Doubly-fed pumped storage low-voltage ride through method based on dynamic demagnetization coefficient

Abstract

The invention relates to the technical field of pumped storage, and particularly discloses a double-fed pumped storage low-voltage ride-through method based on a dynamic demagnetization coefficient, and the method comprises the following steps: detecting whether a power grid has a low-voltage fault or not; when a low-voltage fault is detected, determining a demagnetization coefficient initial value according to a stator flux linkage transient component; based on the attenuation characteristic of the transient component of the stator flux linkage, a demagnetization coefficient dynamically changing along with time is calculated; generating a rotor current reference value according to the dynamic demagnetization coefficient; according to the method, the dynamic distribution strategy of the capacity of the rotor side converter during the fault period is optimized, the demagnetization coefficient is calculated and adjusted in real time based on the transient component of the stator flux linkage through dynamic demagnetization coefficient control, the generated dynamic demagnetization current can effectively inhibit the overcurrent and overvoltage of the rotor, and the over-current and over-voltage of the rotor can be effectively inhibited along with the attenuation of the transient component. And the capacity is gradually released to provide timely reactive power support for the power grid, so that the safety of the converter is protected, and the power grid voltage recovery is assisted.

Description

Technical Field

[0001] This invention belongs to the field of pumped storage technology, specifically relating to a doubly-fed pumped storage low-voltage ride-through method based on dynamic demagnetization coefficient. Background Technology

[0002] Variable-speed pumped storage can operate efficiently over a wider head range and achieve millisecond-level power regulation, but its low-voltage ride-through (LVRT) capability during grid faults still faces challenges, becoming a key factor restricting its application. Existing LVRT control methods mainly fall into two categories: one based on hardware protection technology, and the other based on improved control strategies. However, the relationship between flux decay rate and power support in LVRT control is rarely addressed.

[0003] A search revealed that CN110768296A discloses a system and control method for improving the low-voltage ride-through capability of a doubly-fed pumped storage unit. The system includes a rotor-side controller that receives detection signals from a grid voltage detector. When the grid voltage drops, the rotor-side controller employs a stator flux tracking control strategy; when the grid voltage recovers, the rotor-side controller employs a vector control strategy. A grid-side controller also receives detection signals from the grid voltage detector. When the grid voltage drops, the grid-side controller controls the grid-side converter to operate in fault converter mode; when the grid voltage recovers, the grid-side controller operates in normal converter mode.

[0004] Existing low-voltage ride-through technologies have several limitations in hardware protection and control strategies. For example, Crowbar circuits can lead to converter blockage and system runaway, and cannot provide reactive power support during faults, affecting grid stability. Dynamic series resistance increases system complexity, may introduce additional energy losses, and its response speed is limited by hardware. Series grid-side converters have high hardware costs, complex topologies, and are difficult to maintain. Stator current feedback control relies on accurate current detection, requires high sensor accuracy, and may not effectively suppress rotor overcurrent during deep voltage dips. Virtual resistance technology has complex parameter design, may affect the system's dynamic response performance, and is sensitive to fault types.

[0005] In summary, for large-scale doubly-fed pumped storage units during low-voltage grid faults, existing low-voltage ride-through technologies suffer from contradictions between rotor overcurrent and overvoltage and reactive power support, slow response speed, low capacity utilization of rotor-side converters, and complex hardware with poor adaptability. Summary of the Invention

[0006] The purpose of this invention is to provide a doubly fed pumped storage low voltage ride-through method based on dynamic demagnetization coefficient, in order to solve the problems of rotor overcurrent and overvoltage and reactive power support in the existing low voltage ride-through technology mentioned in the background art, as well as slow response speed, low capacity utilization of rotor-side converter, and complex hardware with poor adaptability.

[0007] To achieve the above objectives, the present invention provides the following technical solution: The low-voltage ride-through method for doubly-fed pumped storage based on dynamic demagnetization coefficient includes the following steps: Detect whether a low voltage fault has occurred in the power grid; When a low voltage fault is detected, the initial value of the demagnetizing coefficient is determined based on the transient component of the stator flux linkage. Based on the attenuation characteristics of the transient component of the stator flux linkage, the demagnetization coefficient that changes dynamically with time is calculated. The rotor current reference value is generated based on the dynamic demagnetization coefficient; The output of the rotor-side converter is controlled based on the rotor current reference value. Once the grid voltage recovers, the control strategy based on the dynamic demagnetization coefficient is discontinued, and low-voltage ride-through is completed.

[0008] In one embodiment, the demagnetizing coefficient that dynamically changes over time is calculated based on the attenuation characteristics of the transient component of the stator flux linkage, specifically including: The dynamic demagnetization coefficient decays exponentially, and its expression is as follows:

[0009] in, k for t The initial value at time 0. α The attenuation coefficient is... t This is the time elapsed since the fault occurred.

[0010] In one embodiment, the attenuation coefficient α The attenuation coefficient is consistent with that of the stator flux linkage, satisfying the requirement. α = L s / R s ,in R s For stator resistance, L s It is the stator inductance.

[0011] Preferably, the process of determining the initial value of the demagnetization coefficient includes: Calculate the maximum value of the initial value of the demagnetizing coefficient based on the maximum allowable current of the rotor-side converter. k max ; Calculate the minimum initial value of the demagnetizing coefficient based on the maximum allowable voltage of the rotor-side converter. k min ; Select the maximum value k max As the initial value of the demagnetization coefficient.

[0012] Preferably, the maximum value of the initial value of the demagnetization coefficient. k max Calculated using the following formula:

[0013] in, I rmax This is the maximum allowable current of the rotor-side converter. L sσ For stator leakage, L rσ For rotor leakage inductance, ψ sn0 This represents the amplitude of the transient component of the stator flux linkage at the initial stage of a power grid fault.

[0014] In one embodiment, the minimum value of the initial value of the demagnetization coefficient k min Calculated using the following formula:

[0015] in, L m For mutual inductance between stator and rotor, L s For stator inductance, ψ sn0 This represents the amplitude of the transient component of the stator flux linkage at the initial stage of a power grid fault. ω r The rotor angular frequency, U rmax This is the maximum allowable voltage of the rotor-side converter. s For slippage, U sp This represents the amplitude of the positive sequence component of the stator voltage.

[0016] In one embodiment, generating a rotor current reference value based on the dynamic demagnetization coefficient specifically includes: The reference value of the demagnetizing current is calculated by multiplying the dynamic demagnetizing coefficient by the transient component of the stator flux linkage. The demagnetizing current reference value is superimposed with the rotor current reference value under normal vector control to generate the final rotor current reference value.

[0017] In one embodiment, exiting the control strategy based on the dynamic demagnetization coefficient specifically includes: When the grid voltage is detected to have returned to the normal range, the dynamic demagnetization coefficient will be gradually reduced to zero within a preset transition time. Smoothly increase the weights of normal vector control until completely switching back to the traditional vector control strategy.

[0018] In a preferred embodiment, the transient component of the stator flux linkage is obtained by a stator flux linkage observer. The stator flux linkage observer calculates the total stator flux linkage based on the measured stator voltage and stator current, and extracts its transient component by high-pass filtering or by subtracting the steady-state component.

[0019] In one embodiment, detecting whether a low-voltage fault has occurred in the power grid specifically includes: Real-time monitoring of the three-phase voltage at the grid connection point, and calculation of its positive sequence voltage amplitude; If the positive sequence voltage amplitude is lower than the preset voltage threshold and this state continues for more than the preset delay, a low voltage fault is determined to have occurred.

[0020] The beneficial effects of this invention are: This invention provides a low-voltage ride-through method for doubly-fed pumped storage based on a dynamic demagnetizing coefficient, comprising the following steps: detecting whether a low-voltage fault has occurred in the power grid; when a low-voltage fault is detected, determining an initial value of the demagnetizing coefficient based on the transient component of the stator flux linkage; calculating the demagnetizing coefficient that dynamically changes over time based on the attenuation characteristics of the transient component of the stator flux linkage; generating a rotor current reference value based on the dynamic demagnetizing coefficient; controlling the output of the rotor-side converter based on the rotor current reference value; and exiting the control strategy based on the dynamic demagnetizing coefficient after the grid voltage recovers, thus completing the low-voltage ride-through. During a doubly-fed induction motor fault, the attenuation of the stator flux linkage is accelerated, shortening the time required for the rotor-side converter to provide reactive power support. Under the control strategy, the demagnetizing coefficient attenuates following the attenuation coefficient of the transient component of the stator flux linkage, reducing both the rotor voltage and the rotor current after the fault to within the allowable range of the generator-side converter, thereby maintaining the controllability of the system during transient processes. Attached Figure Description

[0021] Figure 1 This is a block diagram of the low voltage ride-through control of the present invention.

[0022] Figure 2 This is the equivalent circuit diagram of the doubly fed motor of the present invention.

[0023] Figure 3 This is a circuit diagram of the doubly fed motor control circuit of the present invention.

[0024] Figure 4 Rotor current diagrams obtained by using the control method proposed in this invention and by not using the control method proposed in this invention.

[0025] Figure 5Rotor voltage diagrams obtained by using the control method proposed in this invention and by not using the control method proposed in this invention.

[0026] Figure 6 The stator voltage diagrams are shown for obtaining a control method using the present invention and a control method not using the present invention.

[0027] Figure 7 The present invention provides control methods for obtaining DC bus voltage diagrams, as well as those obtained without using the control methods of the present invention.

[0028] Figure 8 The stator reactive power diagrams are obtained by using the control method proposed in this invention and by not using the control method proposed in this invention.

[0029] Figure 9 The stator active power diagrams are obtained by using the control method proposed in this invention and by not using the control method proposed in this invention. Detailed Implementation

[0030] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0031] Example 1: The low voltage ride-through control method based on dynamic demagnetization coefficient proposed in this example accelerates the attenuation of stator flux linkage during a doubly fed induction motor fault, shortening the time required for the rotor-side converter to provide reactive power support. Under this control strategy, the demagnetization coefficient decreases along with the attenuation coefficient of the transient component of the stator flux linkage, reducing both the rotor voltage and the rotor current after the fault to within the allowable range of the rotor-side converter, thus maintaining the controllability of the system during the transient process.

[0032] like Figure 2 As shown, the low voltage ride-through control method based on dynamic demagnetization coefficient in this embodiment is based on the accurate transient mathematical model of the doubly fed induction motor. When the power grid fails, the electromagnetic transient process of the doubly fed induction motor is described by the following equation.

[0033] The transient equations of the doubly fed generator under grid fault conditions are expressed by equations (1) to (4):

[0034]

[0035]

[0036]

[0037] in, u , i and ψ These represent vectors for voltage, current, and magnetic flux, respectively. R and L Representing resistance and inductance respectively; subscript " s "and" r "" represents the stator and rotor variables respectively; superscript " s "and" r "Indicates the stator and rotor reference frames; L m It's mutual induction.

[0038] The rotor voltage equation can be derived from equations (1) to (4), as shown in equation (5):

[0039] in, σ Representing the leakage flux coefficient, it can be seen from equation (5) that the rotor voltage consists of two parts: one part is the electromotive force induced by the stator flux linkage, and the other part is the rotor resistance. R r and transient inductance σL r Voltage drop U RL .

[0040] Under normal conditions, the induced electromotive force can be expressed by equation (6):

[0041] in, U SN Indicates the rated stator voltage. ω s Let represent the stator angular frequency and s represent the slip; according to formula (6), the amplitude of the induced electromotive force under normal conditions can be obtained as follows: sU SN L m / L S And it is proportional to “s”; since the slip s is usually between -0.3 and 0.3, the induced electromotive force will not exceed 30% of the rated stator voltage.

[0042] When a symmetrical fault occurs, the stator flux contains steady-state and transient components, as shown in equation (7):

[0043] Where h represents the voltage sag depth of the power grid. τ S =LS / R S This represents the time constant of the stator flux linkage.

[0044] Combining equations (5) and (7), the induced electromotive force under a symmetrical power grid fault can be derived as equation (8):

[0045] ω r Represents the rotor angular frequency; 1 / τ is ignored. S Equation (8) can be simplified to Equation (9):

[0046] As shown in equation (9), the first term is the positive-sequence component of the induced electromotive force (EMF), and the second term is the transient component of the EMF. The transient component of the EMF is proportional to "1-s". The initial amplitude of the induced EMF is usually relatively large, which is the root cause of rotor-side overcurrent and overvoltage. For example, under the full voltage drop condition with slip s=−0.3, the initial amplitude of the induced EMF is 1.3. USNL m / L S It is 4.3 times its normal value.

[0047] To address the aforementioned issues, this embodiment introduces a dynamic demagnetizing coefficient to generate the optimal rotor demagnetizing current. The basic principle is that the transient component of the stator flux linkage is canceled out by injecting an additional rotor current containing a corresponding antiphase component.

[0048] The rotor flux linkage equation is shown in equation (10):

[0049] This method of offsetting by injecting an additional rotor current containing a corresponding anti-phase component can significantly weaken the influence of transient flux components on the rotor, retaining only the unchanged positive-sequence stator flux components and alleviating the saturation phenomenon of the rotor-side converter.

[0050] The rotor current required to eliminate the transient component of the stator flux linkage is expressed as equation (11):

[0051] Since converter saturation can be avoided without completely eliminating transient components, a demagnetizing coefficient is introduced for partial cancellation. The required demagnetizing current is expressed by equation (12):

[0052] Where k represents the demagnetization coefficient.

[0053] The demagnetizing coefficient k is not a fixed value, but a variable that decays dynamically over time, as shown in equation (13):

[0054] Equation (13) represents the dynamic demagnetization coefficient, and k is the initial value at time t=0. α t is the attenuation coefficient, and t is the time from the occurrence of the fault.

[0055] The purpose of dynamic demagnetization is to: when the transient component is at its maximum in the early stage of the fault, use the maximum k value to strongly suppress the overcurrent; as the transient component decays, the k value decreases, and the converter capacity is gradually released to provide reactive power support.

[0056] The rotor current reference value obtained thus is expressed by equation (14):

[0057] Furthermore, based on equations (9) and (12), the expression for the rotor voltage (15) can be obtained:

[0058] To protect the converter, the rotor current after a fault should not exceed the maximum permissible rotor current during the entire grid fault period; the initial value of the demagnetizing coefficient k must simultaneously satisfy both the rotor current constraint and the rotor voltage constraint; since the stator flux transient component is the largest at the beginning of the fault, k at the beginning of the fault should satisfy the requirements of equation (16):

[0059] in, I rmax This indicates the maximum permissible rotor current, typically 2.0 pu, which is the maximum value of the IGBT pulse current.

[0060] To achieve controllability of the doubly-fed motor system, the required rotor voltage should not exceed the maximum output voltage of the converter; therefore, the initial time k should also satisfy equation (17):

[0061] in, U rmax This indicates the maximum output voltage of the converter, which is determined by the value of the DC bus voltage.

[0062] By combining equations (16) and (17), the allowable range of values ​​for the demagnetization coefficient k can be determined.

[0063] Equations (18) and (19) respectively give the calculation methods for the maximum and minimum values ​​of the demagnetization coefficient k:

[0064]

[0065] in, ψ sn0 It is the amplitude of the transient component of the stator flux linkage at the initial stage of a power grid fault; L sσ For stator leakage, L rσ For rotor leakage inductance; L m For mutual inductance between stator and rotor, L s For stator inductance, ω r The rotor angular frequency, U rmax This indicates the maximum output voltage of the converter. s For slippage, U sp This represents the amplitude of the positive sequence component of the stator voltage.

[0066] Since the demagnetizing coefficient tracks the attenuation of the transient component of the stator flux linkage, due to the attenuation coefficient... α The attenuation coefficient is consistent with that of the stator flux linkage, satisfying the requirement. α=L S / R S ,in R S Since the stator resistance is the mains resistance and the rotor current needs to be prioritized, the maximum value of the demagnetizing coefficient k should be taken.

[0067] Example 2, as Figure 1 As shown, the low-voltage ride-through method for doubly-fed pumped storage based on dynamic demagnetization coefficient includes the following steps: S1, detects whether a low voltage fault has occurred in the power grid; S2, when a low voltage fault is detected, the initial value of the demagnetization coefficient is determined based on the transient component of the stator flux linkage; S3, based on the attenuation characteristics of the transient component of the stator flux linkage, calculate the demagnetization coefficient that changes dynamically with time; S4, generate a rotor current reference value based on the dynamic demagnetization coefficient; S5 controls the output of the rotor-side converter based on the rotor current reference value; S6, once the grid voltage recovers, exit the control strategy based on the dynamic demagnetization coefficient and complete the low voltage ride-through.

[0068] Specifically, in S1, detecting whether a low-voltage fault has occurred in the power grid includes the following steps: S101, real-time monitoring of the three-phase voltage at the grid connection point, and calculation of its positive sequence voltage amplitude; S102, if the positive sequence voltage amplitude is lower than the preset voltage threshold and this state continues for more than the preset delay, then a low voltage fault is determined to have occurred.

[0069] Specifically, in S2, the transient component of the stator flux linkage is obtained through the stator flux linkage observer. The stator flux linkage observer calculates the total stator flux linkage based on the measured stator voltage and stator current, and extracts its transient component by high-pass filtering or by subtracting the steady-state component.

[0070] Let's take the most commonly used voltage model method as an example for explanation: The stator flux transient component is obtained as follows: the three-phase voltage and three-phase current at the generator terminals are measured by sensors, and the voltage and current components in the two-phase stationary coordinate system are obtained by Clark transformation; the voltage component is subtracted from the stator resistance voltage drop and then integrated to obtain the total stator flux component; subsequently, the required stator flux transient DC component can be separated by passing it through a high-pass filter with a low cutoff frequency, or by subtracting its power frequency sinusoidal steady-state component, for subsequent calculations.

[0071] Specifically, in S2, the process of determining the initial value of the demagnetization coefficient includes: S201, Calculate the maximum value of the initial value of the demagnetizing coefficient based on the maximum allowable current of the rotor-side converter. k max ; S202, based on the maximum allowable voltage of the rotor-side converter, calculate the minimum initial value of the demagnetizing coefficient. k min ; S203, Select the maximum value k max As the initial value of the demagnetization coefficient.

[0072] In the above technical solution, the maximum value of the initial value of the demagnetization coefficient k max The minimum value of the initial value of the demagnetization coefficient is expressed by equation (18). k min This is expressed by equation (19).

[0073] Specifically, in S3, based on the attenuation characteristics of the transient component of the stator flux linkage, the demagnetizing coefficient that dynamically changes with time is calculated, including: The dynamic demagnetization coefficient decays exponentially, and the expression for the dynamic demagnetization coefficient is given by equation (14).

[0074] Specifically, in S4, the rotor current reference value is generated based on the dynamic demagnetization coefficient, which includes the following steps: S401, by multiplying the dynamic demagnetizing coefficient by the transient component of the stator flux linkage, the reference value of the demagnetizing current is calculated; S402 superimposes the demagnetizing current reference value with the rotor current reference value under normal vector control to generate the final rotor current reference value.

[0075] Specifically, in S5, the output of the rotor-side converter is controlled based on the rotor current reference value, which includes the following steps: S501 compares the rotor current reference value with the actual rotor current to obtain the current error signal; S502 adjusts the current error signal through a proportional-integral controller to obtain the rotor voltage reference value; S503 performs pulse width modulation on the rotor voltage reference value to generate a drive signal to control the operation of the power switch in the rotor-side converter and output the corresponding rotor voltage.

[0076] Specifically, in S6, exiting the control strategy based on the dynamic demagnetization coefficient includes the following steps: S601, when the mains voltage is detected to return to the normal range, the dynamic demagnetization coefficient is gradually reduced to zero within a preset transition time; S602 smoothly increases the weight of normal vector control until it completely switches back to the traditional vector control strategy.

[0077] Example 3, as Figure 3 As shown, to verify the feasibility of the above theory, a 1.5MW doubly-fed pumped storage simulation platform was built using MATLAB and Simulink simulation platforms.

[0078] Initially, the DFIM operates at its rated voltage, outputting active power. Ps The reactive power output is 0.7 pu. Qs With a slip of 0 pu, the slip of DFIM is -0.2; At t=0.2 seconds, a symmetrical fault with 80% depth occurred, and the grid fault was cleared at t=1.2 seconds; once the grid fault is detected, LVRT control will be enabled until... t Traditional vector control (VC) is reactivated at 1.4 seconds. Based on the rated DC bus voltage, the maximum output voltage of RSC U rmax Approximately 0.49 pu; meanwhile, according to simulation parameters, ψ sn0 The estimated value is 1.0 pu.

[0079] According to the current constraint formula (16) and the voltage constraint formula (18), the range of k can be calculated as 0.442≤ k≤0.654 pu. Taking the rotor current constraint condition first, k=0.654 is chosen.

[0080] Reference Figure 4 The left side represents the rotor current diagram obtained without the control method of the present invention, and the right side represents the rotor current diagram obtained with the control method of the present invention. Reference Figure 5 The left side represents the rotor voltage diagram obtained without the control method of the present invention, and the right side represents the rotor voltage diagram obtained using the control method proposed in the present invention. Reference Figure 6 The left side represents the stator voltage diagram obtained without the control method of the present invention, and the right side represents the stator voltage diagram obtained using the control method proposed in the present invention. Reference Figure 7 The left side represents the DC bus voltage diagram obtained without the control method of this invention, and the right side represents the DC bus voltage diagram obtained using the control method proposed in this invention. Reference Figure 8 The left side represents the stator reactive power diagram obtained without using the control method of this invention, and the right side represents the stator reactive power diagram obtained using the control method proposed in this invention.

[0081] Reference Figure 9 The left side represents the stator active power diagram obtained without the control method of the present invention, and the right side represents the stator active power diagram obtained using the control method proposed in the present invention. exist Figures 4-9 In the graph, the horizontal axis represents time (s), and the vertical axis represents per-unit values; through... Figures 4-9 Comparative analysis shows that, to avoid converter overheating and damage due to overload, the RSC is considered to have power support capability when its capacity margin exceeds the rated value. The dynamic demagnetizing coefficient (LVRT) strategy exhibits superior performance compared to the traditional fixed demagnetizing coefficient strategy. With the fixed demagnetizing coefficient strategy, the RSC current requires 55ms to drop below 1.0 (pu) and achieve reactive power support capability, while the dynamic demagnetizing coefficient strategy achieves the same effect in only 35ms, significantly improving response speed. Simultaneously, the dynamic demagnetizing coefficient strategy performs better in maintaining DC bus voltage stability, releasing RSC capacity more quickly to provide timely power support for the system and ensure stable DC bus voltage operation. The comparative results fully demonstrate the advantages of the dynamic demagnetizing coefficient strategy in improving system dynamic response performance and operational stability.

[0082] Example 4: Based on the doubly-fed pumped storage low-voltage ride-through method based on dynamic demagnetization coefficient proposed in Example 2, this example proposes a doubly-fed pumped storage low-voltage ride-through control system based on dynamic demagnetization coefficient, specifically including: Signal detection and processing module: When a low voltage fault occurs in the power grid, the stator three-phase voltage and stator three-phase current are first collected through voltage sensors and current sensors; Stator flux linkage observer: Based on the detected stator three-phase voltage and stator three-phase current signals, the total stator flux linkage is calculated in real time using the model described in equation (7) or other flux linkage observation algorithms, and the amplitude and phase information of the transient component of the stator flux linkage are extracted from it; Dynamic demagnetization coefficient calculation module: Receives stator flux transient components, converter capacity parameters, and time signals. Specific steps include: First, calculate the allowable range of the demagnetization coefficient according to equations (17) and (19); Then, the maximum value within this range is selected as the initial value; Finally, the dynamic demagnetization coefficient at the current moment is calculated in real time according to Equation (13). This calculation can be efficiently implemented in the DSP through a recursive algorithm. Rotor current reference value calculation module: receives dynamic demagnetization coefficient and stator flux transient component; calculates demagnetization current reference value according to formula (12), and superimposes the demagnetization current reference value with the original rotor current reference value generated by normal vector control (VC) module to generate the final rotor current comprehensive reference value; Current control and PWM generation module: The current closed-loop controller of the rotor-side converter (RSC) receives the above comprehensive reference value and the actual feedback rotor current, generates a rotor voltage reference value by adjustment, and then generates a pulse signal to drive the switching transistor through PWM modulation, and finally controls the converter to output the required rotor voltage, as shown in equation (15). Mode Management Module: The control system continuously monitors the grid voltage. When the fault is cleared and the voltage recovers to the preset threshold, the mode management module will control the value in the dynamic demagnetization coefficient calculation module to smoothly and gradually decrease to zero within tens of milliseconds, so that the rotor current reference value is generated entirely by the normal vector control module, realizing a shockless switch to the normal operation mode.

[0083] Through the coordinated work of the above modules, the goal of limiting rotor current and voltage within a safe range during a fault and providing rapid reactive power support is achieved.

[0084] This invention optimizes the dynamic allocation strategy of rotor-side converter capacity during faults. By controlling the dynamic demagnetization coefficient, the demagnetization coefficient is calculated and adjusted in real time based on the transient component of stator flux linkage. The generated dynamic demagnetizing current can effectively suppress rotor overcurrent and overvoltage. As the transient component decays, the capacity is gradually released to provide timely reactive power support to the grid, which protects the converter safety and helps the grid voltage recover.

[0085] By adopting a dynamic control strategy, the rotor current can be reduced to a safe level more quickly, enabling the rotor-side converter to provide reactive power support ahead of schedule and improving the overall dynamic response speed.

[0086] By accurately calculating the demagnetizing coefficient using current and voltage constraints, the available capacity of the rotor-side converter is maximized, enabling it to be used simultaneously for transient overcurrent, overvoltage suppression, and reactive power support, thus avoiding the limitations of idle capacity or single-purpose applications.

[0087] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A low-voltage ride-through method for doubly-fed pumped storage based on dynamic demagnetization coefficient, characterized in that, Includes the following steps: Detect whether a low voltage fault has occurred in the power grid; When a low voltage fault is detected, the initial value of the demagnetizing coefficient is determined based on the transient component of the stator flux linkage. Based on the attenuation characteristics of the transient component of the stator flux linkage, the demagnetization coefficient that dynamically changes with time is calculated. The rotor current reference value is generated based on the dynamic demagnetization coefficient; The output of the rotor-side converter is controlled based on the rotor current reference value. Once the grid voltage recovers, the control strategy based on the dynamic demagnetization coefficient is discontinued, and low-voltage ride-through is completed.

2. The method according to claim 1, characterized in that: Based on the attenuation characteristics of the transient component of the stator flux linkage, the demagnetization coefficient, which dynamically changes over time, is calculated, specifically including: The dynamic demagnetization coefficient decays exponentially, and its expression is as follows: in, k for t The initial value at time 0. α The attenuation coefficient is... t This is the time elapsed since the fault occurred.

3. The method according to claim 2, characterized in that: The attenuation coefficient α The attenuation coefficient is consistent with that of the stator flux linkage, satisfying the requirement. α = L s / R s ,in R s For stator resistance, L s It is the stator inductance.

4. The method according to claim 1, characterized in that: The process of determining the initial value of the demagnetization coefficient includes: Calculate the maximum value of the initial value of the demagnetizing coefficient based on the maximum allowable current of the rotor-side converter. k max ; Calculate the minimum initial value of the demagnetizing coefficient based on the maximum allowable voltage of the rotor-side converter. k min ; Select the maximum value k max As the initial value of the demagnetization coefficient.

5. The method according to claim 4, characterized in that: The maximum value of the initial value of the demagnetization coefficient k max Calculated using the following formula: in, I rmax This is the maximum allowable current of the rotor-side converter. L sσ For stator leakage, L rσ For rotor leakage inductance, ψ sn0 This represents the amplitude of the transient component of the stator flux linkage at the initial stage of a power grid fault.

6. The method according to claim 4, characterized in that: The minimum value of the initial value of the demagnetization coefficient k min Calculated using the following formula: in, L m For mutual inductance between stator and rotor, L s For stator inductance, ψ sn0 This represents the amplitude of the transient component of the stator flux linkage at the initial stage of a power grid fault. ω r The rotor angular frequency, U rmax This is the maximum allowable voltage of the rotor-side converter. s For slippage, U sp This represents the amplitude of the positive sequence component of the stator voltage.

7. The method according to claim 1, characterized in that: Based on the dynamic demagnetization coefficient, a rotor current reference value is generated, specifically including: The reference value of the demagnetizing current is calculated by multiplying the dynamic demagnetizing coefficient by the transient component of the stator flux linkage. The demagnetizing current reference value is superimposed with the rotor current reference value under normal vector control to generate the final rotor current reference value.

8. The method according to claim 1, characterized in that: Exiting the control strategy based on dynamic demagnetization coefficient specifically includes: When the grid voltage is detected to have returned to the normal range, the dynamic demagnetization coefficient will be gradually reduced to zero within a preset transition time. Smoothly increase the weight of normal vector control until it completely switches back to the traditional vector control strategy.

9. The method according to claim 1, characterized in that: The transient component of the stator flux linkage is obtained through a stator flux linkage observer. The stator flux linkage observer calculates the total stator flux linkage based on the measured stator voltage and stator current, and extracts its transient component by high-pass filtering or by subtracting the steady-state component.

10. The method according to claim 1, characterized in that: The detection of whether a low-voltage fault has occurred in the power grid specifically includes: Real-time monitoring of the three-phase voltage at the grid connection point, and calculation of its positive sequence voltage amplitude; If the positive sequence voltage amplitude is lower than the preset voltage threshold and this state continues for more than the preset delay, a low voltage fault is determined to have occurred.

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