Method for supporting transient reactive voltage of system for sending out new energy through flexible low-frequency collection

Abstract

The invention provides a method for supporting transient reactive voltage of a new energy collecting and sending-out system through flexible low frequency, and relates to the technical field of transient supporting of a new energy power system. According to the method, voltage and current instantaneous values of a wind power plant, a photovoltaic power station, an M3C frequency conversion station and an SVG are synchronously collected, a positive sequence voltage component is obtained based on an instantaneous symmetric component method, and fault identification is carried out; current vector equal proportion amplitude limiting and capacitor voltage safety domain amplitude limiting are adopted for the M3C frequency conversion station; constructing a segmented reactive current instruction according to the positive sequence voltage amplitude for the wind power plant, and combining the direct current voltage fluctuation to limit the amplitude; for the photovoltaic power station, generating a reactive instruction by using the impedance intensity factor and the adaptive logic coefficient based on the simulation model; and current decoupling and cooperative compensation are realized based on constant AC voltage control for the SVG. According to the method, wind power, photovoltaic, frequency conversion station and SVG collaborative transient reactive power support is realized, and the voltage stability and low voltage ride through capability of the flexible low-frequency collection system are improved.

Description

Technical Field

[0001] This invention relates to the field of transient support technology for new energy power systems, and in particular to a method for supporting transient reactive voltage in a flexible low-frequency collection and transmission system for new energy. Background Technology

[0002] With the rapid development of large-scale renewable energy sources such as offshore wind power, flexible low-frequency transmission technology is becoming an important alternative to traditional power frequency AC or DC transmission schemes due to its excellent techno-economic performance in medium transmission distance and capacity scenarios. A typical flexible low-frequency transmission system usually consists of a wind farm, a photovoltaic power station, an M3C frequency converter station, low-frequency lines, and a power frequency grid.

[0003] However, such systems face severe transient stability challenges during and after fault recovery. On one hand, low-frequency lines have shorter electrical distances than power frequency systems, and their charging power is inversely proportional to frequency, leading to a more significant line capacitance effect. This results in a substantial difference in reactive power distribution during faults compared to power frequency systems, easily causing severe overvoltages or voltage dips. On the other hand, converters such as M3Cs often employ control strategies to suppress fault currents during fault ride-through, drastically altering their reactive power exchange characteristics with the grid. This can exacerbate voltage fluctuations at tie points and even lead to system instability. During fault recovery, stable system voltage recovery depends on the balance between the reactive power support capacity of the power source and load demand. If renewable energy plants or M3Cs cannot provide rapid and sufficient reactive power support, the voltage recovery process will be slow or even lead to voltage collapse.

[0004] Existing reactive power and voltage control strategies are mostly based on power frequency system design or use a fixed QV droop coefficient. They inject reactive current according to a fixed proportion (K coefficient) based on the voltage drop at the grid connection point to support voltage recovery. However, these strategies are ill-suited to the unique frequency characteristics of flexible low-frequency systems and the rapidly changing operating conditions during faults. In severe faults, reactive power overcompensation may lead to synchronous instability or overvoltage. Phase-locked loop (PLL) basic synchronization control relies on traditional SRF-PLLs to detect voltage phase, but under asymmetrical faults, negative sequence interference causes phase detection deviations, exacerbating system oscillations. Negative sequence current suppression strategies suppress negative sequence current through control algorithms, but traditional methods have slow dynamic response and do not consider adaptive adjustment based on fault type, easily leading to capacitor voltage imbalance in flexible low-frequency transmission systems. Controllers with fixed parameters cannot fully utilize the dynamic reactive power capacity of the converter during faults, potentially leading to insufficient support or equipment overload, failing to meet the stringent requirements for transient voltage stability.

[0005] Therefore, there is an urgent need for a method that can accurately analyze the reactive power distribution characteristics during faults in flexible low-frequency systems, quantify the demand for reactive power support during fault recovery, and provide adaptive transient voltage support accordingly. Summary of the Invention

[0006] To overcome the shortcomings of existing technologies, the purpose of this invention is to provide a method for supporting transient reactive voltage in a flexible low-frequency collection and transmission system for new energy sources. By constructing a collaborative transient reactive voltage support mechanism among wind farms, photovoltaic power plants, M3C frequency converters, and SVG, the flexible low-frequency new energy collection system achieves rapid and stable control under voltage disturbances.

[0007] To achieve the above objectives, the present invention provides the following solution:

[0008] A method for supporting transient reactive voltage in a flexible low-frequency collection and transmission system for new energy sources includes:

[0009] The instantaneous values ​​of three-phase voltage and three-phase current at the PCC point of wind farms and photovoltaic power stations, the low-frequency side and power frequency side of M3C frequency converter stations, and the SVG access point are collected simultaneously to form sampling data;

[0010] The instantaneous symmetrical component method is used to obtain the positive sequence voltage component from the instantaneous three-phase voltage values ​​in the sampled data, and the amplitude of the positive sequence voltage component is calculated as the positive sequence voltage amplitude. When the positive sequence voltage amplitude is lower than the voltage drop threshold, a fault is determined and the fault is classified.

[0011] For the M3C frequency converter station, a current vector proportional limiting strategy is adopted based on orthogonal axis decoupling, and the active current command is dynamically limited in combination with the capacitor voltage safety domain to achieve fault current limiting during low voltage ride-through.

[0012] For the wind farm, the wind turbine is switched from maximum power point tracking to constant voltage support mode according to the positive sequence voltage amplitude. Reactive current commands are issued in segments according to the positive sequence voltage amplitude, and a DC voltage fluctuation influence factor is introduced to limit the active current command amplitude.

[0013] For the photovoltaic power station, based on the photovoltaic electromechanical transient simulation model, a segmented reactive current command is constructed using the impedance strength factor and logic coefficients that are adaptively adjusted according to the system strength. The active current command is limited by the DC voltage fluctuation influence factor and then output as reactive support current through the current inner loop controller.

[0014] For the SVG on the grid side, based on the constant AC voltage control strategy, the instantaneous values ​​of the three-phase voltage and the three-phase current at the PCC point are used to achieve decoupling of active and reactive currents and coordinated control of the voltage outer loop and the current inner loop. The SVG modulation voltage and reactive current command are calculated to compensate for reactive power deficit and suppress transient overvoltage.

[0015] Preferably, the instantaneous values ​​of three-phase voltage and three-phase current at the PCC point of wind farms and photovoltaic power stations, the low-frequency side and power frequency side of M3C frequency converter stations, and the SVG access point are collected simultaneously to form sampling data, including:

[0016] The instantaneous values ​​of three-phase voltage and three-phase current at the PCC point of wind farms and photovoltaic power stations, the low-frequency side and power frequency side of M3C frequency converter stations, and the SVG access point are collected in real time using voltage transformers and current transformers.

[0017] The instantaneous values ​​of the three-phase voltage and the instantaneous values ​​of the three-phase current are synchronously sampled at a preset sampling frequency, and the instantaneous voltage value expression and the instantaneous current value expression are calculated; wherein the instantaneous voltage value is characterized by voltage amplitude, angular frequency and initial voltage phase, and the instantaneous current value is characterized by current amplitude, angular frequency and initial current phase;

[0018] The sampled data consists of the samples corresponding to the instantaneous voltage expression and the instantaneous current expression.

[0019] Preferably, the instantaneous symmetrical component method is used to obtain the positive sequence voltage component from the instantaneous three-phase voltage values ​​in the sampled data, and the amplitude of the positive sequence voltage component is calculated as the positive sequence voltage amplitude. When the positive sequence voltage amplitude is lower than the voltage drop threshold, a fault is determined and fault classification is performed, including:

[0020] Using the instantaneous values ​​of three-phase voltage and three-phase current in the sampled data as input, the instantaneous symmetrical component method is adopted. Through Clarke transformation and symmetrical component transformation, the asymmetrical three-phase voltage is decomposed into positive-sequence voltage component, negative-sequence voltage component and zero-sequence voltage component, and the positive-sequence voltage component is obtained.

[0021] The amplitude of the positive-sequence voltage component at each sampling point is calculated using a rotation operator. The amplitude of the positive-sequence voltage component is taken as the positive-sequence voltage amplitude. The positive-sequence voltage amplitude is compared with a voltage drop threshold set based on the rated voltage. When the positive-sequence voltage amplitude is lower than the voltage drop threshold, the system is determined to enter a fault state. Based on the amplitude relationship of the positive-sequence voltage component, the negative-sequence voltage component, and the zero-sequence voltage component, the fault is classified into symmetrical fault, asymmetrical fault, and open-circuit fault.

[0022] Preferably, the steps of using a current vector proportional limiting strategy for orthogonal axis decoupling include:

[0023] The instantaneous values ​​of the three-phase current of the M3C frequency converter station are converted into direct-axis current components and quadrature-axis current components in a rotating coordinate system through Clarke transformation and Park transformation, and the direct-axis current components and the quadrature-axis current components are used to form a current vector.

[0024] The magnitude of the current vector is calculated based on the current vector, and the magnitude of the current vector is compared with a preset current limit. When the magnitude of the current vector is less than the preset current limit, the direct-axis current component and the quadrature-axis current component remain unchanged. When the magnitude of the current vector is greater than the preset current limit, the direct-axis current component and the quadrature-axis current component are scaled proportionally by the ratio of the preset current limit to the magnitude of the current vector, so that the scaled current vector magnitude does not exceed the preset current limit.

[0025] Preferably, the active current command is dynamically limited in conjunction with the capacitor voltage safety domain to achieve fault current limiting during low-voltage ride-through, including:

[0026] Define a capacitor voltage safety range for all M3C submodule capacitor voltages; the capacitor voltage safety range includes a rated operating range, a warning range, and a danger range, wherein the upper limit of the rated operating range is the upper limit of the capacitor voltage allowed for long-term operation, and the upper limit of the danger range is the critical value for equipment protection;

[0027] The average capacitor voltage of all M3C sub-modules is used as the average capacitor voltage, and the average capacitor voltage is used as the input to construct a dynamic current command limiting function;

[0028] When the average capacitor voltage is within the rated operating range, the corresponding maximum allowable active current limit is output. When the average capacitor voltage enters the warning range or the danger range, the active current limit is gradually reduced as the average capacitor voltage increases.

[0029] The active current limit value output by the dynamic limiting function is applied to the active current command to dynamically limit the active current command, so that the M3C frequency converter can meet both the fault current limiting requirement and the capacitor voltage safety constraint during the low voltage ride-through period.

[0030] Preferably, for the wind farm, the wind turbine is switched from maximum power point tracking to constant voltage support mode according to the positive sequence voltage amplitude, reactive current commands are issued in segments according to the positive sequence voltage amplitude, and a DC voltage fluctuation influence factor is introduced to limit the active current command amplitude, including:

[0031] When the positive sequence voltage amplitude is lower than the preset voltage support start-up threshold, the control target of the wind turbine is switched from maximum power point tracking control to transient voltage support mode, and the PCC point voltage is monitored in real time and the positive sequence voltage amplitude is calculated.

[0032] Based on the position of the positive sequence voltage amplitude within the per-unit value range with the rated voltage as the reference, the segmented current command is calculated and used as the reactive current command of the wind turbine. In the normal operation zone, the reactive current command is close to the dispatch setting value, and in the voltage drop zone, it increases as the positive sequence voltage amplitude decreases. In the deep voltage drop, the reactive current command reaches the preset upper limit.

[0033] The active current command of the wind turbine is determined by the maximum current limit of the converter, and a DC voltage fluctuation influence factor is introduced based on the deviation between the DC voltage of the wind farm and the rated DC voltage to limit the active current command so that the active current command decreases accordingly when the DC voltage fluctuation increases.

[0034] Preferably, a segmented reactive current command is constructed using an impedance strength factor and logic coefficients adaptively adjusted according to system strength, including:

[0035] The per-unit values ​​of the positive sequence voltage amplitude and fault current at the grid connection point are calculated based on the photovoltaic electromechanical transient simulation model, the system reference impedance, and the system short-circuit capacity, and the impedance strength factor is determined.

[0036] Based on the impedance strength factor, a logic coefficient is set to be adaptively adjusted according to the system strength. The impedance strength factor and the logic coefficient are substituted into the preset reactive current segmentation command formula to obtain a segmented reactive current command with the positive sequence voltage amplitude at the grid connection point as the independent variable.

[0037] The segmented reactive current command is used as the reactive current command of the photovoltaic grid-connected converter during low voltage ride-through, so that the reactive current output is increased proportionally when the voltage drops slightly, and the reactive current command reaches the upper limit value based on the current rating when the voltage drops deeply.

[0038] Preferably, the active current command is limited by superimposed DC voltage fluctuation influence factors, and the reactive power support current is output through the current inner loop controller, including:

[0039] A DC voltage fluctuation influence factor is constructed based on the difference between the DC side voltage of the photovoltaic grid-connected converter and the rated DC voltage. The photovoltaic active current command is limited by the DC voltage fluctuation influence factor so that the photovoltaic active current command is reduced when the DC side voltage is close to the safe upper limit.

[0040] The limited photovoltaic active current command and the segmented reactive current command are input together into the photovoltaic current inner loop controller to obtain the photovoltaic active current reference value and the photovoltaic reactive current reference value.

[0041] The photovoltaic active current reference value and the photovoltaic reactive current reference value are compared with the actual photovoltaic current. The modulation voltage of the photovoltaic grid-connected converter is adjusted so that the actual photovoltaic current tracks the photovoltaic active current reference value and the photovoltaic reactive current reference value, thereby outputting the reactive power support current.

[0042] Preferably, for the SVG on the grid side, based on a constant AC voltage control strategy, the instantaneous values ​​of the three-phase voltage and three-phase current at the PCC point are used to achieve decoupling of active and reactive currents and coordinated control of the voltage outer loop and current inner loop. The SVG modulation voltage and reactive current commands are calculated to compensate for reactive power deficits and suppress transient overvoltages, including:

[0043] Under a constant AC voltage control strategy, the instantaneous three-phase voltage values ​​at the SVG grid connection point PCC are collected in real time and the effective voltage value is calculated. The difference between the effective voltage value and the grid connection point voltage reference value is used as the voltage outer loop input.

[0044] The instantaneous value of the three-phase current at the PCC point is collected in real time. The current at the PCC point is compared with the reactive current reference value determined by the reactive current command to obtain the current deviation as the current inner loop input. The active current reference value and reactive current reference value are obtained by decoupling the active current and reactive current.

[0045] The outer voltage loop outputs a reactive current reference value based on the voltage deviation, and the inner current loop calculates the SVG modulation voltage based on the reactive current reference value. The SVG modulation voltage is then used to drive the SVG to output the corresponding reactive current, thereby compensating for the reactive power deficit at the PCC point and pulling the PCC point voltage back and stabilizing it within a preset range.

[0046] Preferably, the instantaneous symmetrical component method is used to obtain the positive sequence voltage component from the instantaneous three-phase voltage values ​​in the sampled data, and the amplitude of the positive sequence voltage component is calculated as the positive sequence voltage amplitude. When the positive sequence voltage amplitude is lower than the voltage drop threshold, a fault is determined and the fault is classified. The method further includes:

[0047] The voltage transient conditions are divided into three voltage ranges based on the ratio of the positive sequence voltage amplitude to the rated voltage; wherein, the voltage ranges are: normal operation range, voltage drop range, and deep voltage drop range;

[0048] A set of control parameters is pre-defined for each voltage range. The set of control parameters includes current vector proportional limiting parameters for the M3C frequency converter, segmented reactive current command parameters for the wind farm, segmented reactive current command parameters for the photovoltaic power station, and reactive current command parameters for the SVG.

[0049] When the system is determined to be in a fault state, the corresponding set of control parameters is selected according to the voltage range in which the current positive sequence voltage amplitude is located. The parameters in the set of control parameters are used for the proportional current vector limiting of the M3C frequency converter, the transient voltage support control of the wind farm, the segmented reactive current command construction of the photovoltaic power station, and the constant AC voltage control of the SVG, thereby realizing the coordinated transient reactive power support of the M3C frequency converter, the wind farm, the photovoltaic power station, and the SVG.

[0050] The present invention discloses the following technical effects:

[0051] This invention achieves a unified measurement foundation for flexible low-frequency renewable energy aggregation systems by uniformly acquiring and synchronously processing the instantaneous values ​​of three-phase voltage and three-phase current from wind farms, photovoltaic power plants, M3C frequency converters, and SVG. This avoids the voltage judgment lag and fault identification deviations caused by independent sampling and inconsistent time bases of existing technologies. The positive-sequence voltage component obtained based on the instantaneous symmetrical component method accurately reflects the system fault depth, providing a clear and consistent quantitative basis for the generation of current commands from subsequent devices, thus improving fault identification accuracy and stability.

[0052] The proposed M3C frequency converter's proportional current vector limiting strategy and dynamic active current command limiting method based on capacitor voltage safety domain solve the problem of fixed current limiting strategies and inability to consider DC link stability during low-voltage ride-through in traditional frequency converters. By coordinating the scaling of direct-axis and quadrature-axis current components, the frequency converter can ensure that the current does not exceed the limit and that the module capacitor voltage remains within a safe range during external power grid transient faults, thereby significantly enhancing the transient support capability and operational safety of the M3C frequency converter.

[0053] This invention proposes a segmented reactive current command based on positive sequence voltage components and an active current limiting strategy based on DC voltage fluctuations for wind turbines. This effectively overcomes the technical shortcomings of existing methods, such as delayed switching from MPPT to voltage support mode, coarse segmentation of wind farm support curves, and difficulty in balancing active power output and DC voltage safety. By dynamically adjusting the reactive power support strength according to the voltage drop depth, the wind farm can respond according to the optimal support curve in light, medium, and severe voltage drop ranges, significantly improving the voltage recovery speed.

[0054] This invention addresses the issue of segmented reactive current commands for photovoltaic (PV) power plants based on impedance strength factors and adaptive logic coefficients, superimposed with DC voltage fluctuation factors. This enables PV units to possess voltage support capabilities that match the characteristics of both strong and weak grids, resolving the problem of "excessive support for strong grids and insufficient support for weak grids" in existing LVRT strategies. Its adaptive adjustment mechanism ensures that the PV converter outputs reasonable reactive power support under different grid strengths and voltage drop depths, improving the stability and adaptability of PV power plants in flexible low-frequency collection scenarios.

[0055] This invention unifies the constant AC voltage control of the SVG, the dynamic reactive power commands of wind farms and photovoltaic power stations, and the current limiting mechanism of the M3C frequency converter station under a collaborative framework based on positive sequence voltage amplitude. This achieves hierarchical collaborative voltage control between the renewable energy source and the collection station, overcoming the problems of fragmented strategies, asynchronous responses, and mismatched support quantities among devices in the prior art. The SVG's rapid reactive power compensation capability during voltage transients complements the dynamic reactive power support capability of wind and solar power stations, enabling rapid suppression of voltage fluctuations at the PCC point and significantly improving the overall low-voltage ride-through and voltage stability levels of the system. Attached Figure Description

[0056] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0057] Figure 1 A flowchart of the method provided in an embodiment of the present invention;

[0058] Figure 2 This is a schematic diagram of the technical route provided for an embodiment of the present invention. Detailed Implementation

[0059] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0060] The purpose of this invention is to provide a method for supporting transient reactive voltage in a flexible low-frequency collection and transmission system for new energy sources. By unifying the fault identification basis and hierarchical dynamic current commands, it enables coordinated response of various new energy devices during low-voltage ride-through, thereby significantly improving the voltage stability of the collection system.

[0061] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0062] Figure 1 The method flowchart provided in the embodiments of the present invention is as follows: Figure 1 As shown, this invention provides a method for supporting transient reactive voltage in a flexible low-frequency collection and transmission system for new energy sources, comprising:

[0063] Step 100: Synchronously collect the instantaneous values ​​of three-phase voltage and three-phase current at the PCC point of the wind farm and photovoltaic power station, the low-frequency side and power frequency side of the M3C frequency converter station, and the SVG access point to form sampling data;

[0064] Step 200: The instantaneous symmetrical component method is used to obtain the positive sequence voltage component from the instantaneous three-phase voltage values ​​in the sampled data, and the amplitude of the positive sequence voltage component is calculated as the positive sequence voltage amplitude. When the positive sequence voltage amplitude is lower than the voltage drop threshold, a fault is determined and the fault is classified.

[0065] Step 300: For the M3C frequency converter station, a current vector proportional limiting strategy is adopted based on orthogonal axis decoupling, and the active current command is dynamically limited in combination with the capacitor voltage safety domain to achieve fault current limiting during low voltage ride-through.

[0066] Step 400: For wind farms, switch the wind turbine from maximum power point tracking to constant voltage support mode according to the positive sequence voltage amplitude, issue reactive current commands in segments according to the positive sequence voltage amplitude, and introduce a DC voltage fluctuation influence factor to limit the active current command amplitude.

[0067] Step 500: For photovoltaic power plants, based on the photovoltaic electromechanical transient simulation model, a segmented reactive current command is constructed using the impedance strength factor and logic coefficients that are adaptively adjusted according to the system strength. The active current command is limited by the DC voltage fluctuation influence factor and then output as reactive support current through the current inner loop controller.

[0068] Step 600: For the grid-side SVG, based on the constant AC voltage control strategy, the instantaneous values ​​of the three-phase voltage and the three-phase current at the PCC point are used to achieve decoupling of active and reactive currents and coordinated control of the voltage outer loop and the current inner loop. The SVG modulation voltage and reactive current commands are calculated to compensate for reactive power deficits and suppress transient overvoltages.

[0069] As an optional implementation method, the technical approach of this embodiment is as follows: Figure 2 As shown, it includes the following steps:

[0070] S1. Synchronously sample the voltage and current signals of the new energy collection station grid connection point, frequency converter station and grid test;

[0071] By using voltage transformers and current transformers, the sampling frequency is adjusted in real time. (like Simultaneously sample the three-phase instantaneous voltage and three-phase instantaneous current at each point of the PCC point of the wind farm and photovoltaic power station, both sides of the M3C frequency converter station (low frequency side and power frequency side), and the SVG access point, and calculate the instantaneous value expressions of voltage and current based on the sampled data;

[0072] Among them, three-phase voltage The instantaneous value expression is:

[0073] ;

[0074] In the formula, Voltage amplitude, Angular frequency, The initial phase of the voltage. These are the instantaneous values ​​of the three-phase voltage;

[0075] Current The instantaneous value expression is:

[0076] ;

[0077] In the formula, The current amplitude, Angular frequency, The initial phase of the current. This represents the instantaneous value of the three-phase current.

[0078] S2. Set balanced and unbalanced faults on the low-frequency line side and the M3C power frequency side, and perform rapid fault detection calculation based on the instantaneous symmetrical component method.

[0079] The instantaneous symmetrical component method is employed to directly calculate the sequence components by constructing rotating phasors. After acquiring the instantaneous three-phase values, Clarke transform (αβ0 transform) and symmetrical component transform are performed to decompose the asymmetrical three-phase quantities into independent positive-sequence, negative-sequence, and zero-sequence systems. The mathematical transformation formula is as follows:

[0080] ;

[0081] Where F represents voltage U or current I. As a rotation operator, the positive, negative, and zero-sequence components of the voltage and current at each sampling point can be obtained.

[0082] Set voltage drop threshold (Typically 0.9 pu). If If the system is deemed to have entered a fault state, the subsequent fault handling process will be initiated. This represents the amplitude of the positive sequence component of the voltage.

[0083] Subsequently, through the inverse Clarke transform and the Park transform, the dq-axis components of the positive-sequence, negative-sequence, and zero-sequence voltages in the rotating coordinate system can be obtained. Fault classification: Symmetrical faults will cause negative sequence voltage. Zero-sequence voltage However, the positive sequence voltage drops significantly; asymmetrical faults will cause the negative sequence voltage and zero sequence voltage to increase significantly; open circuit faults manifest as severe voltage imbalance, which can be judged based on the characteristics of the sequence components.

[0084] S3. Design transient reactive power support control strategies for reactive power sources such as wind farms, photovoltaic power plants, and M3C frequency converter stations, including the following steps:

[0085] S31. To address the specific control requirements of the M3C frequency converter station during a fault, a low-frequency ride-through control is designed, which adopts a proportional limiting strategy based on dq axis decoupling.

[0086] This strategy first converts the three-phase AC quantities into DC components in a rotating coordinate system using Clarke and Park transformations. The current vector magnitude is then calculated and compared to a preset limit.

[0087] If the current vector magnitude is less than the preset limit, maintain the current command;

[0088] If the current vector magnitude is less than a preset limit, the d-axis and q-axis currents are scaled proportionally to ensure that the current vector magnitude is strictly limited after scaling, while preserving the original d / q axis ratio. Specifically, the following steps are included:

[0089] S311. Decouple the M3C frequency converter station current signal acquired by S1 using Clarke and Park transforms, and calculate the current d-axis current component Id and q-axis current component Iq. The mathematical transformation formula is as follows:

[0090] ;

[0091] ;

[0092] in, , For the current in The decoupling value of the current signal in the direction of the signal. Here are the three-phase current values, id is the current d-axis current component, and iq is the q-axis current component.

[0093] S312. Calculate the magnitude of the current vector based on the d-axis and q-axis current components:

[0094] ;

[0095] in, Id is the magnitude of the current current vector, Iq is the current d-axis current component, and Id is the current q-axis current component.

[0096] S313. Perform current limiting judgment. When the current amplitude exceeds the maximum allowable current... At that time, the limiter causes the d-axis and q-axis currents to decrease proportionally, and the preset current limit value of the converter station is... Where IN is the system rated current. , These are the limiting values ​​for the d-axis and q-axis, respectively.

[0097] Will With the preset current limit Comparison, among which :

[0098] like Keep the current d-axis and q-axis current commands unchanged;

[0099] like This triggers proportional amplitude limiting control, which means entering step S314.

[0100] S314. Perform proportional amplitude limiting control:

[0101] When the current amplitude exceeds the limit Limit the amplitude using the following formula:

[0102] ;

[0103] in, This refers to the low-frequency side output voltage amplitude. This is the d-axis current command after limiting. This is the q-axis current limit value. All parameters are per-unit values, and their base value is the system rating.

[0104] For d-axis current command To address power redundancy in the M3C system during power frequency side faults, this paper proposes a dynamic current command limiting strategy based on the capacitor voltage safety domain to be introduced into the existing low-voltage ride-through control architecture. The capacitor voltage safety domain is defined for the M3C as: the rated operating range. Warning range and dangerous areas .in, This is the upper limit of the capacitor voltage that allows for long-term operation, and it is usually set to 1.05 to 1.1 times the rated voltage. This is the critical value for equipment protection, typically 1.15 to 1.2 times the rated voltage. Once this value is exceeded, the hardware protection circuit may activate within milliseconds.

[0105] Next, design a dynamic limiting function. ,in This is the average value of the capacitor voltages of all M3C submodules, which can be obtained in real time through the existing voltage equalization control system.

[0106] The mathematical expression for this function is as follows:

[0107]

[0108] in, This is the upper limit of the capacitor voltage that allows for long-term operation. It is the critical value for equipment protection. This is the average value of the capacitor voltages of all submodules in the M3C. The maximum active current is the final, improved active current command. It will be determined by both traditional instructions and dynamic constraints, and its expression is:

[0109] ;

[0110] in, This is the upper limit of the capacitor voltage that allows for long-term operation. It is the critical value for equipment protection. This is the average value of the capacitor voltages of all submodules in the M3C. This is the maximum value of the active current. This is a dynamic limiting function;

[0111] S32. To address the voltage support requirements of wind farms during fault periods, a low-frequency ride-through control design is implemented. This design employs a strategy where, during a fault, the wind farm's control target switches from maximum power point tracking (MPPT) to transient voltage support mode. This mode prioritizes system stability, actively reducing or even completely sacrificing active power output, and utilizing all or as needed the remaining capacity of the converter for reactive power output to help raise the PCC voltage. Specifically, the design includes the following steps:

[0112] S321. Monitor the voltage at the PCC point in real time and calculate its positive sequence voltage amplitude. (at rated voltage) (Based on per-unit values).

[0113] S322. Perform segmented current command calculation, i.e., reactive current. .

[0114] ;

[0115] in, The positive sequence voltage amplitude at the grid connection point. Generally taken , This is the rated current.

[0116] when At that time, that is, in the normal operating range, The voltage support start-up threshold is typically set to 0.9 pu. When the system voltage is normal, no additional support is needed. At this time, the reactive current command is set according to the normal operation settings, usually 0 or the dispatch given value.

[0117] when , The depth support threshold is typically set to 0.2 pu, indicating a voltage sag. The reactive current command increases linearly with the voltage sag depth, with the slope determined by the gain coefficient K. This coefficient is based on the converter's maximum allowable current. Configure and ensure Within safe limits.

[0118] ;

[0119] in, , .

[0120] when When this occurs, it indicates a serious fault and a deep voltage drop. At this time, the reactive current command reaches its upper limit. To provide maximum reactive power support.

[0121] ;

[0122] S323, Issue active current command calculate.

[0123] To increase the safety margin under high reactive power output conditions, a DC voltage fluctuation impact factor is introduced:

[0124] ;

[0125] Right now:

[0126] ;

[0127] in, This is a q-axis current command. This is the maximum allowable current value. .

[0128] S324, the calculated result and The current inner loop controller input to the wind turbine generates a PWM modulation signal, which drives the converter to output the required reactive power support current, thereby achieving rapid and adaptive adjustment of the PCC voltage.

[0129] S33. To address the voltage support requirements of photovoltaic power plants during fault periods, a low-frequency ride-through (LVRT) control design is implemented. This design employs a low-voltage ride-through (LVRT) active and reactive power control process and current limiting strategy based on a photovoltaic electromechanical transient simulation model, specifically including the following steps:

[0130] S331. Monitor the voltage at the PCC point of the photovoltaic power station in real time and calculate its positive sequence voltage amplitude. With fault current (at rated voltage) (Based on per-unit values).

[0131] S332. Determine the impedance factor With adaptive logic coefficients Then, segmented current command calculations are performed, i.e., reactive current. This ensures that the deeper the voltage drop, the greater the reactive current output, with a maximum not exceeding 1.1 times the rated current.

[0132]

[0133]

[0134]

[0135] in, The positive sequence voltage amplitude at the grid connection point. The fault resistor, For fault current, Rated voltage, The system reference impedance, For system short-circuit capacity, This is the impedance intensity factor.

[0136] Then set the logic coefficients that adaptively adjust based on system strength. :

[0137]

[0138] in It is the impedance intensity factor. These are logic coefficients that are adaptively adjusted based on system strength.

[0139] The final formula is:

[0140] ;

[0141] in It is the impedance intensity factor. K is typically set to 1.5, serving as a logic coefficient that adaptively adjusts based on system strength. This is the rated current.

[0142] when At that time, that is, in the normal operating range, The voltage support start-up threshold is typically set to 0.9 pu. When the system voltage is normal, no voltage support is needed. In this case, the reactive current command is set according to the normal operation settings.

[0143] when , The depth support threshold is typically set to 0.5 pu, indicating a voltage drop. The reactive current command increases linearly with the voltage drop depth, and the slope is determined by the gain coefficient K. This coefficient is usually set to... ,default ,make sure Within safe limits.

[0144] ;

[0145] in, This is the rated current of the photovoltaic inverter.

[0146] when When this occurs, it indicates a serious fault and a deep voltage drop. In this case, the current should be 1.1 times the rated current to provide maximum reactive power support.

[0147] ;

[0148] S323, Issue active current command calculate.

[0149] The current is determined by the maximum current limit of the converter, and the influence factor of DC voltage fluctuation is introduced:

[0150] ;

[0151] Right now:

[0152] ;

[0153] Among them, among them, This is a q-axis current command. This is the maximum allowable current value. .

[0154] S324, the calculated result and After being introduced into the photovoltaic power plant's current inner loop controller, the controller generates a PWM modulation signal to drive the converter to output the required reactive power support current, ultimately achieving rapid and adaptive regulation of the PCC voltage.

[0155] S4. To address the voltage support requirements of the power grid during a fault, reactive power compensation is performed. This employs a constant AC voltage control strategy, including the following steps:

[0156] S41: Perform voltage measurement. Real-time acquisition of the instantaneous three-phase voltage values ​​at the SVG grid connection point (PCC point). And calculate its effective value or use it directly. The voltage component in the coordinate system is controlled.

[0157] S42: Construct a mathematical model for constant voltage control.

[0158] For ease of analysis, this paper assumes that the SVG only outputs reactive power. First, define the state variables:

[0159] ;

[0160] In the formula: This is the difference between the converter's DC voltage and its rated DC voltage. This is the difference between the voltage amplitude at the PCC point and the rated voltage amplitude at the PCC point. This is the difference between the active and reactive components of the output current and the rated current. DC side voltage This is the reference value for the DC side voltage. The positive sequence voltage amplitude at the grid connection point. This is the reference value for the grid connection point voltage. This refers to the active current on the power frequency side. This is the reference value for the active current on the power frequency side. This refers to the reactive current on the power frequency side. This is the reference value for reactive current on the power frequency side.

[0161] S43: Perform active and reactive current design.

[0162] Traditional active and reactive current decoupling is as follows:

[0163] ;

[0164] To achieve coordinated optimization of the voltage outer loop and current inner loop and to coordinate across multiple time scales, a nonlinear element is introduced:

[0165] ;

[0166] in, These are the proportional-integral parameters for the voltage loop. This is the difference between the converter's DC voltage and its rated DC voltage.

[0167] S44: Calculate the modulation voltage.

[0168] ;

[0169] ;

[0170] in, The active and reactive components of the modulated voltage; The phase-locked angle of the phase-locked loop. For the proportional-integral parameters of the current loop, This refers to the active current on the power frequency side. This is the reference value for the active current on the power frequency side. This refers to the reactive current on the power frequency side. This is the reference value for reactive current on the power frequency side. It is the difference between the active and reactive components of the output current and the rated current.

[0171] S45: Perform reactive current output and execution.

[0172] The obtained reactive current command is used as the setpoint for the inner loop current control. The control system drives the IGBT bridge arm through PWM modulation, forcing the SVG to output the corresponding reactive current, thereby dynamically compensating for the reactive power deficit of the system and pulling the PCC point voltage back and stabilizing it nearby.

[0173] Furthermore, throughout the entire testing process, the test data is stored in local storage units (such as hard drives) or uploaded to a remote monitoring center in real time, so that maintenance personnel can perform subsequent analysis and troubleshooting.

[0174] Therefore, the present invention adopts the above-mentioned method for supporting transient reactive voltage in a flexible low-frequency collection and transmission system for new energy sources. By integrating new energy sources and M3C low-frequency crossing with grid-side SVG dynamic reactive power support, the voltage stability and crossing reliability of the flexible low-frequency transmission system during faults are significantly enhanced, and the adaptability and robustness under multi-energy collection scenarios are improved.

[0175] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0176] This document uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. Furthermore, those skilled in the art will recognize that, based on the ideas of the present invention, there will be changes in the specific implementation methods and application scope. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for supporting transient reactive voltage of a new energy flexible low-frequency collection and sending-out system, characterized in that, The method comprises the following steps: Synchronously collecting three-phase voltage instantaneous values and three-phase current instantaneous values of PCC points of wind farms and photovoltaic power stations, low-frequency sides and power-frequency sides of M3C frequency conversion stations, and SVG access points to form sampling data; Using the instantaneous symmetrical component method on the three-phase voltage instantaneous values in the sampling data to obtain voltage positive sequence components, and calculating the voltage positive sequence component amplitudes as positive sequence voltage amplitudes, when the positive sequence voltage amplitudes are lower than voltage drop thresholds, determining that a fault is entered and classifying the fault; For the M3C frequency conversion station, using the current vector equal-proportion amplitude limiting strategy based on direct-quadrature axis decoupling, and combining the capacitor voltage safety domain to dynamically limit the active current instruction, fault current limiting during low-voltage ride-through is realized; For the wind farm, according to the positive sequence voltage amplitudes, wind turbine generators are switched from maximum power point tracking to constant voltage support mode, reactive current instructions are issued according to the positive sequence voltage amplitudes, and a direct current voltage fluctuation influence factor is introduced to limit the active current instruction; For the photovoltaic power station, based on a photovoltaic electromechanical transient simulation model, a segmented reactive current instruction is constructed by using an impedance strength factor and a logic coefficient that is adaptively adjusted according to system strength, and a direct current voltage fluctuation influence factor is superimposed to limit the active current instruction, and a reactive support current is output through a current inner loop controller; For the SVG on the grid side, based on a constant alternating voltage control strategy, the three-phase voltage instantaneous values and the three-phase current instantaneous values of the PCC points are used to realize active and reactive current decoupling and voltage outer loop and current inner loop coordinated control, calculate the SVG modulation voltage and the reactive current instruction, to compensate for the reactive power shortage and suppress transient overvoltage.

2. The method according to claim 1, wherein, Synchronously collecting three-phase voltage instantaneous values and three-phase current instantaneous values of PCC points of wind farms and photovoltaic power stations, low-frequency sides and power-frequency sides of M3C frequency conversion stations, and SVG access points to form sampling data, comprising: Real-time collecting three-phase voltage instantaneous values and three-phase current instantaneous values of PCC points of wind farms and photovoltaic power stations, low-frequency sides and power-frequency sides of M3C frequency conversion stations, and SVG access points by using voltage transformers and current transformers; Synchronously sampling the three-phase voltage instantaneous values and the three-phase current instantaneous values through a preset sampling frequency according to the sampled three-phase voltage instantaneous values and the three-phase current instantaneous values, and calculating voltage instantaneous value expressions and current instantaneous value expressions; wherein the voltage instantaneous value is characterized by a voltage amplitude, an angular frequency and a voltage initial phase, and the current instantaneous value is characterized by a current amplitude, an angular frequency and a current initial phase; The samples corresponding to the voltage instantaneous value expressions and the current instantaneous value expressions constitute the sampling data.

3. The method of claim 1, wherein the new energy flexible low frequency collection and transmission system transient state reactive voltage support method is characterized by, Using the instantaneous symmetrical component method on the three-phase voltage instantaneous values in the sampling data to obtain voltage positive sequence components, and calculating the voltage positive sequence component amplitudes as positive sequence voltage amplitudes, when the positive sequence voltage amplitudes are lower than voltage drop thresholds, determining that a fault is entered and classifying the fault, comprising: Taking the three-phase voltage instantaneous values and the three-phase current instantaneous values in the sampling data as inputs, using the instantaneous symmetrical component method, through Clarke transformation and symmetrical component transformation, decomposing the asymmetric three-phase voltage into voltage positive sequence components, voltage negative sequence components and voltage zero sequence components, and obtaining the voltage positive sequence components; The amplitude of the voltage positive sequence component of each sampling point is calculated by using a rotation operator, the amplitude of the voltage positive sequence component is taken as the positive sequence voltage amplitude, the positive sequence voltage amplitude is compared with a voltage drop threshold set based on a rated voltage, when the positive sequence voltage amplitude is lower than the voltage drop threshold, it is determined that the system enters a fault state, and the fault is divided into symmetric fault, asymmetric fault and open circuit fault according to the amplitude relationship of the voltage positive sequence component, the voltage negative sequence component and the voltage zero sequence component.

4. The method of claim 1, wherein the new energy flexible low frequency gathering and sending system transient state reactive voltage support method is characterized by, The step of adopting the current vector equal-proportion limiting strategy for the direct-quadrature axis decoupling comprises: The Clarke transformation and the Park transformation are used to convert the collected M3C frequency conversion station three-phase current instantaneous value into a direct-axis current component and a quadrature-axis current component in a rotating coordinate system, and the direct-axis current component and the quadrature-axis current component constitute a current vector; The current vector amplitude is calculated according to the current vector, and the current vector amplitude is compared with a preset current limit value, when the current vector amplitude is less than the preset current limit value, the direct-axis current component and the quadrature-axis current component remain unchanged, when the current vector amplitude is greater than the preset current limit value, the direct-axis current component and the quadrature-axis current component are scaled in equal proportion by the ratio of the preset current limit value to the current vector amplitude, so that the scaled current vector amplitude does not exceed the preset current limit value.

5. The method of claim 1, wherein the new energy flexible low frequency gathering and sending system transient state reactive voltage support method is characterized by, The active current instruction is dynamically limited in combination with the capacitor voltage safety domain to realize fault current limiting during low voltage ride through, comprising: A capacitor voltage safety domain is defined for the capacitor voltage of all M3C sub-modules; the capacitor voltage safety domain includes a rated working interval, a pre-warning interval and a dangerous interval, wherein the upper limit of the rated working interval is the upper limit value of the capacitor voltage allowed for long-term operation, and the upper limit of the dangerous interval is the critical value of device protection; The average capacitor voltage is taken as the average value of the capacitor voltage of all M3C sub-modules, and the average capacitor voltage is taken as the input to construct a current instruction dynamic limiting function; When the average capacitor voltage is in the rated working interval, the corresponding maximum allowed active current limit value is output, and when the average capacitor voltage enters the pre-warning interval or the dangerous interval, the active current limit value is gradually reduced with the increase of the average capacitor voltage; The active current limit value output by the dynamic limiting function is applied to the active current instruction to dynamically limit the active current instruction, so that the M3C frequency conversion station meets the fault current limiting requirement and the capacitor voltage safety constraint during low voltage ride through.

6. The method of claim 1, wherein the new energy flexible low frequency gathering and sending system transient state reactive voltage support method is characterized by, For the wind farm, the wind turbine is switched from maximum power point tracking to constant voltage support mode according to the positive sequence voltage amplitude, the reactive current instruction is issued in sections according to the positive sequence voltage amplitude, and the active current instruction is limited by introducing a direct current voltage fluctuation influence factor, comprising: When the positive sequence voltage amplitude is lower than a preset voltage support starting threshold, the control target of the wind turbine is switched from maximum power point tracking control to transient voltage support mode, the PCC point voltage is monitored in real time, and the positive sequence voltage amplitude is continuously calculated; According to the position of the positive sequence voltage amplitude in the interval based on the rated voltage, the segmented current command is calculated, the segmented current command is taken as the reactive current command of the wind turbine, the reactive current command is close to the dispatching set value in the normal operation area, increases with the decrease of the positive sequence voltage amplitude in the voltage drop area, and reaches the preset upper limit when the voltage drops deeply; The active current command of the wind turbine is determined by the maximum current limiting of the converter, and a DC voltage fluctuation influence factor is introduced according to the deviation between the DC voltage of the wind farm and the rated DC voltage, so that the active current command is limited, and the active current command decreases correspondingly when the DC voltage fluctuation increases.

7. The method of claim 1, wherein the new energy flexible low frequency gathering and sending system transient state reactive voltage support method is characterized by, The segmented reactive current command is constructed by using the impedance strength factor and the logic coefficient which is adaptively adjusted according to the system strength, including: The positive sequence voltage amplitude and the fault current at the grid connection point are calculated based on the photovoltaic electromechanical transient simulation model, the system reference impedance and the system short-circuit capacity, and the impedance strength factor is determined; The logic coefficient which is adaptively adjusted according to the system strength is set according to the impedance strength factor, the impedance strength factor and the logic coefficient are substituted into the preset reactive current segmented command formula, and the segmented reactive current command taking the positive sequence voltage amplitude at the grid connection point as the independent variable is obtained; The segmented reactive current command is taken as the reactive current command of the photovoltaic grid-connected converter during low-voltage ride-through, the reactive current output is increased in proportion when the voltage drops slightly, and the reactive current command reaches the upper limit value based on the current rated value when the voltage drops deeply. 8.The method for supporting transient state reactive voltage of a new energy flexible low-frequency collection and transmission system according to claim 7, wherein, The active current command is limited by superimposing the DC voltage fluctuation influence factor, and the reactive support current is output by the current inner loop controller, including: The DC voltage fluctuation influence factor is constructed according to the difference between the DC side voltage of the photovoltaic grid-connected converter and the rated DC voltage, and the photovoltaic active current command is limited by the DC voltage fluctuation influence factor, so that the photovoltaic active current command decreases when the DC side voltage is close to the safety upper limit; The limited photovoltaic active current command and the segmented reactive current command are input into the photovoltaic current inner loop controller to obtain the photovoltaic active current reference value and the photovoltaic reactive current reference value; The photovoltaic active current reference value and the photovoltaic reactive current reference value are compared with the actual photovoltaic current, the modulation voltage of the photovoltaic grid-connected converter is adjusted, the actual photovoltaic current tracks the photovoltaic active current reference value and the photovoltaic reactive current reference value, so that the reactive support current is output. 9.The method for supporting transient state reactive voltage of a new energy flexible low-frequency gathering and sending system according to claim 1, wherein, For the SVG on the grid side, based on the constant AC voltage control strategy, the active and reactive current decoupling and voltage outer loop and current inner loop coordinated control are realized by using the three-phase voltage instantaneous value and three-phase current instantaneous value of the PCC point, the SVG modulation voltage and the reactive current command are calculated to compensate for the reactive power shortage and suppress transient overvoltage, including: Under the constant AC voltage control strategy, the three-phase voltage instantaneous value of the PCC point connected with the SVG is collected in real time, the voltage effective value is calculated, and the difference between the voltage effective value and the voltage reference value of the grid connection point is taken as the voltage outer loop input; Real-time acquisition of PCC point three-phase current instantaneous value, comparison of PCC point current with reactive current reference value determined by the reactive current instruction, current deviation as current inner loop input, and active current reference value and reactive current reference value obtained by active current and reactive current decoupling; The voltage outer loop outputs the reactive current reference value according to the voltage deviation, the current inner loop calculates the SVG modulation voltage according to the reactive current reference value, and the corresponding reactive current is output by the SVG driven by the SVG modulation voltage, so that the PCC point reactive power shortage is compensated and the PCC point voltage is pulled back and stabilized in the preset range.

10. The method of claim 3, wherein the new energy flexible low frequency collection and transmission system transient state reactive voltage support method is characterized by, The three-phase voltage instantaneous value in the sampling data is obtained by using the instantaneous symmetrical component method, and the voltage positive sequence component is obtained, and the voltage positive sequence component amplitude is calculated as the positive sequence voltage amplitude, when the positive sequence voltage amplitude is lower than the voltage drop threshold, it is determined to enter the fault and the fault classification is carried out, and further comprising: According to the ratio of the positive sequence voltage amplitude to the rated voltage, the voltage transient state is divided into three voltage intervals; wherein the voltage interval is: normal operation area, voltage drop area and deep voltage drop area; A set of control parameters is set for each voltage interval, including current vector equal proportion limiting parameter for M3C frequency conversion station, segmented reactive current instruction parameter for wind farm, segmented reactive current instruction parameter for photovoltaic power station and reactive current instruction parameter for SVG; When it is determined that the system enters the fault state, the corresponding control parameter set is selected according to the voltage interval where the current positive sequence voltage amplitude is located, and the parameters in the control parameter set are used for current vector equal proportion limiting of M3C frequency conversion station, transient voltage support control of wind farm, segmented reactive current instruction construction of photovoltaic power station and SVG constant AC voltage control, so as to realize the coordinated transient reactive power support of M3C frequency conversion station, wind farm, photovoltaic power station and SVG.

Images