An adaptive parameter adjustment method and system for suppressing negative sequence current under asymmetric fault

Abstract

The application provides an adaptive parameter adjustment method and system for inhibiting negative sequence current under asymmetric fault, and relates to the technical field of power electronic control, and comprises the following steps: adaptively adjusting the proportional coefficient and integral time constant of a proportional integral controller based on the voltage drop depth; setting the positive sequence reactive current instruction value, the negative sequence reactive current instruction value, the positive sequence active current instruction value and the negative sequence active current instruction value during asymmetric fault according to the effective value of the positive sequence voltage, wherein the negative sequence reactive current instruction value and the negative sequence active current instruction value are set to zero; using the adjusted proportional integral controller parameters and the current instruction values to generate the positive sequence d-axis component, the positive sequence q-axis component, the negative sequence d-axis component and the negative sequence q-axis component of the modulation wave voltage to control the new energy unit so as to inhibit the negative sequence current under asymmetric fault. The application can eliminate or greatly weaken the negative sequence current component from the control target.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of power electronic control, in particular to a self-adaptive parameter adjustment method and system for suppressing negative sequence current under asymmetric fault. BACKGROUND

[0002] When the inverter is caused to be unbalanced in three-phase output due to internal fault (such as power device damage, control logic error) or external grid asymmetry (such as single-phase ground short circuit), negative sequence component will appear in the current.

[0003] The existence of negative sequence component will cause the unbalance of inverter bridge arm current, and part of IGBT will bear higher current stress, which may cause overheating and even burnout, affect the service life of the inverter, and also interfere with the PLL and control algorithm of the inverter, causing voltage / frequency tracking error, and even causing oscillation.

[0004] The existence of negative sequence current will cause the asymmetry of three-phase voltage of the grid, affect the normal operation of sensitive equipment (such as precision instruments and medical equipment), and also may trigger negative sequence overcurrent protection, cause mis-trip (such as transformer differential protection misjudgment of internal fault), and also mask the real fault characteristics, so that the protection device is difficult to accurately identify the fault type.

[0005] Therefore, it is necessary to suppress the negative sequence current. SUMMARY

[0006] The technical problem to be solved by the present application is to provide a self-adaptive parameter adjustment system for suppressing negative sequence current under asymmetric fault, which can eliminate or greatly weaken the negative sequence current component from the control target.

[0007] To solve the above technical problems, the technical scheme of the present application is as follows:

[0008] In a first aspect, a self-adaptive parameter adjustment method for suppressing negative sequence current under asymmetric fault is provided, which comprises:

[0009] Step 1, obtaining the positive sequence voltage effective value of the grid-connected point of the new energy unit;

[0010] Step 2, determining the voltage drop depth according to the positive sequence voltage effective value;

[0011] Step 3, adaptively adjusting the proportional coefficient and integral time constant of the proportional integral controller based on the voltage drop depth, wherein the proportional coefficient increases with the increase of the voltage drop depth, and the integral time constant decreases with the increase of the voltage drop depth;

[0012] Step 4, setting positive sequence reactive current instruction value, negative sequence reactive current instruction value, positive sequence active current instruction value and negative sequence active current instruction value during asymmetric fault according to the positive sequence voltage effective value, wherein the negative sequence reactive current instruction value and the negative sequence active current instruction value are set to zero;

[0013] Step 5, generating positive sequence d-axis component, positive sequence q-axis component, negative sequence d-axis component and negative sequence q-axis component of the modulation wave voltage using the adjusted proportional integral controller parameters and the current instruction value to control the new energy unit, so as to suppress the negative sequence current under asymmetric fault.

[0014] The second aspect is an adaptive parameter adjustment system for suppressing negative sequence current under asymmetric fault, comprising:

[0015] The acquisition module is configured to acquire the positive sequence voltage effective value of the new energy unit and the grid-connected point.

[0016] The determination module is configured to determine the voltage drop depth according to the positive sequence voltage effective value.

[0017] The adjustment module is configured to adaptively adjust the proportional coefficient and the integral time constant of the proportional integral controller based on the voltage drop depth, wherein the proportional coefficient increases with the increase of the voltage drop depth, and the integral time constant decreases with the increase of the voltage drop depth.

[0018] The processing module is configured to set positive sequence reactive current instruction value, negative sequence reactive current instruction value, positive sequence active current instruction value and negative sequence active current instruction value during asymmetric fault according to the positive sequence voltage effective value, wherein the negative sequence reactive current instruction value and the negative sequence active current instruction value are set to zero.

[0019] The generation module is configured to generate positive sequence d-axis component, positive sequence q-axis component, negative sequence d-axis component and negative sequence q-axis component of the modulation wave voltage using the adjusted proportional integral controller parameters and the current instruction value to control the new energy unit, so as to suppress the negative sequence current under asymmetric fault.

[0020] The above-mentioned scheme of the present application at least includes the following beneficial effects:

[0021] By setting the negative sequence reactive current instruction value and the negative sequence active current instruction value to zero and combining the adaptive PI controller for accurate tracking, the negative sequence current component can be eliminated or greatly weakened from the control target, thereby solving the problems of uneven equipment current stress, overheating damage and interference to the power grid and protection system caused by negative sequence current.

[0022] By monitoring the positive sequence voltage effective value in real time and determining the voltage drop depth, the parameters (proportion coefficient increases and integral time constant decreases) of the PI controller are dynamically adjusted, so that the control system can automatically adapt to different severity of asymmetric fault conditions. This adaptive mechanism ensures that the controller has faster response speed and stronger adjustment ability in deep voltage drop, effectively avoids the problems of slow response and oscillation instability caused by fixed controller parameters, and enhances the operation stability of new energy units during fault ride-through.

[0023] According to the positive sequence voltage effective value, the instruction value of the positive sequence active and reactive current is reasonably set, so that necessary positive sequence power support can be actively provided according to the grid voltage state while suppressing the negative sequence current; combined with the generated accurate positive and negative sequence d-q axis modulation wave voltage components, the multi-degree-of-freedom and decoupled accurate control of the unit output current is realized, so that good grid current quality and control performance can be maintained under complex asymmetric fault conditions. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The figure is a flowchart of the adaptive parameter adjustment method for suppressing negative sequence current under asymmetric fault of the present application.

[0025] Figure 2 The figure is a schematic diagram of the adaptive parameter adjustment system for suppressing negative sequence current under asymmetric fault of the present application. DETAILED DESCRIPTION

[0026] Exemplary embodiments of the present disclosure will be described in greater detail below with reference to the accompanying drawings. Although exemplary embodiments of the present disclosure are shown in the drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. Rather, these embodiments are provided so that the present disclosure can be more thoroughly understood and the scope of the present disclosure can be accurately conveyed to those skilled in the art.

[0027] As Figure 1 shown, the embodiments of the present application propose an adaptive parameter adjustment method for suppressing negative sequence current under asymmetric fault, comprising:

[0028] Step 1, obtaining the positive sequence voltage effective value of the new energy unit grid-connected point;

[0029] Step 2, determining the voltage drop depth according to the positive sequence voltage effective value;

[0030] Step 3, adaptively adjusting the proportion coefficient and integral time constant of the proportional integral controller based on the voltage drop depth, wherein the proportion coefficient increases with the increase of the voltage drop depth, and the integral time constant decreases with the increase of the voltage drop depth;

[0031] Step 4, setting positive sequence reactive current instruction value, negative sequence reactive current instruction value, positive sequence active current instruction value and negative sequence active current instruction value during asymmetric fault according to the positive sequence voltage effective value, wherein the negative sequence reactive current instruction value and the negative sequence active current instruction value are set to zero;

[0032] Step 5, using the adjusted proportional integral controller parameters and the current instruction value, generating positive sequence d-axis component, positive sequence q-axis component, negative sequence d-axis component and negative sequence q-axis component of the modulation wave voltage to control the new energy unit, so as to suppress the negative sequence current under asymmetric fault.

[0033] In this embodiment, by directly setting the negative sequence current instruction to zero and combining the adaptive adjustment of the controller for accurate tracking, the negative sequence current can be effectively eliminated or greatly weakened, which avoids the problems of unbalanced inverter bridge arm current, over-stress and over-heat damage of power devices caused by negative sequence current, and significantly improves the operation reliability and life of new energy power generation equipment; according to the voltage drop depth, the PI controller parameters are adjusted in real time, so that the control system can automatically adapt to different severity fault conditions, and when the voltage drops deeply, the response speed and robustness of the controller are improved by increasing the proportional coefficient and reducing the integral time constant, which effectively prevents the system from oscillating and losing stability under fault, and ensures the smooth operation and fault ride-through capability of the new energy unit under different asymmetric faults. While suppressing the negative sequence current, the positive sequence current instruction is set according to the positive sequence voltage, which ensures that the new energy unit can provide necessary active and reactive power support according to the grid state. This helps to maintain the voltage stability at the grid connection point, reduces the risk of interference and protection device misoperation of sensitive equipment in the grid, and improves the overall safe and stable operation level of the power system with high proportion of new energy access.

[0034] In a preferred embodiment of the present application, step 1, obtaining the positive sequence voltage effective value of the new energy unit grid-connected point, comprises:

[0035] Step 11, real-time monitoring of the three-phase voltage instantaneous value of the new energy unit grid-connected point, specifically including: taking the doubly-fed wind turbine as the application object, the core monitoring position is the grid-connected point (PCC) connected with the grid, and the voltage state of this node directly reflects whether the grid exists asymmetric fault. First, on the A, B, C three-phase lines at the PCC, deploy voltage collection devices adapted to the grid voltage level (such as low-voltage distribution network 380V, medium-voltage network 10kV, etc.), specifically, Hall voltage sensor or voltage sampling module, to ensure accurate capture of the instantaneous change of three-phase voltage. Secondly, to avoid the time difference of three-phase voltage collection leading to subsequent data deviation, through hardware synchronization mechanism (such as GPS-based clock synchronization or FPGA timing control module), ensure that the collection time deviation of three-phase voltage data does not exceed 1 microsecond, realize synchronous collection. Finally, pre-process the collected original voltage analog signal: filter out the common high-frequency electromagnetic interference in the grid through the RC filter circuit, then convert the analog signal to digital signal through the 16-bit and above precision analog-to-digital conversion (ADC) module, and transmit it to the storage unit of the control system of the new energy unit (such as digital signal processor DSP or programmable logic controller PLC) for temporary storage, providing real and undisturbed original data for subsequent coordinate transformation.

[0036] Step 12, coordinate transformation of the three-phase voltage instantaneous value to obtain the voltage component in the two-phase rotating coordinate system, specifically including: through twice coordinate transformation, the complex three-phase alternating voltage signal is converted into a form convenient for separating the positive sequence component. First step, perform Clark transformation (three-phase static coordinate system to two-phase static coordinate system): based on the three-phase voltage instantaneous value obtained in step 11 (satisfying the sum of three-phase voltage is zero, ignoring the zero sequence component), convert the voltage instantaneous value in the three-phase static coordinate system (abc coordinate system) to the voltage component in the two-phase static coordinate system (αβ coordinate system); the core purpose of this transformation is to eliminate the coupling relationship between the three-phase voltage, and simplify the three-dimensional voltage signal to a two-dimensional orthogonal alternating voltage signal. Second step, perform Park transformation (two-phase static coordinate system to two-phase rotating coordinate system): taking the phase of the grid voltage fundamental positive sequence component as the synchronous reference (the phase is calculated in real time by PLL phase-locked loop, ensuring complete consistency with the positive sequence voltage phase), further convert the voltage component in the two-phase static coordinate system (αβ coordinate system) to the voltage component in the two-phase rotating coordinate system (dq coordinate system). After twice transformation, the positive sequence voltage component in the dq coordinate system presents as direct current or slowly changing value, while the negative sequence voltage component presents as alternating value with 2 times grid fundamental frequency (such as 100Hz for 50Hz grid), the difference between the two is significant, creating key conditions for subsequent separation of positive sequence voltage component.

[0037] Step 13, separating the positive sequence voltage component from the voltage components in the two-phase rotating coordinate system by using a filter, specifically including: this step selects a low-pass filter to separate the positive sequence voltage component according to the characteristic difference between the positive and negative sequence voltage components in the dq coordinate system (the positive sequence is a direct current / slow changing quantity, and the negative sequence is a 2 times frequency alternating quantity). First, determine the filter type and parameters: preferentially select a second-order Butterworth low-pass filter, which has a flat passband characteristic and a steep stopband attenuation capability, can effectively filter out the negative sequence component while preserving the positive sequence component, and avoids distortion of the positive sequence component; set the cutoff frequency of the filter to 5-10 Hz (much lower than 100 Hz, which is 2 times the fundamental frequency of the power grid), to ensure that the 2 times frequency alternating component corresponding to the negative sequence voltage can be completely filtered out, and the positive sequence component transmission delay will not be caused due to too low cutoff frequency, affecting the real-time performance of subsequent calculation. Second, perform the separation operation: input the voltage components in the dq coordinate system obtained in step 12 (containing positive and negative sequence components) into the low-pass filter, the filter performs frequency screening on the input voltage components, filters out the 2 times frequency alternating component (negative sequence component), and finally outputs the pure positive sequence voltage component, which is specifically divided into d-axis component of positive sequence voltage and q-axis component of positive sequence voltage, completely excluding the interference of negative sequence voltage on the analysis of positive sequence voltage.

[0038] Step 14, calculating the effective value of the positive sequence voltage component as the positive sequence voltage effective value, specifically including: this step calculates the effective value reflecting the state of the positive sequence voltage of the power grid based on the positive sequence voltage d-axis and q-axis components separated in step 13. First, since the d-axis and q-axis components of the positive sequence voltage have been processed by low-pass filtering, the alternating current fluctuation interference has been eliminated, and only the direct current or slowly changing values are retained, so the amplitude related parameters of the positive sequence voltage can be obtained by operating the values of these two components. Second, according to the inherent relationship between the effective value and the amplitude of alternating current voltage (effective value is the amplitude divided by ), combined with the operation results of the positive sequence voltage d-axis and q-axis components, the instantaneous effective value of the positive sequence voltage is calculated. To avoid the influence of accidental fluctuations on the calculation results at a single moment, the calculation update period is set to half of the fundamental frequency period of the power grid (e.g. 10 ms for a 50 Hz power grid), and the positive sequence voltage effective value is updated once every interval. At the same time, the effective value results obtained by continuous 3 times calculation are subjected to sliding average processing, further weakening the influence of instantaneous fluctuations, and finally obtaining a stable and accurate positive sequence voltage effective value, which will be used as the core input basis for determining the voltage drop depth in step 2.

[0039] In this embodiment, through accurate node selection, synchronous acquisition and signal preprocessing, the real and synchronous three-phase voltage instantaneous value is ensured, and deviation in subsequent analysis caused by sampling error or interference is avoided; coordinate transformation converts complex three-phase alternating voltage into components in dq coordinate system, converts the difference between positive and negative sequence voltage components into obvious difference between direct current / slow change and 2 times frequency alternating current, greatly reduces the difficulty of positive sequence voltage separation, and the transformation process adapts to the real-time operation ability of the control system, meets the rapid response demand under fault working condition; the selected low-pass filter filters out negative sequence components, can efficiently retain positive sequence voltage components, the purity of the separated positive sequence voltage is high, the calculation accuracy of the positive sequence voltage effective value is avoided from being affected by negative sequence interference, and the subsequent judgment of the asymmetric fault of the power grid is ensured to be accurate; through reasonable calculation period setting and smoothing processing, stable and accurate positive sequence voltage effective value is obtained, which is directly used as the core input of step 2 to determine the voltage drop depth, provides accurate basis for subsequent adaptive adjustment of PI parameters and setting of current command value, and ensures that the entire negative sequence current suppression strategy can accurately match the fault working condition.

[0040] In a preferred embodiment of the application, step 2, determining the voltage drop depth according to the positive sequence voltage effective value, comprises:

[0041] Step 21, comparing the positive sequence voltage effective value with the preset rated positive sequence voltage effective value to obtain a voltage deviation, specifically comprising: determining the definition and source of the preset rated positive sequence voltage effective value; the value is a reference value preset according to the design parameters of the new energy unit (such as a double-fed wind turbine) and the rated voltage standard of the grid-connected power grid (such as a low-voltage distribution network 380V, a medium-voltage power grid 10kV, etc.), represents the ideal effective value of the positive sequence voltage at the point of common coupling (PCC) when the unit is normally connected to the grid, and this value is recorded and stored when the unit control system is initialized, and remains fixed in subsequent operation and does not change with real-time working condition. Then, the real-time positive sequence voltage effective value calculated in step 14 is retrieved, and a numerical comparison is made between the real-time positive sequence voltage effective value and the above-mentioned preset rated positive sequence voltage effective value. Since the voltage drop under asymmetric fault is that the real-time positive sequence voltage effective value is lower than the rated value, the specific comparison method is: subtracting the real-time positive sequence voltage effective value from the preset rated positive sequence voltage effective value, and the difference value obtained is the voltage deviation. The physical meaning of the deviation is the absolute drop amplitude of the real-time positive sequence voltage compared with the rated value, for example, when the rated positive sequence voltage effective value is 10kV and the real-time positive sequence voltage effective value is 6kV, the voltage deviation is 4kV, which directly reflects the specific value of the voltage lower than the rated value.

[0042] Step 22, based on the voltage deviation, calculate its relative size with the rated positive sequence voltage effective value, specifically including: determining the core purpose of calculating the relative size: eliminating the influence of different voltage levels (such as 380V unit and 10kV unit) on voltage drop amplitude judgment, through relative proportion, the voltage drop degree caused by fault is uniformly measured, for example, the voltage deviation is 2kV, which belongs to extreme serious drop in 380V level, and belongs to mild drop in 10kV level, only using absolute deviation cannot accurately reflect the fault severity, which needs to be corrected by relative proportion. The specific calculation process is:

[0043] The voltage deviation obtained in step 21 is called, and it is divided by the preset rated positive sequence voltage effective value, that is, voltage deviation ÷ preset rated positive sequence voltage effective value, the result is a dimensionless proportion value, which is the relative size of voltage deviation and rated positive sequence voltage effective value. For example, the voltage deviation is 4kV, the rated positive sequence voltage effective value is 10kV, and the relative size is 0.4; the voltage deviation is 0.19kV, the rated positive sequence voltage effective value is 0.38kV, and the relative size is also 0.5, both of which can be uniformly determined as medium relative drop, realizing the standardized comparison of drop degree of different voltage level units.

[0044] Step 23, quantize the relative size into specific depth value, which is the voltage drop depth representing the fault severity, specifically including: determining the operation logic of quantization:

[0045] The relative size proportion value obtained in step 22 is directly converted into voltage drop depth value with definite physical meaning, without additional complex operation, only the value consistency needs to be maintained and its meaning needs to be defined, that is, voltage drop depth = relative size proportion value obtained in step 22, and the value range of the value is 0 to 1 (when the real-time positive sequence voltage is equal to the rated value, the deviation is 0, the relative size is 0, and the drop depth is 0, representing no drop; when the real-time positive sequence voltage drops to 0, the deviation is equal to the rated value, the relative size is 1, and the drop depth is 1, representing extreme serious drop). Then, determine the fault representation meaning of the depth value: through the interval division, the fault severity can be directly distinguished, for example, when the drop depth is between 0 and 0.2, it is determined as mild asymmetric fault; when it is between 0.2 and 0.5, it is determined as moderate asymmetric fault; when it is between 0.5 and 1, it is determined as severe asymmetric fault. The quantized depth value will be directly used as the core input basis for step 3 to adaptively adjust the PI controller parameters, for example, when the severe fault (drop depth 0.8) occurs, the proportional coefficient needs to be increased and the integral time constant needs to be reduced, and when the mild fault (drop depth 0.1) occurs, the parameters need to be adjusted slightly, to ensure that the parameter adjustment is accurately matched with the fault severity.

[0046] In this embodiment, the absolute deviation is obtained by comparing the rated value with the real-time value, the step 22 eliminates the voltage level difference by relative ratio calculation, and the step 23 determines the fault severity by numerical quantification. The three steps cooperatively convert the abstract voltage imbalance into a specific value in the interval of 0-1, avoiding the fuzzy judgment of the fault degree. The relative size calculation of the step 22 breaks through the voltage level limitation of different new energy units (such as low-voltage distributed photovoltaic and medium-voltage doubly-fed wind turbine), so that the voltage drop depth becomes a unified measurement standard, and there is no need to design separate drop judgment logic for different voltage level units, thereby improving the universality and adaptability of the method. The voltage drop depth quantified in the step 23 is directly related to the fault severity, so that the PI parameter adjustment in the step 3 can be adapted as needed. In the case of severe fault, the parameters are greatly optimized to strengthen the suppression effect, and in the case of mild fault, the parameters are slightly adjusted to avoid system oscillation, which not only ensures the effectiveness of the negative sequence current suppression, but also takes into account the stability of the control system.

[0047] In a preferred embodiment of the present application, step 3, based on the voltage drop depth, adaptively adjusts the proportional coefficient and the integral time constant of the proportional integral controller, wherein the proportional coefficient increases with the increase of the voltage drop depth, and the integral time constant decreases with the increase of the voltage drop depth, comprising:

[0048] Step 31, input the voltage drop depth into the adjustment function of the proportional coefficient, the adjustment function of the proportional coefficient is configured to output a first adjustment coefficient which monotonically increases with the increase of the voltage drop depth, multiply the original proportional coefficient by the first adjustment coefficient to obtain the adjusted proportional coefficient, specifically comprising: first, from the control system storage unit of the new energy unit (specifically doubly-fed wind turbine), call the pre-set original proportional coefficient. The pre-set original proportional coefficient is the classic initial control parameter adopted by the proportional integral controller when the doubly-fed wind turbine is normally connected to the grid, which is a basic parameter designed based on the rated operating condition of the unit and the normal voltage level of the power grid, and ensures that the unit can stably control the current output when there is no fault. The pre-set original proportional coefficient is pre-stored in the control program parameter library of DSP or PLC, and does not need to be calculated in real time.

[0049] The adjustment function of the proportional coefficient is configured in the control system, and the core characteristic of the function is that the input quantity (voltage drop depth) and the output quantity (adjustment coefficient) are in a monotonically increasing relationship. When designing, the value boundary of the adjustment function of the proportional coefficient needs to be determined: when the voltage drop depth is 0 (i.e. no asymmetric fault, the effective value of the positive sequence voltage is equal to the rated value), the first adjustment coefficient output by the adjustment function of the proportional coefficient is 1, which ensures that the proportional coefficient does not be adjusted additionally when there is no fault; when the voltage drop depth is between 0 and 1 (corresponding to mild to severe asymmetric fault), the first adjustment coefficient increases smoothly with the increase of the drop depth, for example, when the drop depth is 0.2 (mild fault), the first adjustment coefficient is 1.2, when the drop depth is 0.5 (moderate fault), the first adjustment coefficient is 1.5, and when the drop depth is 1 (extreme fault, the effective value of the positive sequence voltage tends to 0), the first adjustment coefficient is controlled to be less than 2 (to avoid excessive coefficient leading to current overshoot); at the same time, the function needs to set an upper threshold of the adjustment coefficient (such as 2) to prevent the adjustment coefficient from increasing unlimitedly due to extreme fault, which may cause oscillation of the control system.

[0050] The voltage drop depth (value range 0-1) calculated in step 2 is taken as the input quantity and input into the above configured adjustment function of the proportional coefficient, and the corresponding first adjustment coefficient is calculated and output in real time, for example, when the voltage drop depth is 0.6 (severe fault), the function outputs the first adjustment coefficient of 1.8, which ensures that the more serious the fault is, the larger the first adjustment coefficient is.

[0051] The adjusted proportional coefficient is calculated by multiplying the original proportional coefficient obtained in step 1 and the first adjustment coefficient, for example, when the original proportional coefficient is 2.0 and the first adjustment coefficient is 1.8, the adjusted proportional coefficient is 2.0x1.8=3.6; the adjusted proportional coefficient will be directly used for the current error signal operation of the subsequent proportional integral controller, which ensures that the controller can quickly respond to the current deviation through a larger proportional coefficient during asymmetric fault, and suppress the negative sequence current.

[0052] In step 32, the voltage drop depth is input into the adjustment function of the integral time constant, the adjustment function of the integral time constant is configured to output a second adjustment coefficient which monotonically increases with the increase of the voltage drop depth, and the original integral time constant is divided by the second adjustment coefficient to obtain an adjusted integral time constant, specifically including: from the parameter library of the doubly-fed wind turbine control system, the pre-set original integral time constant is called, which is also the classic initial parameter of the proportional integral controller when the unit is in normal operation, and the design purpose is to eliminate the current steady-state error through integral action to ensure the accuracy of current control under normal working conditions, which is stored together with the original proportional coefficient and is the basic control parameter loaded when the control system is initialized.

[0053] Configuring the adjustment function of integral time constant: configuring the adjustment function of integral time constant in the control system, the core characteristics of the function are consistent with the adjustment function of proportional coefficient in step 31 (the input quantity is voltage drop depth, the output quantity is the second adjustment coefficient, and the second adjustment coefficient monotonically increases with the drop depth). The value boundary is also set: when the voltage drop depth is 0, the second adjustment coefficient is 1, which ensures that the integral time constant remains the original value when there is no fault; when the drop depth increases, the second adjustment coefficient increases accordingly (for example, when the drop depth is 0.2, the second adjustment coefficient is 1.2; when the drop depth is 0.5, the second adjustment coefficient is 1.5, and when the drop depth is 1, the second adjustment coefficient is less than 2), and the upper limit of the second adjustment coefficient is set to avoid the integral time constant being too small in subsequent calculation, which leads to system instability.

[0054] The voltage drop depth determined in step 2 is input into the above-mentioned adjustment function of integral time constant, and the control system outputs the corresponding second adjustment coefficient in real time according to the function logic. For example, when the voltage drop depth is 0.7 (severe fault), the function outputs the second adjustment coefficient as 1.9, which ensures that the more serious the fault, the larger the second adjustment coefficient.

[0055] The original integral time constant called in step 1 is divided by the obtained second adjustment coefficient to obtain the adjusted integral time constant. For example, when the original integral time constant is 0.05 seconds and the second adjustment coefficient is 1.9, the adjusted integral time constant is 0.05 ÷ 1.9 ≈ 0.026 seconds. The adjusted integral time constant will be used in the operation of the proportional-integral controller in cooperation with the adjusted proportional coefficient obtained in step 31. The reduction of integral time constant can speed up the response speed of integral action, reduce the accumulation time of current error, and further improve the suppression ability of the controller to negative sequence current.

[0056] In this embodiment, through the design that the proportional coefficient increases with the increase of the drop depth, when the fault is severe (such as the voltage drop depth is 0.8 or more), the current error signal can be quickly amplified by a larger proportional coefficient, so that the controller can quickly adjust the output and suppress the growth of negative sequence current; step 32 reduces the integral delay by designing the integral time constant to decrease with the increase of the drop depth, which quickly eliminates the steady-state error of current, and the two work together to ensure that the controller can respond with optimal parameters under different severity of asymmetric faults, avoid the problem of overshoot caused by too strong parameters under light fault or lag caused by insufficient parameters under heavy fault, and greatly improve the accuracy and efficiency of negative sequence current suppression.

[0057] By setting the upper limit of the first adjustment coefficient and the second adjustment coefficient (such as not more than 2), and adjusting based on the classic control parameters of the doubly-fed fan, rather than completely reconstructing the parameters, current overshoot caused by excessively large proportional coefficient and system oscillation caused by excessively small integral time constant are avoided, the stability of the original control framework of the unit is compatible, and the control system can run smoothly during asymmetric fault, without causing new dynamic problems (such as voltage fluctuation and current impact), which meets the stability requirements of the fault ride-through of the doubly-fed fan; The original proportional coefficient and the original integral time constant called in the step are the classic control parameters of the doubly-fed fan, and the adjustment process is based on the optimization of the original parameters, rather than the redesign of the parameter system, which can seamlessly adapt to the existing converter control logic (such as positive and negative sequence decoupling control and modulation wave generation logic) of the doubly-fed fan, without the need for large-scale modification of the original control system of the unit, thereby reducing the difficulty and cost of technology landing; The adjustment function adopts a stable characteristic of monotonic increase, so that even if the voltage drop depth of the power grid appears small fluctuations (such as voltage rebounding temporarily and then dropping again in the initial stage of the fault), the adjustment coefficient can change smoothly, avoiding the jump of the controller output caused by parameter mutation; At the same time, the parameter adjustment is based on the drop depth derived from the effective value of the positive sequence voltage, which can accurately reflect the essential characteristics of the asymmetric fault of the power grid, is not affected by other interference signals (such as harmonics and instantaneous impact) in the power grid, and ensures the robustness of the parameter adjustment of the controller.

[0058] In a preferred embodiment of the present application, step 4, according to the effective value of the positive sequence voltage, the positive sequence reactive current command value, the negative sequence reactive current command value, the positive sequence active current command value and the negative sequence active current command value during asymmetric fault are set, wherein the negative sequence reactive current command value and the negative sequence active current command value are set to zero, comprising:

[0059] Step 41, based on the first proportional coefficient, the effective value of the positive sequence voltage, the rated current of the new energy unit and the positive sequence reactive current of the new energy unit during normal operation, the positive sequence reactive current command value is determined by a first calculation relationship, specifically including: four key parameters are called from the control system parameter library of the doubly-fed fan:

[0060] The first proportional coefficient: this coefficient is a fixed value preset according to the reactive power regulation capability of the doubly-fed fan, the voltage support demand of the power grid and the fault ride-through standard, for example, based on the requirement of the power grid for reactive power support when the voltage of the new energy unit drops, it is stored after simulation and test calibration, to ensure that it can adapt to the reactive power compensation demand of different degrees of voltage drop;

[0061] The positive sequence voltage effective value calculated in step 14: this value reflects the real-time positive sequence voltage state of the grid-connected point (PCC) under asymmetric fault, and is the core basis for judging the size of reactive power support demand;

[0062] Rated current of a doubly fed wind turbine: This value is an inherent parameter in the design of the unit and represents the maximum current output capacity of the wind turbine during safe operation. It is used to limit the reactive current command value from not exceeding the unit's safety threshold.

[0063] Positive sequence reactive current during normal operation of a doubly fed wind turbine: This value is the stable reactive current output value of the wind turbine when there is no fault and the grid voltage is normal. It serves as the basic reference for the reactive power command value during a fault, so as to avoid excessive sudden changes in reactive power output during a fault.

[0064] The first calculation relationship is used to determine the command value: The calculation is performed according to the preset first calculation relationship logic. First, the difference between the current positive sequence voltage effective value and the preset rated positive sequence voltage effective value (the reference value used to calculate the drop depth in step 2) is calculated. This difference reflects the reactive power support demand gap caused by the voltage drop. Then, the gap is amplified or calibrated using the first proportional coefficient to obtain the additional reactive current component that needs to be supplemented. Then, the rated current of the doubly fed wind turbine is combined with the safety limit verification to ensure that the sum of the additional reactive component and the positive sequence reactive current during normal operation does not exceed the upper limit of reactive power output corresponding to the rated current. Finally, the positive sequence reactive current during normal operation is added to the calibrated additional reactive component to obtain the positive sequence reactive current command value under asymmetrical fault. This command value will be transmitted to the subsequent current control link in real time as the control target of the positive sequence reactive current.

[0065] Step 42, setting the negative sequence reactive current command value to zero, specifically includes: based on the core objective of suppressing negative sequence current under asymmetrical faults, negative sequence reactive current is an important component of negative sequence current. If there is a negative sequence reactive current command, the controller will drive the unit to output negative sequence reactive current, which will aggravate the negative sequence current problem. Therefore, its command value needs to be fixed to zero to cut off the generation of negative sequence reactive current from the control source.

[0066] Execution of setting operation: The control system of the doubly fed wind turbine directly defines the negative sequence reactive current command value as zero. This setting does not change with the voltage drop depth or real-time operating conditions and always remains fixed. At the same time, the control system will synchronize this zero command to the negative sequence current control module as the target value of the negative sequence reactive current, ensuring that the subsequent PI controller only aims to eliminate the negative sequence reactive current when dealing with negative sequence current errors, and avoids generating additional negative sequence components.

[0067] Step 43: Based on the second proportional coefficient and the effective value of the positive sequence voltage, determine the positive sequence active current command value through the second calculation relationship. Specifically, this includes retrieving two key parameters from the doubly-fed induction generator (DFIG) control system parameter library:

[0068] Second proportional coefficient: This coefficient is a fixed value set in advance according to the active regulation characteristics of the doubly-fed wind turbine and the active receiving capacity during power grid faults. For example, based on the principle that active output needs to be reduced to avoid overcurrent of the unit when the voltage drop is too deep, it is determined through tests to ensure that the relationship between active output and fault safety operation can be balanced; the positive sequence voltage effective value calculated in step 14: This value is used to judge the receiving capacity of the power grid to active, when the positive sequence voltage drop is deep, the ability of the power grid to withstand active impact decreases, and the power grid and the unit need to be protected by reducing the positive sequence active current command value.

[0069] Execute the second calculation relationship to determine the command value: According to the preset second calculation relationship logic, first, take the rated positive sequence voltage effective value as the reference, calculate the proportional relationship between the current positive sequence voltage effective value and the reference value. This proportion reflects the current active receiving capacity of the power grid (the lower the proportion, the weaker the receiving capacity); then use the second proportional coefficient to calibrate the proportional relationship to obtain the active current output proportion allowed under the current working condition; then combine the rated current of the doubly-fed wind turbine to calculate the upper limit value of the active current corresponding to the proportion; finally, the upper limit value is determined as the positive sequence active current command value under asymmetric fault, ensuring that the command value decreases simultaneously with the decrease of the positive sequence voltage effective value, avoiding excessive active output during voltage drop, which leads to overcurrent of the unit or oscillation of the power grid, while ensuring that a certain active output can be maintained when the voltage drop is shallow, reducing the loss of power generation.

[0070] Step 44, set the negative sequence active current command value to zero, specifically including: the negative sequence active current and the negative sequence reactive current together constitute the negative sequence current, if the negative sequence active current command value is not zero, the controller will drive the unit to output negative sequence active current, leading to unbalanced current of the inverter bridge arm, additional stress on power devices, and interference with the three-phase power balance of the power grid, therefore the command value needs to be fixed to zero, completely eliminating the active component of the negative sequence current.

[0071] Perform the setting operation: the control system of the doubly-fed wind turbine directly defines the negative sequence active current command value as zero, which is consistent with the negative sequence reactive current command value and does not change with any working condition; at the same time, the control system will transmit this zero command to the negative sequence current control module, which will work together with the negative sequence reactive current zero command to make the control target of the negative sequence current always zero, ensuring that the subsequent modulation wave generation link will not generate a voltage signal containing negative sequence active component, and suppressing the negative sequence current from the control source.

[0072] In this embodiment, the negative sequence reactive and active current command values are both set to zero, so that the controller aims to eliminate all negative sequence current components, cutting off the generation path of negative sequence current from the control source, effectively avoiding the problems of unbalanced inverter bridge arm current, IGBT over-stress and over-heat damage caused by negative sequence current, and significantly improving the operation reliability and service life of the double-fed fan; the positive sequence reactive current command value is dynamically adjusted by the first proportional coefficient and the real-time positive sequence voltage effective value, so as to automatically increase the reactive output when the voltage drops, provide reactive support for the power grid, help the grid-connected point voltage to recover to a reasonable range, reduce the interference of voltage asymmetry on sensitive equipment in the power grid (such as precision instruments and medical equipment), and reduce the risk of power grid voltage instability; the positive sequence active current command value is adjusted based on the second proportional coefficient and the real-time positive sequence voltage effective value, so as to reduce the active output when the voltage drop is deep to avoid over-current of the unit, and maintain a certain active output when the voltage drop is shallow to reduce power generation loss, which not only meets the fault ride-through requirements (off-grid operation) of the double-fed fan, but also avoids the aggravation of active impact on the power grid fault; the first and second proportional coefficients are both fixed values set in advance according to the unit characteristics and the power grid standard, and the calculation of the positive sequence command value is automatically completed depending on the real-time parameters, without the need for manual adjustment by the operation and maintenance personnel, thereby reducing the cost of manual intervention; at the same time, the setting logic of all command values is seamlessly compatible with the control framework of the double-fed fan, without the need to modify the original control system; by suppressing negative sequence current and supporting positive sequence voltage, the problems of negative sequence over-current protection triggered by negative sequence current, transformer differential protection misjudgment, etc. are reduced, and the protection device mis-trip caused by the unit off-grid or power grid power failure is avoided, thereby improving the overall safety and stability of the power system under high proportion of new energy access.

[0073] In a preferred embodiment of the present application, step 5, using the adjusted proportional integral controller parameters and the current command values, the positive sequence d-axis component, the positive sequence q-axis component, the negative sequence d-axis component and the negative sequence q-axis component of the modulation wave voltage are generated to control the new energy unit, thereby suppressing the negative sequence current under asymmetric fault, comprising:

[0074] Step 51, comparing the positive sequence active current command value and the positive sequence reactive current command value with the measured positive sequence active current value and the positive sequence reactive current value respectively to obtain a positive sequence current error signal, specifically including: calling the determined positive sequence active current command value and the positive sequence reactive current command value from the instruction storage unit of the double-fed fan control system, which are calculated based on the real-time positive sequence voltage effective value, reflecting the target value of the positive sequence current output by the double-fed fan under asymmetric fault, ensuring that the power grid can be supported without exceeding the safety range of the unit.

[0075] Collect and process the measured positive sequence current value: deploy a current sensor (such as a Hall current sensor) at the grid-connected point (PCC) of the doubly-fed wind turbine or the output end of the rotor-side converter, and collect the three-phase current instantaneous value in real time; according to the same coordinate transformation logic as step 12 (first Clark transformation to convert to αβ coordinate system components, and then Park transformation to convert to dq coordinate system components), and through the same low-pass filter as step 13, the positive sequence current component is separated, and finally the measured positive sequence active current value (corresponding to the dq coordinate system d-axis component) and the measured positive sequence reactive current value (corresponding to the dq coordinate system q-axis component) are obtained, ensuring that the coordinate dimensions of the measured value and the command value are consistent, and avoiding comparison deviation.

[0076] Calculate the positive sequence current error signal: compare the positive sequence active current command value with the measured positive sequence active current value, subtract the measured value from the command value to obtain the positive sequence active current error signal; at the same time, compare the positive sequence reactive current command value with the measured positive sequence reactive current value, and also subtract the measured value from the command value to obtain the positive sequence reactive current error signal; these two error signals together constitute the positive sequence current error signal, which is used to reflect the deviation degree of the current actual output and the target command.

[0077] Step 52, compare the negative sequence active current command value and the negative sequence reactive current command value with the measured negative sequence active current value and the measured negative sequence reactive current value respectively to obtain the negative sequence current error signal, which specifically includes: from the instruction storage unit of the doubly-fed wind turbine control system, call the negative sequence reactive current command value (zero) set in step 42 and the negative sequence active current command value (zero) set in step 44. These two command values are fixed at zero, which is the core target of suppressing negative sequence current, and ensures that the controller adjusts in the direction of eliminating negative sequence current.

[0078] Collect and process the measured negative sequence current value: based on the three-phase current instantaneous value collected in step 51, after completing the Clark transformation and the Park transformation, through a specific negative sequence component separation logic (cooperating with the low-pass filter for separating the positive sequence component in step 13, and using the frequency difference between the positive sequence and the negative sequence in the dq coordinate system, the negative sequence is 2 times the fundamental frequency), the measured negative sequence active current value (corresponding to the negative sequence dq coordinate system d-axis component) and the measured negative sequence reactive current value (corresponding to the negative sequence dq coordinate system q-axis component) are separated from the transformed current components, ensuring that the measured negative sequence current value can accurately reflect the actual size of the current negative sequence current.

[0079] Calculate the negative sequence current error signal: compare the measured negative sequence active current value with the negative sequence active current command value (zero), and subtract the measured value from the command value (zero) to obtain the negative sequence active current error signal; at the same time, compare the measured negative sequence reactive current value with the negative sequence reactive current command value (zero), and subtract the measured value from the command value (zero) to obtain the negative sequence reactive current error signal; these two error signals together constitute the negative sequence current error signal, which directly reflects the deviation of the current negative sequence current from the zero target, and is the key input for subsequent elimination of negative sequence current.

[0080] Step 53, input the positive sequence current error signal to the proportional-integral controller with adjusted parameters, calculate the positive sequence d-axis modulation voltage component and the positive sequence q-axis modulation voltage component, specifically including: from the parameter storage unit of the double-fed wind turbine control system, call the adjusted proportional coefficient in step 31 and the adjusted integral time constant in step 32, these two parameters are optimized based on the voltage drop depth, ensuring that the PI controller can quickly respond to errors and avoid oscillation under current fault conditions.

[0081] PI controller calculates the positive sequence modulation voltage component: input the positive sequence active current error signal obtained in step 51 to the proportional-integral (PI) controller loaded with adjusted parameters; the proportional part of the PI controller will quickly output a voltage adjustment amount according to the error size, and the integral part will accumulate the error and output an adjustment amount to eliminate the steady-state deviation, and the superposition of the two obtains the positive sequence d-axis modulation voltage component (since the positive sequence active current corresponds to the d-axis of the dq coordinate system, the error signal operation result corresponds to the d-axis voltage); at the same time, input the positive sequence reactive current error signal obtained in step 51 to the same set of PI controller (or independent PI control channel, parameters consistent with the former) with adjusted parameters, after the same proportional-integral operation, obtain the positive sequence q-axis modulation voltage component (the positive sequence reactive current corresponds to the q-axis of the dq coordinate system, so the error signal operation result corresponds to the q-axis voltage); these two components together constitute the core of the positive sequence modulation voltage, which is used to control the positive sequence current to track the command value.

[0082] Step 54, input the negative sequence current error signal to the proportional-integral controller with adjusted parameters, calculate the negative sequence d-axis modulation voltage component and the negative sequence q-axis modulation voltage component, specifically including: consistent with step 53, call the adjusted proportional coefficient in step 31 and the adjusted integral time constant in step 32, no need to set new parameters, ensure that the positive sequence and negative sequence current control use unified optimized parameters, avoid control discord caused by parameter difference, at the same time simplify system logic.

[0083] PI controller operation of negative sequence modulation voltage component: the negative sequence active current error signal obtained in step 52 is input into the PI controller loaded with the adjusted parameters, and after the proportional part quickly responds to the error and the integral part eliminates the steady-state deviation, the negative sequence d-axis modulation voltage component is obtained (the negative sequence active current corresponds to the d-axis of the negative sequence dq coordinate system, so the error signal operation result corresponds to the negative sequence d-axis voltage); at the same time, the negative sequence reactive current error signal obtained in step 52 is input into the same set of PI controller with adjusted parameters, and after the same proportional-integral operation, the negative sequence q-axis modulation voltage component is obtained (the negative sequence reactive current corresponds to the q-axis of the negative sequence dq coordinate system, so the error signal operation result corresponds to the negative sequence q-axis voltage), the target of the two components is to eliminate the negative sequence current error through voltage regulation, so that the measured negative sequence current tends to zero.

[0084] Step 55, the positive sequence d-axis modulation voltage component, the positive sequence q-axis modulation voltage component, the negative sequence d-axis modulation voltage component and the negative sequence q-axis modulation voltage component are synthesized to generate the final modulation wave signal, which is used to control the switching devices of the converter in the new energy unit to realize the suppression of the negative sequence current; the new energy unit is a doubly-fed wind turbine, which specifically includes: the positive sequence d-axis modulation voltage component and the positive sequence q-axis modulation voltage component obtained in step 53 are input into the coordinate inverse transformation module of the doubly-fed wind turbine control system: first, the positive sequence and negative sequence dq coordinate system components are converted into positive sequence and negative sequence components in the αβ coordinate system through inverse Park transformation (with the grid fundamental phase as the reference, the negative sequence part corresponds to the reverse phase); then the comprehensive components in the αβ coordinate system are converted into three-phase (A, B, C) modulation wave voltage signals through inverse Clark transformation; during the synthesis process, it is necessary to ensure that the positive sequence and negative sequence components are accurately superimposed in phase and amplitude to avoid distortion of the modulation wave caused by inverse transformation error.

[0085] Control the switching devices of the doubly-fed wind turbine converter: the synthesized three-phase modulation wave signal is transmitted to the rotor-side converter PWM (pulse width modulation) control unit of the doubly-fed wind turbine, which generates IGBT trigger pulse signals meeting the switching frequency requirements (usually several kHz to tens of kHz) according to the ratio of the modulation wave signal to the converter DC side voltage; the trigger pulse signal directly drives the IGBT switching devices in the rotor-side converter to control the time proportion of its conduction and shutdown: when the IGBT is turned on, the converter outputs the voltage of the corresponding phase to the rotor winding; when the IGBT is turned off, the voltage output of the phase is cut off; by dynamically adjusting the switching state of the IGBT, the voltage output by the converter is accurately matched with the synthesized modulation wave voltage, and then the positive sequence component of the rotor current of the doubly-fed wind turbine is controlled to track the instruction, and the negative sequence component tends to zero, finally realizing effective suppression of the negative sequence current under asymmetric fault.

[0086] In this embodiment, by direct comparison of the command value and the measured value, and ensuring that the command value and the measured value are consistent in the coordinate dimension, the positive and negative sequence current error signals obtained can truly reflect the deviation of the current output from the target, avoiding control inaccuracy caused by inaccurate error calculation; steps 53 and 54 reuse the PI parameters optimized based on the voltage drop depth, so that the PI controller can quickly respond to the error (the proportional coefficient is increased) and eliminate the steady-state deviation (the integral time constant is reduced) under different fault severity, avoiding current overshoot caused by overstrong parameters in light faults, and solving the suppression lag caused by insufficient parameters in heavy faults, significantly improving the timeliness and accuracy of negative sequence current suppression; steps 53 and 54 respectively perform PI regulation on the positive and negative sequence current error signals, the positive sequence part focuses on tracking the command and supporting the power grid, and the negative sequence part focuses on eliminating the error and suppressing the negative sequence, the control target is clear and does not interfere with each other, effectively avoiding the problem of low suppression efficiency caused by positive and negative sequence component coupling, and ensuring that the negative sequence current can accurately approach zero; step 55 synthesizes three-phase modulation waves through coordinate inverse transformation, and directly drives the converter IGBT, forming a complete closed-loop control of error detection, PI regulation, voltage modulation, device control and current correction, so that the logic of suppressing the negative sequence current is from the command to the actual action, ensuring that the control strategy can really act on the doubly-fed wind turbine.

[0087] When applied specifically, a certain wind farm doubly-fed induction wind turbine (hereinafter referred to as "doubly-fed wind turbine") is connected to the designated voltage level regional power grid, and the rated parameters of the grid-connected point (PCC) are as follows:

[0088] Rated positive sequence voltage effective value U pcc+N ; doubly-fed wind turbine rated current I N (based on the rated power and rated voltage of the unit); maximum allowable current of doubly-fed wind turbine I max = 1.5 I N ; positive sequence reactive current during normal operation I q0+ (maintain the specified power factor of the power grid).

[0089] Set key coefficients to adapt to the characteristics of the doubly-fed wind turbine:

[0090] positive sequence reactive current regulation coefficient K1 (calibrated through simulation to ensure effective reactive power support during voltage drop); positive sequence active current regulation coefficient K2 (balance active output and fault safety to avoid overcurrent); PI parameter proportional coefficient adjustment coefficient K3 (control the increasing amplitude of the proportional coefficient with the drop depth); PI parameter integral time constant adjustment coefficient K4 (control the decreasing amplitude of the integral time constant with the drop depth); original PI controller parameters (normal working condition): proportional coefficient k p , integral time constant ki ; second-order Butterworth low-pass filter, cutoff frequency 5-10 Hz (filter out 2 times the fundamental frequency of the negative sequence component).

[0091] The grid causes the three-phase voltage at the PCC to be unbalanced due to external faults (such as phase-to-phase short circuit, single-phase ground fault), the positive sequence voltage effective value drops, the negative sequence voltage component is generated, and the negative sequence current component of the doubly-fed wind turbine is generated, triggering the negative sequence current suppression control logic of the method.

[0092] The case implementation steps are as follows:

[0093] Step 1, obtain the positive sequence voltage effective value at the PCC:

[0094] Deploy Hall voltage sensors of appropriate voltage level at the PCC A, B, and C three-phase lines respectively, realize synchronous acquisition of three-phase voltage through hardware synchronization mechanism (such as GPS clock synchronization, FPGA timing control), sampling frequency 2-5 kHz; the collected raw voltage analog signals are preprocessed by RC filter circuit (to filter out high-frequency electromagnetic interference), then converted into digital signals by 16-bit and above precision ADC module, and transmitted to the control system (such as DSP, PLC) of the doubly-fed wind turbine for storage.

[0095] Coordinate transformation:

[0096] Clark transformation: convert the collected three-phase voltage instantaneous value into voltage component in two-phase stationary coordinate system (αβ coordinate system), eliminate the coupling relationship between three-phase voltage.

[0097] Park transformation: through PLL phase-locked loop, real-time track the grid fundamental positive sequence phase, convert the voltage component in αβ coordinate system into voltage component in two-phase rotating coordinate system (dq coordinate system), make the positive sequence voltage component present as DC or slowly changing quantity, and the negative sequence voltage component present as 2 times the fundamental frequency of the alternating quantity.

[0098] Separate positive sequence voltage component:

[0099] Input the voltage component in dq coordinate system into the preset second-order Butterworth low-pass filter, filter out the negative sequence alternating component of 2 times the fundamental frequency, and output the pure positive sequence voltage component.

[0100] Calculate the positive sequence voltage effective value:

[0101] According to the positive sequence voltage d-axis and q-axis components, calculate the positive sequence voltage effective value U pcc+ (in per unit) according to the inherent relationship between the effective value and the amplitude of alternating voltage; set the calculation update period to half of the grid fundamental period, and perform sliding average processing on the continuous 3 calculation results to obtain the positive sequence voltage effective value U pcc+ .

[0102] Step 2, determine the voltage drop depth:

[0103] Call the preset rated positive sequence voltage effective value U in the control system pcc+N (for the unit value), calculate the voltage deviation amount, where the voltage deviation amount = U pcc+N -U pcc+ .

[0104] Calculate the relative size of the voltage deviation amount and the rated positive sequence voltage effective value, eliminate the influence of different voltage levels on the drop amplitude judgment, the relative size = voltage deviation amount / U pcc+N , the above relative size is directly used as the voltage drop depth, the numerical range is 0-1; the voltage drop depth = the relative size, through the numerical interval division, the fault severity is divided (such as 0-0.2 for mild fault, 0.2-0.5 for moderate fault, and 0.5-1 for severe fault), which provides a basis for subsequent PI parameter adjustment.

[0105] Step 3, self-adaptive adjustment of PI controller parameters:

[0106] Call the original proportional coefficient stored in the control system , calculate the adjusted proportional coefficient through the PI parameter adjustment formula , , wherein, is the positive sequence voltage effective value, is the preset proportional coefficient, is the positive sequence voltage threshold value (for the unit value), is the preset proportional coefficient, used to control the amplification range of the original proportional coefficient , to ensure that the adjusted proportional coefficient monotonically increases with the drop depth, and does not exceed the safety upper limit (such as 2 times of the original proportional coefficient); the value of K3 needs to be calibrated through simulation or test in combination with the dynamic characteristics (such as inertia and converter bandwidth) of the double-fed wind turbine, and the typical range is 1.2-2.0:

[0107] If K3 is too small (such as <1.2), the increase of the adjusted proportional coefficient is insufficient, and it is difficult to quickly suppress the negative sequence current in moderate and severe faults; if K3 is too large (such as >2.0), the adjusted proportional coefficient may be far beyond the safety upper limit in extreme drop, leading to current overshoot or system oscillation; in actual application, the preferred K3 = 1.5 (considering response speed and stability).

[0108] Call the original integral time constant stored in the control system ; calculate the adjusted integral time constant according to the PI parameter adjustment formula: , wherein, is the positive sequence voltage effective value, is the preset integral time constant, used to control The reduction amplitude of the original integral time constant ensures that the adjusted integral time constant monotonically decreases with the depth of the drop (speeds up the integral response) and is not lower than the lower limit of stability (such as 1 / 2 of the original integral time constant), and the specific value range is:

[0109] The value of K4 needs to match the proportional coefficient adjustment logic, and the typical range is 1.2-2.0 (consistent with K3, simplifying parameter design): if K4 is too small (such as <1.2), the integral time constant is not reduced enough, and it is difficult to quickly eliminate the steady-state error in moderate and severe faults; if K4 is too large (such as >2.0), the adjusted integral time constant may be too small in extreme drops, causing the integral action to be too strong and causing system oscillation; in actual application, the preferred K4=1.5 (matched with K3, ensuring the coordination of positive and negative sequence control).

[0110] Step 4, set the current command value:

[0111] Set the positive sequence reactive current command value , set according to the positive sequence reactive current command value calculation formula: , wherein, is the positive sequence reactive current regulation coefficient, is the unit rated current, is the positive sequence reactive current in normal operation; the calculation result is safety checked to ensure that does not exceed the upper limit of unit reactive power output.

[0112] Set the negative sequence reactive current command value , based on the core goal of suppressing negative sequence current, set: ; synchronize the command to the negative sequence current control module to ensure that the controller does not drive the unit to output negative sequence reactive current.

[0113] Set the positive sequence active current command value , set according to the positive sequence active current command value calculation formula: , wherein, is the positive sequence active current regulation coefficient; the calculation result is safety checked to ensure that: , wherein, is the maximum allowable current of the unit.

[0114] Set the negative sequence active current command value , based on the core goal of suppressing negative sequence current, set: ; synchronize the command to the negative sequence current control module to cut off the active component path of the negative sequence current in cooperation with .

[0115] Step 5, generate a modulated wave voltage and control the doubly-fed wind turbine:

[0116] A current sensor is arranged at the output end of the double-fed wind turbine converter to collect three-phase currents and obtain measured positive sequence active current and measured positive sequence reactive current through Clark transformation, Park transformation and low-pass filtering. .

[0117] Error calculation, positive sequence active current error ;

[0118] Positive sequence reactive current error ;

[0119] The above two error signals jointly constitute a positive sequence current error signal.

[0120] Based on the three-phase current collection data at the output end of the converter, measured negative sequence active current and measured negative sequence reactive current are obtained through negative sequence component separation logic (cooperating with positive sequence filtering, utilizing the frequency difference between positive and negative sequence components). .

[0121] Error calculation:

[0122] Negative sequence active current error ;

[0123] Negative sequence reactive current error ;

[0124] The above two error signals jointly constitute a negative sequence current error signal.

[0125] Calculate positive sequence d / q-axis modulation voltage components, input the positive sequence current error signal into a PI controller with adjusted parameters:

[0126] Positive sequence d-axis modulation voltage component ;

[0127] Positive sequence q-axis modulation voltage component ;

[0128] Calculate negative sequence d / q-axis modulation voltage components, reuse the adjusted PI parameters, and input the negative sequence current error signal into a PI controller:

[0129] Negative sequence d-axis modulation voltage component ;

[0130] Negative sequence q-axis modulation voltage component .

[0131] Synthesize modulation waves and control the converter, coordinate inverse transformation: input the positive sequence modulation voltage components ( ) and the negative sequence modulation voltage components ( ) into a coordinate inverse transformation module:​​​​

[0132] Inverse Park transform: Based on the fundamental phase of the power grid (the negative sequence component corresponds to the opposite phase), it is converted into components in the αβ coordinate system;

[0133] Inverse Clark transform: converts the components in the αβ coordinate system into a three-phase (A, B, C) modulated wave voltage signal.

[0134] Converter control: The three-phase modulation wave signal is transmitted to the PWM control unit of the doubly fed wind turbine converter to generate IGBT trigger pulses that meet the switching frequency requirements, control the IGBT to turn on and off, so that the converter output voltage matches the synthesized modulation wave voltage, and realizes positive sequence current tracking command and negative sequence current approaching zero.

[0135] By setting the negative sequence current command to zero and coordinating the control of adaptive PI parameters, the negative sequence component in the doubly-fed induction generator (DFIG) current is effectively weakened or eliminated, avoiding inverter arm current imbalance, power device (IGBT) overstress and overheating damage, and extending equipment life. During faults, the DFIG maintains grid-connected operation, and the positive sequence active and reactive currents are output according to the command values, meeting the grid fault ride-through standard requirements and preventing accidental grid disconnection. The positive sequence reactive current is dynamically adjusted according to the voltage drop, providing reactive power support to the grid, helping the positive sequence voltage at the PCC to recover, reducing voltage asymmetry, reducing interference to grid-sensitive equipment, and preventing maloperation of protection devices. The PI parameters are adaptively adjusted according to the voltage drop depth, so even if there are small fluctuations in the fault condition (such as a brief voltage rebound), the control remains stable without current overshoot, system oscillation, or other problems.

[0136] like Figure 2 As shown, an adaptive parameter adjustment system for suppressing negative sequence current under asymmetrical faults includes:

[0137] The acquisition module is used to acquire the effective value of the positive sequence voltage at the grid connection point of the new energy generating unit;

[0138] The determination module is used to determine the voltage drop depth based on the positive sequence voltage RMS value;

[0139] An adjustment module is used to adaptively adjust the proportional coefficient and integral time constant of the proportional-integral controller based on the voltage drop depth, wherein the proportional coefficient increases with the increase of the voltage drop depth, and the integral time constant decreases with the increase of the voltage drop depth.

[0140] The processing module is used to set the positive-sequence reactive current command value, negative-sequence reactive current command value, positive-sequence active current command value and negative-sequence active current command value for asymmetrical faults according to the positive-sequence voltage effective value, wherein the negative-sequence reactive current command value and the negative-sequence active current command value are set to zero.

[0141] The generating module is configured to generate a positive sequence d-axis component, a positive sequence q-axis component, a negative sequence d-axis component and a negative sequence q-axis component of the modulation wave voltage using the adjusted proportional-integral controller parameters and the current instruction value, so as to control the new energy unit and thereby suppress the negative sequence current under the asymmetric fault.

[0142] The above is the preferred embodiment of the present application. It should be pointed out that, for those skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, which should also be considered as the protection scope of the present application.

Claims

1. An adaptive parameter adjustment method for suppressing negative sequence current under asymmetrical faults, characterized in that, The method includes: Step 1: Obtain the effective value of the positive sequence voltage at the grid connection point of the new energy unit; Step 2: Determine the voltage drop depth based on the positive sequence voltage RMS value; Step 3: Based on the voltage drop depth, adaptively adjust the proportional coefficient and integral time constant of the proportional-integral controller, wherein the proportional coefficient increases with the increase of the voltage drop depth, and the integral time constant decreases with the increase of the voltage drop depth. Step 4: Based on the positive-sequence voltage RMS value, set the positive-sequence reactive current command value, negative-sequence reactive current command value, positive-sequence active current command value, and negative-sequence active current command value for asymmetrical faults, wherein the negative-sequence reactive current command value and the negative-sequence active current command value are set to zero. This includes: determining the positive-sequence reactive current command value through a first calculation relationship based on a first proportional coefficient, the positive-sequence voltage RMS value, the rated current of the new energy unit, and the positive-sequence reactive current during normal operation of the new energy unit; setting the negative-sequence reactive current command value to zero; determining the positive-sequence active current command value through a second calculation relationship based on a second proportional coefficient and the positive-sequence voltage RMS value; and setting the negative-sequence active current command value to zero. Step 5: Using the adjusted proportional-integral controller's proportional coefficient and integral time constant, positive-sequence reactive current command value, negative-sequence reactive current command value, positive-sequence active current command value, and negative-sequence active current command value, generate the positive-sequence d-axis component, positive-sequence q-axis component, negative-sequence d-axis component, and negative-sequence q-axis component of the modulated wave voltage to control the new energy unit, thereby suppressing the negative-sequence current under asymmetrical faults.

2. The adaptive parameter adjustment method for suppressing negative sequence current under asymmetrical faults according to claim 1, characterized in that, Step 1: Obtain the effective value of the positive sequence voltage at the grid connection point of the new energy unit, including: Real-time monitoring of the instantaneous three-phase voltage at the grid connection point of new energy generating units; The instantaneous values ​​of the three-phase voltages are transformed by coordinates to obtain the voltage components in a two-phase rotating coordinate system; A filter is used to separate the positive sequence voltage component from the voltage components in the two-phase rotating coordinate system; The effective value of the positive sequence voltage component is calculated and taken as the effective value of the positive sequence voltage.

3. The adaptive parameter adjustment method for suppressing negative sequence current under asymmetrical faults according to claim 2, characterized in that, Step 2, determining the voltage drop depth based on the positive sequence voltage RMS value, including: The voltage deviation is obtained by comparing the positive sequence voltage RMS value with the preset rated positive sequence voltage RMS value. Based on the voltage deviation, calculate its relative magnitude with the effective value of the rated positive sequence voltage; The relative size is quantified into a specific depth value, which is the voltage drop depth that characterizes the severity of the fault.

4. The adaptive parameter adjustment method for suppressing negative sequence current under asymmetrical faults according to claim 3, characterized in that, Step 3, based on the voltage drop depth, adaptively adjust the proportional coefficient and integral time constant of the proportional-integral controller, wherein the proportional coefficient increases with the increase of the voltage drop depth, and the integral time constant decreases with the increase of the voltage drop depth, including: The voltage drop depth is input into the adjustment function of the proportional coefficient, which is configured to output a first adjustment coefficient that monotonically increases with the voltage drop depth. The original proportional coefficient is multiplied by the first adjustment coefficient to obtain the adjusted proportional coefficient. The voltage drop depth is input into the adjustment function of the integral time constant. The adjustment function of the integral time constant is configured to output a second adjustment coefficient that monotonically increases with the voltage drop depth. The original integral time constant is divided by the second adjustment coefficient to obtain the adjusted integral time constant.

5. The adaptive parameter adjustment method for suppressing negative sequence current under asymmetrical faults according to claim 4, characterized in that, Step 5: Using the adjusted proportional-integral controller's proportional coefficient and integral time constant, positive-sequence reactive current command value, negative-sequence reactive current command value, positive-sequence active current command value, and negative-sequence active current command value, generate the positive-sequence d-axis component, positive-sequence q-axis component, negative-sequence d-axis component, and negative-sequence q-axis component of the modulated wave voltage to control the new energy unit, thereby suppressing the negative-sequence current under asymmetrical faults, including: The positive sequence active current command value and the positive sequence reactive current command value are compared with the measured positive sequence active current value and positive sequence reactive current value, respectively, to obtain the positive sequence current error signal. The negative sequence active current command value and negative sequence reactive current command value are compared with the measured negative sequence active current value and negative sequence reactive current value, respectively, to obtain the negative sequence current error signal. The positive sequence current error signal is input to a proportional-integral controller with adjusted parameters to calculate the positive sequence d-axis modulated voltage component and the positive sequence q-axis modulated voltage component. The negative sequence current error signal is input to a proportional-integral controller with adjusted parameters to calculate the negative sequence d-axis modulated voltage component and the negative sequence q-axis modulated voltage component. The positive-sequence d-axis modulated voltage component, the positive-sequence q-axis modulated voltage component, the negative-sequence d-axis modulated voltage component, and the negative-sequence q-axis modulated voltage component are synthesized to generate the final modulated wave signal, which is used to control the switching devices of the converter in the new energy unit to suppress the negative-sequence current.

6. The adaptive parameter adjustment method for suppressing negative sequence current under asymmetrical faults according to claim 5, characterized in that, The new energy unit is a doubly fed wind turbine.

7. An adaptive parameter adjustment system for suppressing negative sequence current under asymmetrical faults, the system implementing the method as described in any one of claims 1 to 6, characterized in that, include: The acquisition module is used to acquire the effective value of the positive sequence voltage at the grid connection point of the new energy generating unit; The determination module is used to determine the voltage drop depth based on the positive sequence voltage RMS value; An adjustment module is used to adaptively adjust the proportional coefficient and integral time constant of the proportional-integral controller based on the voltage drop depth, wherein the proportional coefficient increases with the increase of the voltage drop depth, and the integral time constant decreases with the increase of the voltage drop depth. The processing module is configured to set positive-sequence reactive current command values, negative-sequence reactive current command values, positive-sequence active current command values, and negative-sequence active current command values ​​for asymmetrical faults based on the positive-sequence voltage RMS value, wherein the negative-sequence reactive current command values ​​and the negative-sequence active current command values ​​are set to zero, including: determining the positive-sequence reactive current command value through a first calculation relationship based on a first proportional coefficient, the positive-sequence voltage RMS value, the rated current of the new energy unit, and the positive-sequence reactive current during normal operation of the new energy unit; setting the negative-sequence reactive current command value to zero; determining the positive-sequence active current command value through a second calculation relationship based on a second proportional coefficient and the positive-sequence voltage RMS value; and setting the negative-sequence active current command value to zero. The generation module is used to generate the positive-sequence d-axis component, positive-sequence q-axis component, negative-sequence d-axis component, and negative-sequence q-axis component of the modulated wave voltage using the adjusted proportional-integral controller's proportional coefficient and integral time constant, positive-sequence reactive current command value, negative-sequence reactive current command value, positive-sequence active current command value, and negative-sequence active current command value, in order to control the new energy unit and thus suppress the negative-sequence current under asymmetrical faults.

8. A computing device, characterized in that, include: One or more processors; A storage device for storing one or more programs that, when executed by one or more processors, cause the one or more processors to implement the method as described in any one of claims 1 to 6.

9. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores a program that, when executed by a processor, implements the method as described in any one of claims 1 to 6.

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