Method for calculating short-circuit current of new energy double-fed unit during low voltage ride through

Abstract

The application discloses a calculation method of short-circuit current of a new energy double-fed unit during low-voltage penetration, and is specifically implemented according to the following steps: a short-circuit current simplified model is established; an RT-LAB semi-physical simulation platform is used to build a new energy double-fed unit grid-connected system, and short-circuit currents of the unit under different fault scenes are obtained; simulation test is carried out; a least square method is used for data fitting of the short-circuit current during the low-voltage penetration stage, key parameters of the short-circuit current model are obtained, thereby forming calculation models of the short-circuit current during the Crowbar input stage and the RSC reactive power control stage, and finally forming a practical short-circuit current calculation model of the new energy double-fed unit. The calculation model is clear, has strong universality and operability, through the RT-LAB semi-physical simulation combined with the control characteristics of the actual new energy unit converter, the actual operation condition of the field is more approached, the calculation amount is reduced, the time required for calculation is shortened, and the calculation efficiency is improved.

Description

Technical Field

[0001] This invention belongs to the field of power system fault analysis technology, and relates to a method for calculating the short-circuit current of new energy doubly fed generator units during low voltage ride-through. Background Technology

[0002] To mitigate the impact of large-scale grid disconnection during faults in renewable energy units, grid connection standards require units to meet low-voltage ride-through (LVRT) requirements during fault transients. Simultaneously, influenced by converter control strategies, a comprehensive analysis of the short-circuit characteristics of renewable energy units is necessary, considering both LVRT strategies and fault response characteristics. Therefore, it is essential to establish a short-circuit current calculation method that considers the LVRT phase of renewable energy doubly-fed induction generator (DFIG) units during faults. DFIG units offer advantages such as small inverter capacity and independent decoupling control of active and reactive power.

[0003] However, when a fault occurs in the external power grid, the terminal voltage of the new energy doubly-fed induction generator (DFIG) drops, and a large inrush current is generated in the rotor winding. To avoid damage to the converter, engaging the crowbar circuit and locking the rotor-side converter (RSC) is a commonly used low-voltage ride-through method. Since wind turbines need to remain connected to the grid and provide reactive current during grid faults, after surviving the rotor inrush current, the new energy DFIG usually deactivates the crowbar and restarts the RSC, thereby controlling the rotor current to achieve reactive power output. There are two stages during the fault: the crowbar engagement stage and the reactive power control stage. However, existing research either does not consider factors such as converter control strategies and the degree of voltage drop during the fault, leading to deviations between theoretical models and actual fault conditions, or the short-circuit current expressions for the two stages are extremely complex, computationally intensive, and have low accuracy and efficiency, prolonging the calculation and analysis time of the short-circuit current, which is not conducive to the optimization of subsequent protection configuration and control strategies, and has an adverse impact on grid stability. Summary of the Invention

[0004] The purpose of this invention is to provide a method for calculating the short-circuit current of new energy doubly-fed generator units during low-voltage ride-through, which solves the problems of high computational complexity, large computational load, and low accuracy of existing models.

[0005] The technical solution adopted in this invention is a method for calculating the short-circuit current of a new energy doubly-fed generator unit during low-voltage ride-through, specifically implemented according to the following steps:

[0006] Step 1: Consider the control strategy of the doubly fed wind turbine and establish a simplified model of the short-circuit current.

[0007] Step 2: Use the RT-LAB hardware-in-the-loop simulation platform to build a grid-connected system for a new energy doubly-fed generator unit and obtain the short-circuit current of the unit under different fault scenarios;

[0008] Step 3: Use the hardware-in-the-loop simulation system from Step 2 to conduct simulation tests on the simplified short-circuit current model of the new energy doubly-fed generator unit from Step 1.

[0009] Step 4: Use the least squares method to fit the short-circuit current data during the low-voltage ride-through stage, obtain the key parameters of the short-circuit current model, and thus form the calculation model of the short-circuit current during the Crowbar commissioning stage and the RSC reactive power control stage, and finally form the calculation model of the practical short-circuit current of the new energy doubly-fed generator unit.

[0010] The invention is further characterized in that,

[0011] Step 1 is implemented in the following steps:

[0012] Step 1.1: The wind power absorbed by the new energy doubly-fed generator is converted into electrical power by the asynchronous generator and transmitted to the grid through the converters on the stator and rotor sides. When a grid fault occurs, the converter control system of the doubly-fed generator gives a trigger signal to the Crowbar circuit of the generator-side converter, puts the Crowbar resistor in the rotor winding and locks the reactive power control RSC of the generator-side converter; at this time, the transient inrush current generated in the rotor winding will flow through the Crowbar resistor.

[0013] Step 1.2: When the rotor current is lower than the threshold, the Crowbar circuit is disconnected and the RSC is restarted. The RSC resumes control of the rotor winding. The DFIG outputs reactive power during the reactive power control phase of the RSC, which will raise its own terminal voltage and cause the output current to change. After avoiding the rotor inrush current, the Crowbar is disconnected and the reactive power control of the rotor converter is restarted.

[0014] Step 1.1 is as follows: Based on the principle of flux conservation, in order to keep the stator flux unchanged at the moment of the fault, the stator flux after the fault consists of two parts: one part is the steady-state part of the stator flux; the other part is the transient flux induced in the stator core by the voltage drop, and the transient flux decays with the stator time decay constant.

[0015] In a two-phase synchronous rotating coordinate system, the time-domain analytical expression of the rotor flux linkage of a doubly-fed generator is:

[0016] (1)

[0017] In the formula, , , The rotor flux linkage coefficient;

[0018] Substituting into the stator short-circuit current equation, the expression for the stator short-circuit current after Crowbar protection operation, in a two-phase synchronous rotating coordinate system, is:

[0019] (2)

[0020] In the formula: , , These represent the amplitudes of the power frequency cycle component, DC component, and speed difference frequency cycle component input to the Crowbar, respectively.

[0021] The short-circuit current expression in the three-phase stationary coordinate system is:

[0022] (3)

[0023] in, , , These represent the amplitudes of the DC component, power frequency cycle component, and speed difference frequency cycle component input to the Crowbar, respectively. , , These are the attenuation factors for the DC component, the power frequency periodic component, and the speed difference frequency periodic component, respectively. , These are the angular frequencies of the power frequency periodic component and the speed difference frequency periodic component, respectively. , These are the initial phases of the power frequency periodic component and the speed difference frequency periodic component, respectively.

[0024] Step 1.2 specifically involves the following: According to the law of conservation of flux linkage, the stator flux linkage does not change abruptly at the moment of the fault. The short-circuit current during the RSC reactive power control phase is:

[0025] (4)

[0026] In the formula, , , These are the times when the Crowbar is cut off and the converter reactive power control is started ( ). The amplitudes of the DC component, the power frequency cycle component, and the main harmonic components parasitic in the converter inner loop control at that time. , , These are the attenuation factors for the DC component, the power frequency periodic component, and the main harmonic components parasitic in the converter inner loop control, respectively. , These are the angular frequencies of the power frequency periodic component and the harmonic component, respectively. , These are the initial phases of the power frequency periodic component and the harmonic component, respectively.

[0027] Step 3 specifically involves: calculating the short-circuit current during the Crowbar activation phase and the RSC reactive power control phase of the new energy doubly-fed generator unit under fault scenarios with different voltage sags. , Sampling is performed; within a short data window of 5~10ms, the attenuation terms of the DC component and the speed difference component are selected. , , , Simplified to , , , ; , Taking 1; the simplified expressions for the short-circuit current during the Crowbar activation stage and the RSC reactive power control stage are obtained as follows:

[0028] (5)

[0029] (6).

[0030] Step 4 specifically involves: the short-circuit current during the Crowbar activation phase. Sample values , … The following relationship is satisfied, where , , For the sampling period, take ;

[0031] (7)

[0032] Equation (7) can be expressed in matrix form. ,in:

[0033] (8)

[0034] (9)

[0035] (10)

[0036] In the formula, The matrix contains 8 parameters to be determined. The coefficient matrix was obtained offline using simulation test sample values; It is a constant matrix, derived from the corresponding Crowbar input stage short-circuit current. The sampled values ​​are combined with data redundancy methods to expand the matrix. The size of the matrix can be Seeking, It is a generalized inverse matrix; similarly, the short-circuit current in the reactive power control stage of RSC can be obtained by the above calculation method. The relevant parameter calculation values.

[0037] The beneficial effects of this invention are as follows: Based on the establishment of mathematical models for the short-circuit current provided by new energy units at different fault stages, this invention utilizes the RT-LAB hardware-in-the-loop simulation system to simulate different fault scenarios. Based on the tested short-circuit current data, it fits the key parameters for short-circuit calculation of different types of new energy doubly-fed induction generators at different stages of low-voltage ride-through, forming a simplified method for calculating the short-circuit current of new energy doubly-fed induction generators, thereby simplifying the model scale. This invention is applicable to different types of new energy units and considers the logarithmic influence of different fault scenarios and low-voltage ride-through strategies, demonstrating strong universality and operability. By using RT-LAB hardware-in-the-loop simulation to consider the control characteristics of actual new energy unit converters, it can better approximate the actual operating conditions on-site, reduce calculation time, improve the completeness, accuracy, and efficiency of calculation analysis, facilitate calculation, and provide a basis for the subsequent optimization and configuration of protection for new energy units. Attached Figure Description

[0038] Figure 1 This is a flowchart of the calculation method for short-circuit current of the new energy doubly-fed generator unit during low-voltage ride-through according to the present invention;

[0039] Figure 2 This is a control principle diagram of the new energy doubly-fed generator unit during low voltage ride-through of the present invention;

[0040] Figure 3 This is a schematic diagram of the RT-LAB hardware-in-the-loop simulation test system for calculating the short-circuit current of the new energy doubly-fed generator unit during low-voltage ride-through, as described in this invention. Detailed Implementation

[0041] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0042] This invention provides a method for calculating the short-circuit current of new energy doubly-fed induction generator (DFIG) units during low-voltage ride-through. It establishes mathematical models of different types of DFIG units at different fault stages, and utilizes the RT-LAB semi-physical testing system of the actual generator-side and grid-side converters of the DFIG units for simulation. This allows for the acquisition of short-circuit test data under different fault scenarios, ultimately fitting key parameters of the short-circuit current for different types of DFIG units at different fault stages. This leads to a simplified method for calculating the short-circuit current of different types of DFIG units, providing a basis for parameter optimization of DFIG unit protection.

[0043] The present invention provides a method for calculating the short-circuit current of a new energy doubly-fed induction generator unit during low-voltage ride-through, the process of which is as follows: Figure 1 As shown, please follow these steps:

[0044] Step 1: Consider the control strategy of the doubly fed wind turbine and establish a simplified short-circuit current model; Step 1: Based on the operating characteristics of the new energy doubly fed wind turbine, obtain a simplified short-circuit current model of the new energy doubly fed wind turbine at different stages of the fault.

[0045] Step 1.1: The wind power absorbed by the new energy doubly-fed induction generator is converted into electrical power by an asynchronous generator, and then transmitted to the power grid through converters on the stator and rotor sides, such as... Figure 2 As shown, when a grid fault occurs, the doubly fed generator converter control system sends a trigger signal to the Crowbar circuit of the generator-side converter, puts a Crowbar resistor in the rotor winding, and locks the reactive power control RSC of the generator-side converter. At this time, the transient inrush current generated in the rotor winding will flow through the Crowbar resistor, thereby achieving the purpose of protecting the converter.

[0046] Based on the principle of flux conservation, in order to keep the stator flux constant at the moment of the fault, the stator flux after the fault consists of two parts: one part is the steady-state part of the stator flux; the other part is the transient flux induced in the stator core by the voltage drop, and the transient flux decays with the stator time decay constant.

[0047] In a two-phase synchronous rotating coordinate system, the time-domain analytical expression of the rotor flux linkage of a doubly-fed generator is:

[0048] (1)

[0049] In the formula, , , The rotor flux linkage coefficient;

[0050] Substituting into the stator short-circuit current equation, the expression for the stator short-circuit current after Crowbar protection operation, in a two-phase synchronous rotating coordinate system, is:

[0051] (2)

[0052] In the formula: , , These represent the amplitudes of the power frequency cycle component, DC component, and speed difference frequency cycle component input to the Crowbar, respectively.

[0053] The short-circuit current expression in the three-phase stationary coordinate system is:

[0054] (3)

[0055] in, , , These represent the amplitudes of the DC component, power frequency cycle component, and speed difference frequency cycle component input to the Crowbar, respectively. , , These are the attenuation factors for the DC component, the power frequency periodic component, and the speed difference frequency periodic component, respectively. , These are the angular frequencies of the power frequency periodic component and the speed difference frequency periodic component, respectively. , These are the initial phases of the power frequency periodic component and the speed difference frequency periodic component, respectively.

[0056] Step 1.2: When the rotor current is lower than the threshold, the Crowbar circuit is disconnected and the RSC is restarted. The RSC resumes control of the rotor winding. The DFIG outputs reactive power during the reactive power control phase of the RSC, which will raise its own terminal voltage and cause the output current to change. After avoiding the rotor inrush current, the Crowbar is disconnected and the reactive power control of the rotor converter is restarted.

[0057] Since the grid-side converter has a small capacity, the AC side current has little impact on the short-circuit current fed out by the doubly-fed wind turbine. Therefore, the influence of the GSC on the short-circuit current of the doubly-fed wind turbine is ignored here, taking into account the rotor-side converter control.

[0058] According to the law of conservation of flux linkage, the stator flux linkage does not change abruptly at the moment of fault. The short-circuit current during the reactive power control phase of RSC is:

[0059] (4)

[0060] In the formula, , , These are the times when the Crowbar is cut off and the converter reactive power control is started ( ). The amplitudes of the DC component, the power frequency cycle component, and the main harmonic components parasitic in the converter inner loop control at that time. , , These are the attenuation factors for the DC component, the power frequency periodic component, and the main harmonic components parasitic in the converter inner loop control, respectively. , These are the angular frequencies of the power frequency periodic component and the harmonic component, respectively. , These are the initial phases of the power frequency periodic component and the harmonic component, respectively.

[0061] Step 2: Use the RT-LAB hardware-in-the-loop simulation platform to build a grid-connected system for a new energy doubly-fed induction generator (DFIG) and obtain the short-circuit current of the unit under different fault scenarios; for example... Figure 3As shown, the grid-connected system includes the operation control strategy of the new energy doubly-fed induction generator unit. At the same time, the control system of the generator-side and grid-side converter devices used on site of the new energy doubly-fed induction generator unit is introduced to form a semi-physical simulation system that considers the converter control characteristics used on site of the doubly-fed induction generator.

[0062] Step 3: Use the hardware-in-the-loop simulation system from Step 2 to conduct simulation tests on the simplified short-circuit current model of the new energy doubly-fed generator unit from Step 1.

[0063] For new energy doubly-fed generator units, under fault scenarios with different voltage sags, the short-circuit current during the Crowbar commissioning phase and the RSC reactive power control phase is analyzed. , Sampling is performed; within a short data window of 5~10ms, the attenuation terms of the DC component and the speed difference component are selected. , , , Simplified to , , , ; , Taking 1; the simplified expressions for the short-circuit current during the Crowbar activation stage and the RSC reactive power control stage are obtained as follows:

[0064] (5)

[0065] (6).

[0066] Step 4: Use the least squares method to fit the short-circuit current data during the low-voltage ride-through stage, obtain the key parameters of the short-circuit current model, and thus form the calculation model of the short-circuit current during the Crowbar commissioning stage and the RSC reactive power control stage, and finally form the calculation model of the practical short-circuit current of the new energy doubly-fed generator unit.

[0067] For the short-circuit current during the Crowbar input phase Sample values , … The following relationship is satisfied, where , The sampling period is Pick ;

[0068] (7)

[0069] Equation (7) can be expressed in matrix form. ,in:

[0070] (8)

[0071] (9)

[0072] (10)

[0073] In the formula, The matrix contains 8 parameters to be determined. The coefficient matrix was obtained offline using simulation test sample values; It is a constant matrix, derived from the corresponding Crowbar input stage short-circuit current. The sampled values ​​are combined with data redundancy methods to expand the matrix. The size of the matrix can be Seeking, It is a generalized inverse matrix; similarly, the short-circuit current in the reactive power control stage of RSC can be obtained by the above calculation method. The relevant parameter calculation values.

[0074] This invention proposes a method for calculating the short-circuit current of new energy doubly-fed generator units based on hardware-in-the-loop (HIL) simulation test data. It fully considers factors such as actual converter control strategies and constructs simplified short-circuit current models applicable to different fault scenarios. By establishing mathematical models of the short-circuit current of different types of new energy generator units during faults, the impact of low-voltage ride-through (LVRT) on the short-circuit process of different types of new energy generator units is analyzed, resulting in a phased mathematical model of the fault process and short-circuit current of grid-connected new energy generator units under different grid short-circuit conditions. Using the RT-LAB hardware-in-the-loop simulation system connected to the converter control system of the new energy generator units, different fault scenarios are simulated. Key parameters for calculating the short-circuit current of different types of new energy generator units at different fault stages are fitted using test data, ultimately achieving simplified calculation of the short-circuit current of the new energy generator units. By simplifying the model, the computational load is reduced, and the processing speed and accuracy of the model are improved, providing a direct basis for further analysis of the impact of low-voltage ride-through characteristics of new energy generator units on protection performance and optimized configuration.

Claims

1. A method for calculating the short-circuit current of a new energy doubly-fed induction generator unit during low-voltage ride-through, characterized in that, The specific steps are as follows: Step 1: Consider the control strategy of the doubly fed wind turbine and establish a simplified model of the short-circuit current. Step 2: Use the RT-LAB hardware-in-the-loop simulation platform to build a grid-connected system for a new energy doubly-fed generator unit and obtain the short-circuit current of the unit under different fault scenarios; Step 3: Use the hardware-in-the-loop simulation system from Step 2 to conduct simulation tests on the simplified short-circuit current model of the new energy doubly-fed generator unit from Step 1. Step 4: Use the least squares method to fit the short-circuit current data during the low-voltage ride-through stage, obtain the key parameters of the short-circuit current model, and thus form the calculation model of the short-circuit current during the Crowbar commissioning stage and the RSC reactive power control stage, and finally form the calculation model of the practical short-circuit current of the new energy doubly-fed generator unit. Step 1 is implemented in the following steps: Step 1.1: The wind power absorbed by the new energy doubly-fed generator is converted into electrical power by the asynchronous generator and transmitted to the grid through the converters on the stator and rotor sides. When a grid fault occurs, the converter control system of the doubly-fed generator gives a trigger signal to the Crowbar circuit of the generator-side converter, puts the Crowbar resistor in the rotor winding and locks the reactive power control RSC of the generator-side converter; at this time, the transient inrush current generated in the rotor winding will flow through the Crowbar resistor. Step 1.2: When the rotor current is lower than the threshold, the Crowbar circuit is disconnected and the RSC is restarted. The RSC resumes control of the rotor winding. The DFIG outputs reactive power during the reactive power control phase of the RSC, which will raise its own terminal voltage and cause the output current to change. After avoiding the rotor inrush current, the Crowbar is disconnected and the reactive power control of the rotor converter is restarted. Step 1.1 specifically involves: based on the principle of flux conservation, in order to keep the stator flux unchanged at the moment of the fault, the stator flux after the fault consists of two parts: one part is the steady-state part of the stator flux; the other part is the transient flux induced in the stator core by the voltage drop, and the transient flux decays with the stator time decay constant. In a two-phase synchronous rotating coordinate system, the time-domain analytical expression of the rotor flux linkage of a doubly-fed generator is: （1） In the formula, , , The rotor flux linkage coefficient; Substituting into the stator short-circuit current equation, the expression for the stator short-circuit current after Crowbar protection operation, in a two-phase synchronous rotating coordinate system, is: （2） In the formula: , , These represent the amplitudes of the power frequency cycle component, DC component, and speed difference frequency cycle component input to the Crowbar, respectively. The short-circuit current expression in the three-phase stationary coordinate system is: （3） in, , , These represent the amplitudes of the DC component, power frequency cycle component, and speed difference frequency cycle component input to the Crowbar, respectively. , , These are the attenuation factors for the DC component, the power frequency periodic component, and the speed difference frequency periodic component, respectively. , These are the angular frequencies of the power frequency periodic component and the speed difference frequency periodic component, respectively. , These are the initial phases of the power frequency periodic component and the speed difference frequency periodic component, respectively.

2. The method for calculating the short-circuit current of a new energy doubly-fed generator unit during low-voltage ride-through according to claim 1, characterized in that, Step 1.2 specifically involves the following: According to the law of conservation of flux linkage, the stator flux linkage does not change abruptly at the moment of the fault. The short-circuit current during the RSC reactive power control stage is: （4） In the formula, , , These are the times when the Crowbar is cut off and the converter reactive power control is started ( ). The amplitudes of the DC component, the power frequency cycle component, and the main harmonic components parasitic in the converter inner loop control at that time. , , These are the attenuation factors for the DC component, the power frequency periodic component, and the main harmonic components parasitic in the converter inner loop control, respectively. , These are the angular frequencies of the power frequency periodic component and the harmonic component, respectively. , These are the initial phases of the power frequency periodic component and the harmonic component, respectively.

3. The method for calculating the short-circuit current of a new energy doubly-fed generator unit during low-voltage ride-through according to claim 2, characterized in that, Step 3 specifically involves: analyzing the short-circuit current during the Crowbar activation phase and the RSC reactive power control phase of the new energy doubly-fed generator unit under fault scenarios with different voltage sags. , Sampling is performed; within a short data window of 5~10ms, the attenuation terms of the DC component and the speed difference component are selected. , , , Simplified to , , , ; , Taking 1; the simplified expressions for the short-circuit current during the Crowbar activation stage and the RSC reactive power control stage are obtained as follows: （5） （6）。 4. The method for calculating the short-circuit current of a new energy doubly-fed generator unit during low-voltage ride-through according to claim 3, characterized in that, Step 4 specifically involves: addressing the short-circuit current during the Crowbar activation phase. Sample values , … The following relationship is satisfied, where , , The sampling period is Pick ; （7） Equation (7) can be expressed in matrix form. ,in: （8） （9） （10） In the formula, The matrix contains 8 parameters to be determined. The coefficient matrix was obtained offline using simulation test sample values; It is a constant matrix, derived from the corresponding Crowbar input stage short-circuit current. The sampled values ​​are combined with data redundancy methods to expand the matrix. The size of the matrix Depend on Seeking, It is a generalized inverse matrix; similarly, the short-circuit current in the reactive power control stage of RSC can be obtained by the above calculation method. The relevant parameter calculation values.

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