transient voltage sag process. Compared with the maximum error of 21.8% in the peak value calculation assuming instantaneous voltage sag, the error of the proposed method is less than 6%. Under instantaneous voltage sag, the peak time is around one-third of the power frequency cycle, which is significantly shifted forward after considering the influence of the voltage transient process.

[0175] The main reason for the overall larger peak time is the neglect of the attenuation effect and the approximation of the rotational speed. However, the main reason for the relatively small impact on the peak value error is that the vector rotational speed and the attenuation effect on the peak value cancel each other out near the peak time, meaning that the current change is relatively flat at this time. The main reason for the increased calculation error of the instantaneous drop when the fault steady-state voltage and the crowbar resistance increase is that the crowbar resistance is positively correlated with the magnitude of the voltage transient response current. An increase in the fault steady-state voltage further increases the proportion of the transient voltage response current, and ignoring this will increase the calculation error.

[0176] The proposed approximate voltage transient process method can characterize the impact of fault conditions, network topology parameters, and wind turbine grid connection. It also has certain reference value for calculations of single-unit and distributed doubly-fed induction generator (DFIG) grid-connected systems with severe voltage transient processes. The proposed calculation method can support the study of peak time law. By quantifying the transient change process of voltage drop, the mechanism determines that the peak time of the fault point current can be taken as half a power frequency cycle, and that the peak time of the short-circuit current at the DFIG turbine terminal is assumed to shift forward compared to the instantaneous voltage drop. The proposed method has high computational efficiency and good computational convergence. By comparing the proposed short-circuit current and peak value calculation methods with IEC standards, simulation examples, and measured waveforms, the error in the peak value calculated by the proposed method is verified to be within 6%.

[0177] It is worth noting that all contents not described in detail in this invention are existing technologies and are well known to those skilled in the art.

[0178] Therefore, the present invention adopts the above-mentioned method for calculating the short-circuit current and peak value of new energy grid connection that takes into account voltage drop transients, avoiding full-time-domain iterative calculation, taking into account both calculation accuracy and convergence, and is applicable to doubly-fed wind power generation systems, which can provide important reference for equipment selection and protection setting calculation.

[0179] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions 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 the short-circuit current and peak value of new energy grid-connected systems, taking into account voltage dip transients, characterized in that... Includes the following steps: Step 1: Import the topology and electrical parameters of each component of the doubly-fed wind power generation system; Step 2: Establish the electromagnetic differential equations in the synchronous rotating coordinate system of the doubly fed wind turbine. Based on the voltage drop ratio at the turbine terminals after the fault and the crowbar engagement state, derive the expression for the initial value of the fault voltage at the turbine terminals. Step 3: Construct and solve the matrix equations for the initial values ​​of the node voltages and the initial values ​​of the derivatives of the loop currents in the doubly fed wind power system. Step 4: Calculate the equivalent decay time constant after a fault in the doubly-fed wind power generation system; 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 peak short-circuit current at the fault point based on the voltage expression from Step 5; Step 7: Solve for the stator-side short-circuit current component of the doubly-fed induction generator 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 composite vector rotational speed and the corresponding average rotational speed; Step 10: Calculate the peak time and peak value of the short-circuit current under voltage step using the average rotational speed; Step 11: Establish the current response of the voltage transient component excitation, take the average value of the peak time of the voltage step response and the peak time of the transient response, and substitute it into the total current expression to obtain the final peak value of the short-circuit current of the doubly-fed wind turbine. In step 2, the electromagnetic differential equations of the doubly-fed wind turbine are first established: ； in, , , These represent the stator voltage, current, and flux linkage in complex form in a synchronous rotating coordinate system, respectively. , , Represent the rotor voltage, current, and flux linkage in complex form in a synchronous rotating coordinate system, respectively. , These represent the stator and rotor resistances, respectively. Indicates the synchronization angular frequency. Indicates the slip angular frequency. Indicates the rotor angular frequency. Indicates the mutual inductance between the stator and rotor. Indicates stator inductance, Indicates rotor inductance, , These represent the stator and rotor leakage inductance, respectively. Represents the imaginary unit; The voltage at the generator terminals momentarily drops to the level before the fault. The crowbar is immediately activated, putting the unit into an uncontrolled state. The flux linkage equation in the synchronous rotating coordinate system is: ； in, Represents the stator coupling coefficient. Indicates the rotor coupling coefficient. Represents the stator subtransient time constant. Represents the rotor's subtransient time constant. Indicates the rotor equivalent resistance. This indicates the resistance of the crowbar, which has been factored into the stator side. Indicates the rotor transient inductance. Indicates the stator transient inductance. This represents the stator voltage before the fault in a synchronous rotating coordinate system. The stator voltage expression for a doubly-fed induction generator (DFIG) is rewritten as follows: ； After troubleshooting Substituting the stator current and stator voltage into the expressions representing the stator and rotor flux linkages at time t, we obtain: ； Transform to phase A coordinate system: ； in, Indicates the slip ratio. This represents the initial value of the stator current derivative in complex form in a synchronous rotating coordinate system after a fault. , Let represent the initial values ​​of the voltage and current derivatives of phase A immediately after the fault. This represents the instantaneous voltage value of phase A before the fault. This represents the initial value of the stator voltage after a fault in a synchronous rotating coordinate system. This indicates the transformer turns ratio of the doubly-fed induction generator unit. This indicates the initial phase of the orientation angle of the synchronous rotating coordinate system at the moment of the fault. Represents the natural constant. This indicates the operation of taking the real part. This represents the initial value of the stator current in complex form before a fault, in a synchronous rotating coordinate system. In step 4, the decay time constant is as follows: ； in, Represents the system decay time constant. The Thevenin impedance is shown as a view past the fault point. Indicates the transition resistance. This represents the operation of taking the imaginary part; In step 5, the approximate analytical expression for the node voltage drop process is as follows: ； in, Indicates the first The phase voltage of a node changes with time. The instantaneous value, This indicates the initial value of the phase voltage fault at that node. This indicates the magnitude of the steady-state phase voltage after the fault. Indicates the phase of the steady-state phase voltage after a fault; In step 6, the peak short-circuit current at the fault point is as follows: ； in, This indicates the peak value of the short-circuit current at the fault point. It represents the amplitude of the steady-state phase voltage at the fault point after the fault occurs.

2. The method for calculating the short-circuit current and peak value of new energy grid-connected devices considering voltage dip transients as described in claim 1, characterized in that, In step 3, the matrix equation is: ； in, Represents the back-branch correlation matrix. Indicates transpose. , Each represents a diagonal matrix with the number of branches as its dimension. The instantaneous initial value of the derivative of the loop current after the fault is represented. , This represents a one-dimensional matrix consisting of the equivalent post-fault voltage transient initial values ​​provided by the doubly-fed induction generator (DFIG). If the branch has a DFIG, the corresponding elements are... Otherwise, take 0. A diagonal matrix representing resistances as elements. This represents a one-dimensional matrix formed by the original voltage sources in the branch. This represents a one-dimensional matrix composed of the instantaneous values ​​of the three-phase currents of each branch before the fault.

3. The method for calculating the short-circuit current and peak value of new energy grid-connected devices considering voltage dip transients as described in claim 2, characterized in that, In step 7, the stator-side short-circuit current components are as follows: ； in, This represents the stator current in a two-phase stationary coordinate system. , and These represent the initial values ​​of the stator frequency component, DC component, and rotor frequency component in the two-phase stationary coordinate system, respectively.

4. The method for calculating the short-circuit current and peak value of new energy grid-connected devices considering voltage dip transients as described in claim 3, characterized in that, In step 8, the instantaneous rotational speed of the synthesized vector is as follows: ； in, Indicates the instantaneous rotational speed of the composite vector. express and The included angle, This represents the combined rotor and stator frequency components.

5. The method for calculating the short-circuit current and peak value of new energy grid-connected devices considering voltage dip transients as described in claim 4, characterized in that, In step 9, the average rotational speed is as follows: ； in, Indicates the average rotational speed. express Take the value at the time the fault occurred. This represents the operation of taking the complex phase.

6. The method for calculating the short-circuit current and peak value of new energy grid-connected devices considering voltage dip transients as described in claim 5, characterized in that, In step 10, the moment when the sum of the initial values ​​of the two DC components and the double power frequency component coincides is taken as the approximate peak time: ； in, express Approximate peak time, Indicates the short-circuit current as a function of time under the voltage transient component. The response.

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