New energy equivalent modeling method and system for asymmetric fault short circuit calculation

Abstract

The present invention discloses a new energy equivalent modeling method and system for asymmetric fault short circuit calculation, including: for a new energy unit adopting a negative sequence current suppression control strategy, a negative sequence current suppression type new energy equivalent model is constructed according to the corresponding relationship between the positive sequence voltage drop and the positive sequence current output of the new energy unit, and the positive sequence voltage axis component of the machine end; for a new energy unit adopting a positive and negative sequence coupling control type strategy, a positive and negative sequence coupling control type new energy equivalent model is constructed according to the control priority of the positive sequence component and the negative sequence component during the fault ride-through of the new energy unit, and the corresponding relationship between the sequence voltage and the sequence current; for a new energy unit adopting a positive and negative sequence decoupling control strategy, a positive and negative sequence decoupling control type new energy equivalent model is constructed according to the corresponding relationship between the positive sequence voltage and the positive sequence current of the new energy unit, and the corresponding relationship between the negative sequence voltage and the negative sequence current. The setting accuracy of the protection setting value and the safety of the new power system are improved.

Description

Technical Field

[0001] The present invention relates to the field of relay protection setting calculation, and in particular to a new energy equivalent modeling method and system for asymmetric fault short circuit calculation. Background Art

[0002] As the first line of defense for power system, relay protection relies on setting calculation to provide the basis for setting fixed values, so as to achieve efficient coordination between protection levels, ensure rapid fault isolation and safe and stable operation of the system. Compared with conventional power sources, new energy power sources show strong controllability characteristics during faults, and the types and control strategies of new energy sources are diverse, so it is difficult to accurately characterize them with a single fixed modeling method.

[0003] In current practical projects, the main grid protection setting calculation is assisted by computer software, but most software still simplifies the renewable energy station into synchronous power supply, load or constant current source. Simplifying it into synchronous power supply ignores the influence of renewable energy control, simplifying it into load ignores the fault current provided by renewable energy as power supply, and simplifying it into constant current source simplifies the current change of renewable energy under different voltage drop levels and control strategies. The above simplification method may not have much impact in the system with small-scale access to renewable energy, but when the proportion of renewable energy access is high, the errors caused by unreasonable renewable energy modeling will have a significant impact on the short-circuit calculation results of local areas or even the entire network, making the protection setting, especially the backup protection setting accuracy, decrease, which brings risks to the safe operation of new power systems. Summary of the invention

[0004] In view of the above technical problems, the present invention provides a new energy equivalent modeling method for asymmetric fault short circuit calculation, comprising:

[0005] Obtain the corresponding control strategy for the new energy units when an asymmetric fault occurs in the power grid;

[0006] For new energy units that adopt negative sequence current suppression control strategy, a negative sequence current suppression type new energy equivalent model is constructed based on the corresponding relationship between the positive sequence voltage drop and the positive sequence current output of the new energy units, as well as the positive sequence voltage axis component at the machine end;

[0007] For new energy units that adopt positive-negative sequence coupling control strategy, a positive-negative sequence coupling control type new energy equivalent model is constructed according to the control priority of positive-sequence component and negative-sequence component during fault ride-through of new energy units, as well as the corresponding relationship between sequence voltage and sequence current;

[0008] For new energy units that adopt the positive-negative sequence decoupling control strategy, a positive-negative sequence decoupling control type new energy equivalent model is constructed according to the corresponding relationship between the positive-sequence voltage and the positive-sequence current of the new energy units, as well as the corresponding relationship between the negative-sequence voltage and the negative-sequence current.

[0009] Furthermore, according to the corresponding relationship between the positive sequence voltage drop and the positive sequence current output of the new energy unit, and the positive sequence voltage axis component of the machine end, the negative sequence current suppression type new energy equivalent modeling is carried out, including:

[0010] The d-axis component of the positive sequence voltage at the generator end Construct a negative-sequence current suppression type new energy equivalent model, calculate the reference current of the d and q axes according to the model, output the positive-sequence current and negative-sequence current of the negative-sequence current suppression type new energy, and the model is specifically expressed as:

[0011]

[0012] Among them, I + and I - Respectively represent the positive and negative sequence currents of the renewable energy output, and They represent the d-axis and q-axis components of the positive sequence current respectively.

[0013] Furthermore, it also includes:

[0014] According to the current output control requirements of new energy units at different voltage drop levels, determine is a piecewise function;

[0015] New energy units that adopt negative sequence current suppression control strategy, according to their control strategy, The expression is as follows:

[0016]

[0017] In the formula, I n and U n is the rated current and rated voltage of the new energy unit; P ref is the reference active power of the power outer loop control strategy; I max The maximum output current allowed by the current limiting module; For control The coefficient of the level.

[0018] Furthermore, according to the control priority of positive-sequence components and negative-sequence components during the fault ride-through of the new energy unit, and the corresponding relationship between sequence voltage and sequence current, a positive-negative sequence coupling control type new energy equivalent model is constructed, including:

[0019] During the fault ride-through of the new energy unit, under the action of the limiting link, the priority component is controlled to be output first, and the non-priority component is output after being clamped by the priority component, thereby forming positive and negative sequence coupling;

[0020] According to the corresponding relationship between sequence voltage and sequence current, a positive-negative sequence coupling control type new energy equivalent model is constructed. The specific model is:

[0021]

[0022] Furthermore, it also includes:

[0023] During low voltage ride-through, when the positive sequence voltage drops below the first preset threshold, the new energy unit will preferentially absorb the negative sequence reactive current and suppress the negative sequence voltage;

[0024] When the positive sequence power drop is greater than the second preset threshold, the positive sequence reactive current is output first;

[0025] According to the positive and negative sequence coupling control strategy and parameters, an expression reflecting the output characteristics of the positive and negative sequence coupling control type new energy equivalent model is established as follows:

[0026]

[0027] Where: and are the positive and negative sequence voltages at the grid connection point, and are the positive and negative sequence d-axis and q-axis currents of the grid connection point, respectively; and is the coefficient, and are the maximum positive sequence current and the maximum negative sequence current respectively;

[0028] The maximum value of positive and negative sequence current in the positive and negative sequence coupling control strategy and Total limiting current I under different voltage drop levels max The vector difference constraint with the priority component is:

[0029]

[0030] In the formula, and They all represent the phasors of the corresponding components.

[0031] Furthermore, according to the corresponding relationship between the positive sequence voltage and the positive sequence current of the new energy unit, and the corresponding relationship between the negative sequence voltage and the negative sequence current, a positive and negative sequence decoupling control type new energy equivalent model is constructed, including:

[0032] According to the positive and negative sequence currents and the vector sum of the positive and negative sequence currents within the positive and negative sequence voltage drop interval during the fault ride-through of the new energy unit, and the positive and negative sequence currents do not reach the limit, and the positive and negative sequence currents do not clamp each other, the constructed positive and negative sequence decoupling control type new energy equivalent model is expressed as:

[0033]

[0034] Furthermore, it also includes:

[0035] When an asymmetric fault occurs in a doubly-fed wind turbine, the terminal voltage and short-circuit current when the Crowbar resistor is put into operation and when it is not put into operation are expressed as follows:

[0036]

[0037]

[0038] In the formula, and All are constant coefficients; It indicates the per unit voltage of the machine end when the Crowbar resistor is put into use. It is the constant admittance value reflected by the doubly-fed wind generator in the negative sequence network after the Crowbar resistor is put into operation.

[0039] The present invention also provides a new energy equivalent modeling system for asymmetric fault short circuit calculation, comprising:

[0040] A control strategy acquisition module is used to acquire the corresponding control strategy for the new energy unit when an asymmetric fault occurs in the power grid;

[0041] The first equivalent model building module is used to build a negative-sequence current suppression type new energy equivalent model for a new energy unit adopting a negative-sequence current suppression control strategy according to the corresponding relationship between the positive-sequence voltage drop and the positive-sequence current output of the new energy unit and the positive-sequence voltage axis component at the unit end;

[0042] The second equivalent model building module is used to build a positive-negative sequence coupling control type new energy equivalent model for the new energy unit adopting the positive-negative sequence coupling control strategy according to the control priority of the positive-sequence component and the negative-sequence component during the fault ride-through of the new energy unit, and the corresponding relationship between the sequence voltage and the sequence current;

[0043] The third equivalent model construction module is used to construct a positive- and negative-sequence decoupling control type new energy equivalent model for new energy units that adopt a positive- and negative-sequence decoupling control strategy according to the corresponding relationship between the positive-sequence voltage and the positive-sequence current of the new energy units, and the corresponding relationship between the negative-sequence voltage and the negative-sequence current.

[0044] Furthermore, the first equivalent model construction module includes:

[0045] The model building submodule is used to construct the d-axis component of the positive sequence voltage at the generator end Construct a negative-sequence current suppression type new energy equivalent model, calculate the reference current of the d and q axes according to the model, output the positive-sequence current and negative-sequence current of the negative-sequence current suppression type new energy, and the model is specifically expressed as:

[0046]

[0047] Among them, I + and I - Respectively represent the positive and negative sequence currents of the renewable energy output, and They represent the d-axis and q-axis components of the positive sequence current respectively.

[0048] Furthermore, the second model building module includes:

[0049] The positive-negative sequence coupling forming submodule is used for the fault ride-through of the new energy unit. Under the action of the limiting link, the priority component is controlled to be output first, and the non-priority component is output after being clamped by the priority component, thereby forming positive-negative sequence coupling;

[0050] The model building submodule is used to build a positive-negative sequence coupling control type new energy equivalent model according to the corresponding relationship between the sequence voltage and the sequence current. The specific model is:

[0051]

[0052] Furthermore, the third model building module includes:

[0053] The model building submodule is used to construct the positive and negative sequence decoupling control type new energy equivalent model according to the positive and negative sequence currents and the vector sum of the positive and negative sequence currents within the positive and negative sequence voltage drop interval during the fault ride-through of the new energy unit, which do not reach the limit, and the positive and negative sequence currents do not clamp each other.

[0054]

[0055] The present invention provides a new energy equivalent modeling method and system for asymmetric fault short-circuit calculation, which is a practical equivalent modeling method that reflects the output characteristics of new energy machines with short-circuit calculation as the goal orientation. According to the different processing methods commonly used in new energy control strategies during asymmetric faults, the new energy control strategies can be generally divided into three categories, and equivalent modeling methods for new energy output characteristics in the form of correlation functions between new energy machine-end sequence voltage and output sequence current are established respectively; the setting accuracy of protection settings and the safety of new power systems are improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 It is a flow chart of a new energy equivalent modeling method for asymmetric fault short circuit calculation provided by the first embodiment of the present invention;

[0057] Figure 2 It is a schematic diagram of a simulation scenario of a new energy delivery system ADPSS involved in an embodiment of the present invention;

[0058] Figure 3is a diagram of the relationship between the positive sequence voltage and current VI of a photovoltaic model A using a negative sequence current suppression strategy according to an embodiment of the present invention;

[0059] Figure 4 The maximum positive sequence current in the B-type photovoltaic control strategy using the positive and negative sequence coupling control strategy involved in the embodiment of the present invention The value range of Figure 4 (a) indicates Get the maximum value The scene at that time, Figure 4 (b) indicates Get the minimum value The scene at that time;

[0060] Figure 5 It is a sequence voltage-current-VI relationship diagram of a B-type photovoltaic device adopting a positive-negative sequence coupling control strategy in ADPSS simulation and theoretical analysis according to an embodiment of the present invention;

[0061] Figure 6 It is a sequence voltage-current VI relationship diagram of a C-type DFIG adopting a positive-negative sequence decoupling control strategy in ADPSS simulation and theoretical analysis according to an embodiment of the present invention;

[0062] Figure 7 It is a positive and negative sequence equivalent circuit when different control strategies and new energy sources involved in the embodiments of the present invention are applied to short circuit calculation, wherein Figure 7 (a) is the positive sequence equivalent circuit, Figure 7 (b) is the negative sequence equivalent circuit;

[0063] Figure 8 It is the calculation process of the new energy equivalent model involved in the embodiment of the present invention in the short-circuit calculation;

[0064] Fig. 9 It is a structural schematic diagram of a new energy equivalent modeling system for asymmetric fault short circuit calculation provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0065] Many specific details are set forth in the following description to facilitate a full understanding of the present invention. However, the present invention can be implemented in many other ways than those described herein, and those skilled in the art can make similar generalizations without violating the connotation of the present invention, so the present invention is not limited to the specific implementation disclosed below.

[0066] Example 1

[0067] Figure 1 This is a flow chart of a new energy equivalent modeling method for asymmetric fault short circuit calculation provided by the present invention. Figure 1 The technical solution of the present invention is described in detail.

[0068] Step S101, obtaining a corresponding control strategy for the new energy generating unit when an asymmetric fault occurs in the power grid.

[0069] When an asymmetric fault occurs in the power grid, the control strategies adopted for the renewable energy units mainly include three types of control strategies: negative sequence current suppression, positive and negative sequence coupling, and positive and negative sequence decoupling. The following describes the renewable energy single-machine equivalent method suitable for setting calculation for the above three control strategies.

[0070] Step S102, for a new energy generating unit adopting a negative sequence current suppression control strategy, a negative sequence current suppression type new energy equivalent model is constructed according to the corresponding relationship between the positive sequence voltage drop and the positive sequence current output of the new energy generating unit and the positive sequence voltage axis component at the generating unit end.

[0071] Negative sequence current suppression type renewable energy uses power electronic controllers to suppress the negative sequence current output of renewable energy when an asymmetric fault occurs. It is a common control strategy for full power inverter renewable energy such as photovoltaic and direct drive wind turbines, and is also a common equivalent method for renewable energy in short circuit calculation. The modeling of negative sequence current suppression type renewable energy units is relatively simple, and only the corresponding relationship between positive sequence voltage drop and positive sequence current output needs to be considered. The d-axis component of the positive sequence voltage at the machine end is used as the As a reference to calculate the reference current of the d and q axes, the positive sequence current of the output negative sequence current suppression type new energy is output. The controlled output current theoretically only contains the positive sequence component, and the negative sequence current output can be regarded as zero, that is:

[0072]

[0073] In the formula, I + and I - Respectively represent the positive and negative sequence currents of the renewable energy output, and They represent the d-axis and q-axis components of the positive sequence current respectively.

[0074] According to relevant technical regulations, for new energy units, such as photovoltaics, the current output control requirements at different voltage drop levels are as follows: The actual on-site new energy is produced by different manufacturers of different models. Its control characteristics have a certain degree of autonomy on the basis of meeting the requirements of national standards. This autonomy will be specifically manifested as The differences in expression form and parameter values.

[0075] New energy units that adopt negative sequence current suppression strategy are generally full-power inverter new energy units, such as photovoltaic and direct-drive wind turbines. Taking A-type photovoltaic unit as an example, its control strategy reflects The expression is as follows:

[0076]

[0077] Where: I n and U n is the rated current and rated voltage of the new energy unit; P ref is the reference active power of the power outer loop control strategy; I max The maximum output current allowed by the current limiting module; For control The coefficient of the level, generally The photovoltaic controller takes hour, That is, the active current output depends on the active output during normal operation; in addition, in this photovoltaic control strategy, I max In case of symmetrical and asymmetrical faults, respectively Switching is done by sensing the negative sequence voltage.

[0078] The simulation system ADPSS includes packaging models of renewable energy control strategies used in actual sites produced by multiple manufacturers. The fault control characteristics of the packaging models have been compared with the fault characteristics of the actual physical devices, including the A-type photovoltaic unit that adopts the negative sequence current suppression strategy mentioned above. The photovoltaic station transmission system based on the photovoltaic model is built in the ADPSS simulation platform. Figure 2 As shown, the equivalent method of its output characteristics mentioned above is verified by simulation results under different faults. The capacity of a single photovoltaic unit is 3.0MW, which is stepped up to a 35kV collector line through a 35kV / 0.36kV transformer, and then connected to a 220kV bus and a large grid equivalent power source through a step-up transformer. Asymmetric faults of different types, transition resistances, short-circuit ratios and other working conditions are set on the 220kV line. At the same time, the photovoltaic output level is adjusted, that is, α in formula (2) is 0.3, 0.8 and 1.0 respectively, and a series of faults are set respectively, so as to obtain the positive-sequence current amplitude corresponding to different positive-sequence voltage amplitudes at the photovoltaic end in the steady-state stage of the fault, and the relationship between the positive-sequence voltage and the positive-sequence current is obtained as follows: Figure 3 As shown in the scattered points. At each output level, different transition resistances, different fault types and fault locations are included, but they all show strong regularity and trend, which shows that the PV output current amplitude is only related to the port voltage amplitude, and has nothing to do with the external topology and fault scenario. At the same time, substituting the value of α into formula (2), Figure 3 The theoretical curve of the relationship between voltage and current is drawn. By comparison, it can be seen that the trend and value level of the theoretical results are close to those of the simulation results, which verifies the correctness of the above equivalence method.

[0079] Step S103, for the new energy generating set adopting the positive-negative sequence coupling control strategy, a positive-negative sequence coupling control type new energy equivalent model is constructed according to the control priority of the positive-sequence component and the negative-sequence component during the fault ride-through of the new energy generating set, and the corresponding relationship between the sequence voltage and the sequence current.

[0080] The output characteristics of new energy units using negative sequence current suppression strategies are simple, but may cause fluctuations in output power. Therefore, existing new energy control strategies will also adopt a method of controlling the positive and negative sequence components at the same time. According to whether there is a correlation between the positive and negative sequence components, they can be divided into positive and negative sequence coupling type and positive and negative sequence decoupling type. The essential connotation of the positive and negative sequence coupling type is that new energy has different control priorities for the positive and negative sequence components during fault ride-through. Under the action of the limiting link, the output of the priority component must be guaranteed, and the non-priority component is clamped by it, thereby forming a positive and negative sequence coupling phenomenon. The corresponding relationship function of the sequence voltage and sequence current can be used as formula (3) to equate its output characteristics:

[0081]

[0082] This modeling method is applicable to both photovoltaic and wind turbines with corresponding control strategies for new energy units with low limit levels. During low voltage ride-through, when the positive sequence voltage drop is less than the first preset threshold (high threshold), the new energy unit will give priority to absorbing negative sequence reactive current and suppressing negative sequence voltage; when the positive sequence voltage drop is greater than the second preset threshold (low threshold), the positive sequence reactive current will be output first.

[0083] Taking photovoltaic units as an example, according to relevant technical regulations, it is required that when photovoltaic asymmetric faults occur, positive sequence reactive current should be injected into the grid to support the recovery of positive sequence voltage and absorb negative sequence reactive current to suppress the increase of negative sequence voltage. When the positive sequence voltage at the grid connection point is less than 60% of the rated voltage, photovoltaics should support the positive sequence voltage and suppress the negative sequence voltage based on the actual control capability without increasing the voltage imbalance at the grid connection point.

[0084] For a certain B-type photovoltaic, during the low voltage ride-through (LVRT) period, when the positive sequence voltage drops slightly, it will give priority to absorbing the negative sequence reactive current and suppressing the negative sequence voltage. When the positive sequence voltage drops severely, it will give priority to ensuring the positive sequence reactive current output to support the positive sequence voltage. Combined with its control strategy and parameters, the mathematical expression reflecting its output characteristics is established as follows:

[0085]

[0086]

[0087] Where: and are the positive and negative sequence voltages at the grid connection point, and are the positive and negative sequence d-axis and q-axis currents of the grid connection point, respectively; and is the coefficient, and are the maximum positive sequence current and the maximum negative sequence current respectively. For the B type photovoltaic,

[0088] The maximum value of positive and negative sequence current in the positive and negative sequence coupling control strategy and Total limiting current I under different voltage drop levels max The vector difference constraint with the priority component is:

[0089]

[0090] In the formula, and They all represent the phasors of the corresponding components.

[0091] With the control and The phase change, in formula (5) and There is a certain range of variation in the value of and The value of is approximated.

[0092] By estimation For example, assume that the total phase current limit I max 1.2I n , then the negative sequence current phasor is When the maximum positive sequence current Depends on the angle between the positive and negative sequence current components. The smaller the angle, the The smaller the current is. Since the new energy limiting link is for each phase current, It is mainly determined by the phase that reaches the limiting level first (i.e. the phase with the smallest positive and negative sequence angle). Assume that the angle between the positive sequence current of phase A and the negative sequence current of phase A is θ, then the angles between the positive and negative sequence currents of phases B and C are θ+120° and θ-120° respectively. The amplitude of the phase sum of the positive and negative sequence currents of any phase does not exceed the current limiting amplitude of 1.2I. n It is known that no matter what value θ takes, the range of the phase with the smallest positive and negative sequence current angle is [-60°, 60°], The value is determined by this phase. Figure 4 As shown, The maximum and minimum values ​​are obtained when the positive and negative sequence current angles are 60° and 0° respectively, and are recorded as and in:

[0093]

[0094] In the formula, As a result, It can be estimated as the average of the two boundaries to reduce the equivalence error, that is, right The estimation of can be achieved in the same way, but the B type photovoltaic The negative sequence current limit is set to a fixed value of 0.6I n .

[0095] Combining expressions (3) and (4), and Substituting the calculation method of the two boundaries, the theoretical relationship curve and regional range of sequence voltage-sequence current corresponding to the B-type photovoltaic control strategy can be obtained. At the same time, the B-type photovoltaic packaging model is selected in ADPSS to set faults of different types, transition resistances and locations. Figure 5 The relationship between positive sequence voltage and positive sequence current and negative sequence voltage and negative sequence current is shown in the form of scattered points. It can be seen that the theoretical relationship between negative sequence voltage and current is consistent with the simulation results. For the positive sequence component, when When the voltage and positive sequence current correspond to the theoretical curve and the simulation result, When , the simulation results of the corresponding relationship between the positive sequence voltage and the positive sequence current are within the range of theoretical derivation. In summary, the simulation analysis results can verify the accuracy of the equivalent modeling method described above.

[0096] Step S104, for the new energy generating set adopting the positive-negative sequence decoupling control strategy, a positive-negative sequence decoupling control type new energy equivalent model is constructed according to the corresponding relationship between the positive-sequence voltage and the positive-sequence current of the new energy generating set, and the corresponding relationship between the negative-sequence voltage and the negative-sequence current.

[0097] The main difference between positive and negative sequence decoupling type new energy and positive and negative sequence coupling type is that due to the large current limit or the control strategy adopted, the positive and negative sequence current and their vector sum do not reach the limit within a large voltage drop range, and the positive and negative sequence currents do not clamp each other. Therefore, the output characteristics of positive and negative sequence decoupling type new energy can be expressed as:

[0098]

[0099] This modeling method is aimed at new energy units with strong overcurrent resistance and high limit level settings, generally doubly fed wind turbines (DFIGs), whose current limit can reach 1.5 times the rated current. Taking DFIG as an example, according to the high to low terminal voltage after the fault, the control of DFIG output current can be divided into three cases: outer loop control, rotor side inverter (RSC) excitation reactive support, and Crowbar resistor input. When the voltage drop is extremely small, DFIG adopts the outer loop power control method; when the voltage drop is large and enters the RSC excitation process, reactive current is injected into the power grid based on relevant technical regulations to provide voltage support; when the voltage drop is serious, the Crowbar resistor is put into use, and the short-circuit current and terminal voltage in the stable state are linearly related. Therefore, in the case of asymmetric faults, the corresponding relationship between the terminal voltage and short-circuit current when the Crowbar resistor is put into use and not put into use can be expressed as follows:

[0100]

[0101]

[0102] In the formula, some parameters have been defined above, in addition, and All are constant coefficients; It indicates the unit value of the terminal voltage when the Crowbar resistor is put into use. It depends on the current impact level after the fault, which is generally about 0.3~0.5, and is affected by the DFIG parameters. For C type DFIG, I max =1.5I n , It is the constant admittance value reflected by DFIG in the negative sequence network after the Crowbar resistor is put into use; during the RSC excitation reactive support period The magnitude of the active current depends on the pre-fault current, i.e. Where α=0.5.

[0103] Reference Figure 2 A wind farm transmission system is established in ADPSS. Eight groups of single-phase and two-phase asymmetric faults are set at different locations (0-60 km) on the 220 kV transmission line to obtain the relationship between sequence voltage and current. Figure 6 As shown in the circle “○” in (b), the trend and value level are similar to those of the theoretical equivalent curves in equations (8) and (9). In 6 of the 8 asymmetric fault cases, the Crowbar is short-term activated during the fault. The relationship between the positive and negative sequence voltage and current amplitude during the Crowbar activation period is Figure 6(b) is shown as dots “■”, which are close to linear. It can be seen that when Crowbar is put into operation, the positive and negative sequence equivalent impedance of DFIG, which can be regarded as an asynchronous machine, is constant. However, since the Crowbar is put into operation for a short time, its influence does not need to be considered in the setting and verification of backup protection.

[0104] The ADPSS simulation verification generally proves the correctness of the equivalent expressions written in the above steps based on the DFIG asymmetric fault control strategy to simulate the DFIG output characteristics.

[0105] In summary, the photovoltaic / wind turbines in various situations can be equivalent to the voltage-controlled current source model in the positive sequence network, and can be equivalent to the open circuit, constant current source and constant impedance in the negative sequence network according to different control strategies and voltage drop levels. The equivalent circuits of the three types of new energy models are as follows: Figure 7 shown.

[0106] Example 2

[0107] The present invention builds a new energy station transmission system in the ADPSS simulation system, adopts three actual new energy packaging models A, B, and C, which represent the negative sequence current suppression type, positive and negative sequence coupling type, and positive and negative sequence decoupling type control strategies respectively, sets different types of symmetrical and asymmetrical faults at different locations to obtain the simulation results of new energy output characteristics under various scenarios, and respectively Figure 3 , Figure 5 and Figure 6 The simulation results were compared with the theoretical output characteristics in the new energy equivalent model, and the correctness and feasibility of the equivalent method were verified.

[0108] In addition, in order to further verify the accuracy of the new energy equivalent method in the setting calculation application, the MATLAB short-circuit calculation results based on the equivalent model proposed in this invention are compared with the ADPSS simulation system calculation results of the actual engineering photovoltaic and wind turbine packaging model. Figure 8 The iterative short-circuit calculation process is shown.

[0109] Still using Figure 2 The ADPSS new energy transmission system shown in the figure performs short-circuit calculation and verification on the new energy equivalent models of three types A, B, and C. Figure 2 There are two central collection lines, each connected to 7 and 6 new energy units, and connected to the 35kV collection busbar via a 0.69kV / 35kV box transformer. According to the equivalent modeling method of the three types of new energy A, B, and C in formulas (1), (3), and (7), their positive and negative sequence circuits in short-circuit calculation are as follows: Figure 7As shown in the figure, the positive sequence circuit can be equivalent to a voltage-controlled current source, the negative sequence circuit is equivalent to an open circuit when the negative sequence current suppression strategy is used, and can be equivalent to a constant impedance when the positive and negative sequence decoupling control strategy is used. When the positive and negative sequence coupling control strategy is used, it can be equivalent to a constant impedance and a constant current source according to the coupling state. and They represent the positive and negative sequence equivalent impedance, positive and negative sequence current, positive and negative sequence voltage at the machine end and negative sequence equivalent constant impedance of the new energy box transformer. The rated capacity of the three new energy units of model A, B and C are 3.0MW, 0.5MW and 2.0MW respectively. The impedance of 220kV step-up transformer and 35kV box transformer are 0.06j and 1.2j (per unit value, the same below), the total impedance of 220kV line is (0.90+3.75j)×10-2 (total length 60km), and the total impedance of 35kV collector line is 0.21+0.085j (total length 7.6km).

[0110] The A, B, and C models of new energy equivalent value were selected for testing. Different types of faults were set on the 220kV line at a distance of 20km from the 220kV step-up transformer, and the positive and negative sequence current values ​​measured on the new energy transmission line (220kV step-up transformer low voltage side) were summarized as shown in Table 1. Figure 8 The short-circuit calculation current results under the same working conditions are compared with the results. The amplitude errors of A and C models of new energy (negative sequence current suppression type and positive and negative sequence decoupling type) do not exceed 9%, and the calculation errors of symmetrical faults are generally small and do not exceed 3%. The error rate of B model photovoltaic (positive and negative sequence coupling control) is slightly higher due to the estimation process and the small current value. The comparison results generally verify the effectiveness of the equivalent method in this paper in the protection setting calculation considering the symmetrical and asymmetrical fault characteristics of actual new energy stations, and can meet the engineering application requirements of setting calculation.

[0111] Table 1 Comparison and verification of short-circuit current calculation results and ADPSS simulation results

[0112]

[0113] Note: The phase angles are all based on the voltage at the large grid terminal as the 0° reference phase. A was operating at 0.3 times the rated power before the PV failure, and B and C were both operating at full power.

[0114] Example 3

[0115] Based on the same inventive concept, the present invention also provides a new energy equivalent modeling system 900 for asymmetric fault short circuit calculation, comprising:

[0116] The control strategy acquisition module 910 is used to acquire the corresponding control strategy for the new energy unit when an asymmetric fault occurs in the power grid;

[0117] The first equivalent model building module 920 is used to build a negative-sequence current suppression type new energy equivalent model for the new energy unit adopting the negative-sequence current suppression control strategy according to the corresponding relationship between the positive-sequence voltage drop and the positive-sequence current output of the new energy unit and the positive-sequence voltage axis component at the unit end;

[0118] The second equivalent model building module 930 is used to build a positive-negative sequence coupling control type new energy equivalent model for the new energy unit adopting the positive-negative sequence coupling control strategy according to the control priority of the positive-sequence component and the negative-sequence component during the fault ride-through of the new energy unit, and the corresponding relationship between the sequence voltage and the sequence current;

[0119] The third equivalent model construction module 940 is used to construct a positive- and negative-sequence decoupling control type new energy equivalent model for the new energy unit adopting the positive- and negative-sequence decoupling control strategy according to the corresponding relationship between the positive-sequence voltage and the positive-sequence current of the new energy unit, and the corresponding relationship between the negative-sequence voltage and the negative-sequence current.

[0120] Furthermore, the first model building module includes:

[0121] The model building submodule is used to construct the d-axis component of the positive sequence voltage at the generator end Construct a negative-sequence current suppression type new energy equivalent model, calculate the reference current of the d and q axes according to the model, output the positive-sequence current and negative-sequence current of the negative-sequence current suppression type new energy, and the model is specifically expressed as:

[0122]

[0123] Among them, I + and I - Respectively represent the positive and negative sequence currents of the renewable energy output, and They represent the d-axis and q-axis components of the positive sequence current respectively.

[0124] Furthermore, the second model building module includes:

[0125] The positive-negative sequence coupling forming submodule is used for the fault ride-through of the new energy unit. Under the action of the limiting link, the priority component is controlled to be output first, and the non-priority component is output after being clamped by the priority component, thereby forming positive-negative sequence coupling;

[0126] The model building submodule is used to build a positive-negative sequence coupling control type new energy equivalent model according to the corresponding relationship between the sequence voltage and the sequence current. The specific model is:

[0127]

[0128] Furthermore, the third model building module includes:

[0129] The model building submodule is used to construct the positive and negative sequence decoupling control type new energy equivalent model according to the positive and negative sequence currents and the vector sum of the positive and negative sequence currents within the positive and negative sequence voltage drop interval during the fault ride-through of the new energy unit, which do not reach the limit, and the positive and negative sequence currents do not clamp each other.

[0130]

[0131] The present invention provides a new energy equivalent modeling method and system for asymmetric fault short-circuit calculation, which is a practical equivalent modeling method that reflects the output characteristics of new energy machines with short-circuit calculation as the goal orientation. According to the different processing methods commonly used in new energy control strategies during asymmetric faults, the new energy control strategies can be generally divided into three categories, and equivalent modeling methods for new energy output characteristics in the form of correlation functions between new energy machine-end sequence voltage and output sequence current are established respectively; the setting accuracy of protection settings and the safety of new power systems are improved.

[0132] Those skilled in the art will appreciate that embodiments of the present invention may be provided as methods, systems, or computer program products. Therefore, the present invention may take the form of a complete hardware embodiment, a complete software embodiment, or an embodiment combining software and hardware. Moreover, the present invention may take the form of a computer program product implemented on one or more computer-usable storage media (including but not limited to disk storage, CD-ROM, optical storage, etc.) containing computer-usable program code.

[0133] The present invention is described with reference to flowcharts and / or block diagrams of methods, devices (systems), and computer program products according to embodiments of the present invention. It should be understood that each process and / or block in the flowchart and / or block diagram, as well as the combination of processes and / or blocks in the flowchart and / or block diagram, can be implemented by computer program instructions. These computer program instructions can be provided to a processor of a general-purpose computer, a special-purpose computer, an embedded processor, or other programmable data processing device to produce a machine, so that the instructions executed by the processor of the computer or other programmable data processing device generate instructions for implementing the processes in the flowchart and / or block diagram. Figure 1 A process or multiple processes and / or boxes Figure 1 A device that provides the functions specified in a block or multiple blocks.

[0134] These computer program instructions may also be stored in a computer-readable memory capable of directing a computer or other programmable data processing device to operate in a specific manner, so that the instructions stored in the computer-readable memory produce an article of manufacture comprising an instruction device, which implements the process Figure 1 A process or multiple processes and / or boxes Figure 1 A function specified in one or more boxes.

[0135] These computer program instructions can also be loaded onto a computer or other programmable data processing device so that a series of operating steps are executed on the computer or other programmable device to produce a computer-implemented process, thereby providing instructions for implementing the process. Figure 1 A process or multiple processes and / or boxes Figure 1 The steps for the functions specified in one or more boxes.

[0136] 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 it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalents that do not depart from the spirit and scope of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A new energy equivalent modeling method for asymmetric fault short circuit calculation, characterized in that: include: Obtain the corresponding control strategy for the new energy units when an asymmetric fault occurs in the power grid; For new energy units that adopt negative sequence current suppression control strategy, a negative sequence current suppression type new energy equivalent model is constructed based on the corresponding relationship between the positive sequence voltage drop and the positive sequence current output of the new energy units, as well as the positive sequence voltage axis component at the machine end; For new energy units that adopt positive-negative sequence coupling control strategy, a positive-negative sequence coupling control type new energy equivalent model is constructed according to the control priority of positive-sequence component and negative-sequence component during fault ride-through of new energy units, as well as the corresponding relationship between sequence voltage and sequence current; For new energy units that adopt the positive-negative sequence decoupling control strategy, a positive-negative sequence decoupling control type new energy equivalent model is constructed based on the corresponding relationship between the positive-sequence voltage and the positive-sequence current of the new energy units, as well as the corresponding relationship between the negative-sequence voltage and the negative-sequence current; According to the corresponding relationship between the positive sequence voltage drop and the positive sequence current output of the new energy unit, and the positive sequence voltage axis component at the machine end, a negative sequence current suppression type new energy equivalent model is constructed, including: taking the positive sequence voltage d axis component at the machine end of the new energy unit Construct a negative-sequence current suppression type new energy equivalent model, calculate the reference current of the d and q axes according to the model, output the positive-sequence current and negative-sequence current of the negative-sequence current suppression type new energy, and the model is specifically expressed as: Among them, I + and I - Respectively represent the positive and negative sequence currents of the renewable energy output, and They represent the d-axis and q-axis components of the positive sequence current output by the renewable energy source respectively; According to the current output control requirements of new energy units at different voltage drop levels, determine is a piecewise function; New energy units that adopt negative sequence current suppression control strategy, according to their control strategy, The expression is as follows: In the formula, I n and U n is the rated current and rated voltage of the new energy unit; P ref is the reference active power of the power outer loop control strategy; I max The maximum output current allowed by the current limiting module; For control The coefficient of the level, It is the d-axis component of the positive sequence voltage at the generator end of the new energy unit; According to the control priority of positive-sequence components and negative-sequence components during the fault ride-through of the new energy unit, and the corresponding relationship between the sequence voltage and the sequence current, a positive-negative sequence coupling control type new energy equivalent model is constructed, including: during the fault ride-through of the new energy unit, under the action of the limiting link, the priority component is controlled to be output first, and the non-priority component is output after being clamped by the priority component, thereby forming positive-negative sequence coupling; According to the corresponding relationship between sequence voltage and sequence current, a positive-negative sequence coupling control type new energy equivalent model is constructed. The specific model is: In the formula, and are the positive-sequence voltage and negative-sequence voltage d-axis components at the new energy generator end; I + and I - Respectively represent the positive-sequence and negative-sequence currents of the renewable energy output; According to the corresponding relationship between the positive sequence voltage and positive sequence current of the new energy unit, and the corresponding relationship between the negative sequence voltage and negative sequence current, a positive and negative sequence decoupling control type new energy equivalent model is constructed, including: According to the positive and negative sequence currents and the vector sum of the positive and negative sequence currents within the positive and negative sequence voltage drop interval during the fault ride-through of the new energy unit, and the positive and negative sequence currents do not reach the limit, and the positive and negative sequence currents do not clamp each other, the constructed positive and negative sequence decoupling control type new energy equivalent model is expressed as: In the formula, and They represent the d-axis and q-axis components of the positive sequence current output by the renewable energy source respectively; and They represent the d-axis and q-axis components of the negative sequence current output by the renewable energy source, respectively; and They are the d-axis components of the positive-sequence voltage and negative-sequence voltage at the new energy machine end respectively.

2. The method according to claim 1, characterized in that: Also includes: During low voltage ride-through, when the positive sequence voltage drops below the first preset threshold, the new energy unit will preferentially absorb the negative sequence reactive current and suppress the negative sequence voltage; When the positive sequence power drop is greater than the second preset threshold, the positive sequence reactive current is output first; According to the positive and negative sequence coupling control strategy and parameters, an expression reflecting the output characteristics of the positive and negative sequence coupling control type new energy equivalent model is established as follows: Where: and They are the d-axis components of the positive-sequence voltage and negative-sequence voltage at the new energy generator end, respectively; and They represent the d-axis and q-axis components of the positive sequence current output by the renewable energy source respectively; and They represent the d-axis and q-axis components of the negative sequence current output by the renewable energy source, respectively; and is the coefficient, and are the maximum positive sequence current and the maximum negative sequence current respectively; The maximum value of positive and negative sequence current in the positive and negative sequence coupling control strategy and Total limiting current I under different voltage drop levels max The vector difference constraint with the priority component is: In the formula, is the phasor of the total limiting current component, and are the phasors of the maximum positive sequence current and the maximum negative sequence current components, and They are the phasors of the q-axis current components of the positive and negative sequence of the new energy output respectively.

3. The method according to claim 1, characterized in that Also includes: When an asymmetric fault occurs in a doubly-fed wind turbine, the corresponding relationship between the terminal voltage and the short-circuit current when the Crowbar resistor is put into use and not put into use is expressed as follows: In the formula, is the d-axis component of the positive sequence voltage at the new energy generator end; and They represent the d-axis and q-axis components of the positive sequence current output by the renewable energy source respectively; and They represent the d-axis and q-axis components of the negative sequence current output by the renewable energy source, respectively; and All are constant coefficients; It indicates the per unit voltage of the machine end when the Crowbar resistor is put into use. is the constant admittance value reflected by the doubly fed wind turbine in the negative sequence network after the Crowbar resistor is put into use, U n is the rated voltage of the new energy unit, I max It is the maximum output current allowed by the current limiting module.

4. A new energy equivalent modeling system for asymmetric fault short circuit calculation, characterized in that: include: A control strategy acquisition module is used to acquire the corresponding control strategy for the new energy generating unit when an asymmetric fault occurs in the power grid; The first equivalent model building module is used to build a negative-sequence current suppression type new energy equivalent model for a new energy unit adopting a negative-sequence current suppression control strategy according to the corresponding relationship between the positive-sequence voltage drop and the positive-sequence current output of the new energy unit and the positive-sequence voltage axis component at the unit end; The second equivalent model building module is used to build a positive-negative sequence coupling control type new energy equivalent model for the new energy unit adopting the positive-negative sequence coupling control strategy according to the control priority of the positive-sequence component and the negative-sequence component during the fault ride-through of the new energy unit, and the corresponding relationship between the sequence voltage and the sequence current; The third equivalent model building module is used to build a positive-negative sequence decoupling control type new energy equivalent model for the new energy unit adopting the positive-negative sequence decoupling control strategy according to the corresponding relationship between the positive-sequence voltage and the positive-sequence current of the new energy unit, and the corresponding relationship between the negative-sequence voltage and the negative-sequence current; According to the corresponding relationship between the positive sequence voltage drop and the positive sequence current output of the new energy unit, and the positive sequence voltage axis component at the machine end, a negative sequence current suppression type new energy equivalent model is constructed, including: taking the positive sequence voltage d axis component at the machine end of the new energy unit Construct a negative-sequence current suppression type new energy equivalent model, calculate the reference current of the d and q axes according to the model, output the positive-sequence current and negative-sequence current of the negative-sequence current suppression type new energy, and the model is specifically expressed as: Among them, I + and I - Respectively represent the positive and negative sequence currents of the renewable energy output, and They represent the d-axis and q-axis components of the positive sequence current output by the renewable energy source respectively; According to the current output control requirements of new energy units at different voltage drop levels, determine is a piecewise function; New energy units that adopt negative sequence current suppression control strategy, according to their control strategy, The expression is as follows: In the formula, I n and U n is the rated current and rated voltage of the new energy unit; P ref is the reference active power of the power outer loop control strategy; I max The maximum output current allowed by the current limiting module; For control The coefficient of the level, It is the d-axis component of the positive sequence voltage at the generator end of the new energy unit; According to the control priority of positive-sequence components and negative-sequence components during the fault ride-through of the new energy unit, and the corresponding relationship between the sequence voltage and the sequence current, a positive-negative sequence coupling control type new energy equivalent model is constructed, including: during the fault ride-through of the new energy unit, under the action of the limiting link, the priority component is controlled to be output first, and the non-priority component is output after being clamped by the priority component, thereby forming positive-negative sequence coupling; According to the corresponding relationship between sequence voltage and sequence current, a positive-negative sequence coupling control type new energy equivalent model is constructed. The specific model is: In the formula, and are the positive-sequence voltage and negative-sequence voltage d-axis components at the new energy generator end; I + and I - Respectively represent the positive-sequence and negative-sequence currents of the renewable energy output; According to the corresponding relationship between the positive sequence voltage and positive sequence current of the new energy unit, and the corresponding relationship between the negative sequence voltage and negative sequence current, a positive and negative sequence decoupling control type new energy equivalent model is constructed, including: According to the positive and negative sequence currents and the vector sum of the positive and negative sequence currents within the positive and negative sequence voltage drop interval during the fault ride-through of the new energy unit, and the positive and negative sequence currents do not reach the limit, and the positive and negative sequence currents do not clamp each other, the constructed positive and negative sequence decoupling control type new energy equivalent model is expressed as: In the formula, and They represent the d-axis and q-axis components of the positive sequence current output by the renewable energy source respectively; and They represent the d-axis and q-axis components of the negative sequence current output by the renewable energy source, respectively; and They are the d-axis components of the positive-sequence voltage and negative-sequence voltage at the new energy machine end respectively.

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