Fault ride-through control method and system based on photovoltaic power generation grid-connected system

Abstract

The application provides a fault ride-through control method and system based on a photovoltaic power generation grid-connected system, and the method comprises the following steps: establishing a model of the photovoltaic power generation grid-connected system, setting an inverter circuit at a rear stage of a DC bus, and setting two DC / DC conversion circuits at a front stage of the DC bus; judging whether a low-voltage ride-through occurs on a grid-connected side of the photovoltaic power generation grid-connected system, detecting voltage drop depth and photovoltaic power high-low degree; according to the detected voltage drop depth and photovoltaic power high-low degree, different control strategies are adopted in different scenes to realize fault ride-through. The application solves the technical problems that the photovoltaic power generation grid-connected system is prone to off-grid operation or slow response in an asymmetric fault condition when the grid voltage drop occurs, is easily affected by a negative sequence component, and the response and multi-target control requirements of the system in different fault scenes are not fully considered, resulting in insufficient fault ride-through capability.

Description

Technical Field

[0001] This invention relates to the field of power systems and their control technology, specifically to a fault ride-through control method and system based on a photovoltaic power generation grid-connected system. Background Technology

[0002] With the rapid development of distributed power sources and microgrids, grid-connected photovoltaic (PV) power generation systems have become an important component of renewable energy. However, as a clean and renewable energy solution, the intermittent, random, and fluctuating nature of PV output, leading to power quality issues such as grid voltage fluctuations, has become a critical problem that urgently needs to be addressed.

[0003] Furthermore, asymmetrical faults frequently occur during actual grid operation, leading to asymmetrical voltage drops at the grid connection point of photovoltaic (PV) power generation systems. In this situation, on the one hand, the poor tolerance of power electronic device interfaces may cause the PV power generation system to disconnect from the grid, thus affecting the effective utilization of renewable energy; on the other hand, the presence of negative sequence components can cause fluctuations in the output power of the PV power generation system, thereby triggering second harmonics in the DC-side voltage of the inverter. Therefore, it is urgent to research a control strategy that can ensure the PV power generation system does not disconnect from the grid under asymmetrical fault conditions while eliminating the influence of negative sequence components. Currently, research on inverters mainly focuses on: supplying reactive power to the grid to support voltage, supplying active power to reduce instantaneous power imbalance, suppressing active power oscillations to reduce DC bus capacitor voltage oscillations, and limiting the inverter output current, but it does not fully consider the real-time changes in PV power and the simultaneous achievement of these objectives.

[0004] The existing invention patent application document CN116742691A, entitled "A Method and Device for Implementing Fault Ride-through Control Strategy in a Photovoltaic-Storage Combined Generation System," describes a method where, under normal grid-connected operation of the photovoltaic-storage combined generation system, the grid-connected inverter employs a pre-set maximum power point tracking control strategy, and the energy storage converter employs a constant power control strategy to achieve stable operation. In the event of an asymmetrical fault in the grid-connected photovoltaic-storage combined generation system, the grid-connected inverter outputs active and reactive power based on actual reactive current reference values ​​and actual active current reference values ​​to achieve fault ride-through; the energy storage converter outputs active and reactive power based on its actual reactive current reference values ​​and actual active current reference values ​​to support fault ride-through. Furthermore, a negative-sequence voltage compensation control strategy is employed to reduce the impact of PCC (Point of Common Coupling) negative-sequence voltage on the grid-connected inverter. However, this existing solution does not completely eliminate the negative-sequence component's influence, has limited fault ride-through capability, and lacks a fast enough response speed to voltage dips, dynamic adjustment capability, and sufficient consideration for equipment lifespan and system losses.

[0005] The existing invention patent application document with publication number CN114784866A, entitled "An Overall Coordinated Low Voltage Ride-Through Control Method and Its Application", includes the following methods: combining the mechanism of reactive power supporting voltage, analyzing the influence mechanism of reactive power output of multiple DC / AC converter stations on the port voltage of near-fault DC / AC converter stations; based on the above, adopting a two-stage coordinated control strategy, and performing generalized setting of reactive current output of DC / AC converter stations on the basis of refining the coordination mechanism in different scenarios.

[0006] However, the existing solution has limited applicability, lacks processing of negative sequence components, has a high degree of complexity in its control strategy, does not fully consider the dynamic changes in photovoltaic power, and has insufficient consideration for equipment protection.

[0007] The existing invention patent application document CN117791480A, entitled "Equivalent Modeling Method and System for Asymmetric Fault Short Circuit Calculation of New Energy Units," includes the following methods: For new energy units employing a negative-sequence current suppression control strategy, a negative-sequence current suppression type new energy equivalent model is constructed based on the correspondence between the positive-sequence voltage drop and the positive-sequence current output of the new energy unit, as well as the positive-sequence voltage axis component at the generator terminal; For new energy units employing a positive-negative-sequence coupled control strategy, a positive-negative-sequence coupled control type new energy equivalent model is constructed based on the control priority of the positive-sequence and negative-sequence components during fault ride-through, as well as the correspondence between sequence voltage and sequence current; For new energy units employing a positive-negative-sequence decoupling control strategy, a positive-negative-sequence decoupling control type new energy equivalent model is constructed based on the correspondence between the positive-sequence voltage and positive-sequence current, and the correspondence between negative-sequence voltage and negative-sequence current.

[0008] However, the existing scheme has high complexity and computational cost, lacks dynamic adaptability, does not fully handle negative order components, does not adequately consider equipment protection and system economy, and has limited applicability.

[0009] The method in the existing invention patent application with publication number CN117791480A is mainly for asymmetrical fault short-circuit calculation and is applicable to the field of relay protection setting calculation. However, its applicability to other problems in photovoltaic power generation grid-connected systems, such as voltage fluctuations and power balance, is insufficient.

[0010] The existing invention patent application document with publication number CN110071528A, entitled "A Method for Constructing a Fault Model of Inverter-Grid-Connected Distributed Power Generation", includes the following steps: establishing a fault ride-through control model representing the fault ride-through strategy based on single or multiple scenarios set by the fault ride-through control strategy; dividing the inverter model into independent model stages under each operating condition according to the differences in operating conditions, and establishing an inverter characteristic model representing the operating characteristics of the inverter; and combining the fault ride-through control model and the multi-stage inverter characteristic model to obtain a multi-stage inverter-grid-connected distributed power generation symmetrical fault model.

[0011] However, the existing scheme has the following technical defects: high model complexity and computational cost, lack of dynamic adaptability, and insufficient handling of negative-order components.

[0012] In summary, existing technologies suffer from several technical problems: slow response to grid voltage dips, leading to grid disconnection or delayed response of photovoltaic power generation grid-connected systems under asymmetrical fault conditions; susceptibility to negative sequence components; and insufficient fault ride-through capability due to failure to fully consider system response and multi-objective control requirements under different fault scenarios. Summary of the Invention

[0013] The technical problem to be solved by the present invention is: how to solve the technical problems in the prior art where the grid voltage drop is slow to respond, causing the photovoltaic power generation grid-connected system to disconnect from the grid or respond slowly in the case of asymmetrical faults, is easily affected by negative sequence components, and fails to fully consider the system's response and multi-objective control requirements under different fault scenarios, resulting in insufficient fault ride-through capability.

[0014] This invention solves the above-mentioned technical problems by employing the following technical solution: a fault ride-through control method based on a photovoltaic power generation grid-connected system includes:

[0015] S1. Establish a system model of the photovoltaic power generation grid-connected system, and set the system model to an inverter circuit after the DC bus and two DC / DC (Direct Current) conversion circuits before the DC bus.

[0016] S2. Using the system model, determine whether a low voltage ride-through occurs on the grid-connected side of the photovoltaic power generation grid-connected system, obtain and detect the voltage drop depth and photovoltaic power level based on the judgment result;

[0017] S3. Based on the voltage drop depth and the photovoltaic power level, select a different control strategy in at least two different scenarios to perform fault ride-through operation.

[0018] This invention divides six fault scenarios and dynamically adjusts the control strategy based on the combination of voltage drop depth and photovoltaic power level, thereby realizing adaptive control of the photovoltaic power generation grid-connected system under various fault conditions.

[0019] This invention employs precise multi-objective coordinated control to comprehensively consider the performance requirements of a photovoltaic (PV) grid-connected system under fault conditions. The optimized control strategy not only ensures system safety but also effectively extends the service life of the PV grid-connected system and related equipment. By avoiding equipment overcurrent, overvoltage, and power fluctuations, it reduces losses and equipment load on the PV grid-connected system, thereby improving its long-term stability and economic efficiency.

[0020] In a more specific technical solution, in S2, voltage dip detection is performed based on a sliding mode observer and a decoupled dual-synchronous reference system phase-locked loop. The decoupled dual-synchronous reference system phase-locked loop is used to transform the grid connection point voltage using Clarke transformation. Voltage in a stationary reference frame.

[0021] This invention employs a combination of a sliding mode observer and a decoupled dual-synchronous reference system phase-locked loop (PLL) to quickly and accurately detect voltage dips and separate positive-sequence and negative-sequence voltage components. Traditional fault detection methods are slow to react when voltage dips occur, easily leading to grid disconnection or delayed response of the photovoltaic power generation grid-connected system. However, by introducing a sliding mode observer, the system can instantly capture abnormal voltage changes, reducing the system's response time to faults.

[0022] Meanwhile, the decoupled dual-synchronous reference system phase-locked loop used in this invention can accurately extract positive-sequence and negative-sequence voltage components, especially for analyzing asymmetric faults in the system. By effectively suppressing the negative-sequence voltage component, it avoids fluctuations in the power output and DC bus voltage of the photovoltaic power generation grid-connected system, thereby improving the stability and reliability of the photovoltaic power generation grid-connected system and avoiding grid disconnection or unstable power output caused by asymmetric faults.

[0023] In a more specific technical solution, in S2, the grid connection point voltage on the grid-connected side is obtained through symmetrical component processing, and the positive sequence component is defined in... Expressions in a stationary reference frame, negative order components Expression in a stationary reference frame;

[0024] According to the positive order components in The expression in the stationary reference frame, the negative order component in The expression in the stationary reference frame defines the total voltage. exist Expression in a stationary frame of reference:

[0025] According to the total voltage exist Expressions in a stationary reference frame are used to design nonlinear sliding surfaces for dynamic response enhancement.

[0026] For the nonlinear sliding surface, a dynamic adjustment of the sliding gain is designed to perform system voltage drop sensitivity and chatter suppression operations; a sliding mode switching function is designed to perform high-frequency chatter reduction operations.

[0027] Using a sliding mode observer, the positive-sequence and negative-sequence voltage components are quickly separated, and the state equation of the sliding mode observer is defined.

[0028] In a more specific technical solution, in S2, the dual synchronous reference system phase-locked loop is decoupled, and the sliding mode observer estimates... Plane voltage components Obtained through Park transformation Rotating reference frame Axis voltage components, Axis voltage components;

[0029] According to the above The axis voltage component, the The axis voltage components are processed to obtain positive-sequence and negative-sequence components;

[0030] Based on the expressions for the positive-sequence component and the negative-sequence component, the expressions for the positive-sequence voltage amplitude and the negative-sequence voltage amplitude are obtained.

[0031] The voltage drop is detected by the dynamic change of the positive sequence voltage amplitude, and the judgment result is obtained.

[0032] In a more specific technical solution, in S2, the level of photovoltaic power is detected, and the output current vector of the inverter is divided into positive sequence current vector and negative sequence current vector, as well as active current vector and reactive current vector; the maximum allowable active power of the inverter is injected into the grid.

[0033] In a more specific technical solution, in S3, the expression for the positive sequence voltage of the power grid is obtained based on the system model;

[0034] By maximizing the positive sequence voltage to support operation, a reference value for the positive sequence voltage is obtained through processing.

[0035] The first positive sequence voltage reference value in the reference values ​​of the positive sequence voltage. Substituting this into the expression for the positive sequence voltage of the power grid, we obtain the reference value for the first positive sequence reactive current.

[0036] This invention achieves coordinated control among multiple objectives during fault periods, including supplying reactive power to the grid to support voltage, supplying active power to reduce instantaneous power imbalance, suppressing active power oscillations to reduce DC bus capacitor voltage oscillations, limiting inverter output current, and eliminating negative sequence components, thereby optimizing the overall performance of the photovoltaic power generation grid-connected system.

[0037] In a more specific technical solution, in S3, the DC bus capacitor voltage oscillation is controlled within a preset safe range of normal voltage. The following logic is used to express the expression for the amplitude of the active power oscillation:

[0038]

[0039] In equation (22), This represents the amplitude of active power oscillation. This represents the DC bus capacitance. This represents the DC bus capacitor voltage. Indicates the synchronization frequency of the power grid. Represents the positive-sequence voltage component and the negative-sequence voltage component. This represents the positive-sequence active current vector. This represents the positive-sequence reactive current vector. This represents the negative-sequence active current vector. This represents the negative sequence reactive current vector;

[0040] Based on the expression (22) for the amplitude of the active power oscillation, the second positive sequence reactive current reference value is obtained using the following logic:

[0041]

[0042] In equation (23), This represents the second reference value of the positive sequence reactive current.

[0043] In a more specific technical solution, in S3, the reference value of the second positive sequence active current is obtained by using the following logic:

[0044]

[0045] In equation (24), This represents the reference value for the second positive sequence active current. This represents the amplitude of active power oscillation. Represents the negative sequence voltage component. This represents the reference value for the first positive sequence reactive current.

[0046] In a more specific technical solution, in S3, the maximum amplitude of the phase current in each phase is controlled to a preset multiple of the inverter's rated current to obtain the third positive sequence reactive current reference value. :

[0047]

[0048] In equation (25), This indicates the maximum allowable current of the inverter. This represents the reference value for the third positive sequence reactive current.

[0049] The third positive sequence active current reference value is obtained by using the following logic. :

[0050] .

[0051] This invention addresses the problem that traditional static control methods fail to adequately consider the system's response requirements under different fault scenarios, resulting in insufficient fault ride-through capability. By taking into account the diversity of fault scenarios, the control strategy can be flexibly adjusted to ensure that the system can maintain stable operation under complex fault conditions.

[0052] In a more specific technical solution, the fault ride-through control system based on the photovoltaic power generation grid-connected system includes:

[0053] The grid-connected system modeling module is used to establish a system model of the photovoltaic power generation grid-connected system. The system model is set up with an inverter circuit after the DC bus and two DC / DC conversion circuits before the DC bus.

[0054] The voltage and power status detection module is used to determine whether a low voltage ride-through has occurred on the grid-connected side of the photovoltaic power generation grid-connected system using the system model. It detects the voltage drop depth and the level of photovoltaic power. The voltage and power status detection module is connected to the grid-connected system modeling module.

[0055] The fault ride-through module is used to select different control strategies for fault ride-through operation based on the voltage drop depth and photovoltaic power level, according to at least two different scenarios. The fault ride-through module is connected to the voltage and power status detection module.

[0056] The present invention has the following advantages over the prior art:

[0057] This invention divides six fault scenarios and dynamically adjusts the control strategy based on the combination of voltage drop depth and photovoltaic power level, thereby realizing adaptive control of the photovoltaic power generation grid-connected system under various fault conditions.

[0058] This invention employs precise multi-objective coordinated control to comprehensively consider the performance requirements of a photovoltaic (PV) grid-connected system under fault conditions. The optimized control strategy not only ensures system safety but also effectively extends the service life of the PV grid-connected system and related equipment. By avoiding equipment overcurrent, overvoltage, and power fluctuations, it reduces losses and equipment load on the PV grid-connected system, thereby improving its long-term stability and economic efficiency.

[0059] This invention employs a combination of a sliding mode observer and a decoupled dual-synchronous reference system phase-locked loop (PLL) to quickly and accurately detect voltage dips and separate positive-sequence and negative-sequence voltage components. Traditional fault detection methods are slow to react when voltage dips occur, easily leading to grid disconnection or delayed response of the photovoltaic power generation grid-connected system. However, by introducing a sliding mode observer, the system can instantly capture abnormal voltage changes, reducing the system's response time to faults.

[0060] Meanwhile, the decoupled dual-synchronous reference system phase-locked loop used in this invention can accurately extract positive-sequence and negative-sequence voltage components, especially for analyzing asymmetric faults in the system. By effectively suppressing the negative-sequence voltage component, it avoids fluctuations in the power output and DC bus voltage of the photovoltaic power generation grid-connected system, thereby improving the stability and reliability of the photovoltaic power generation grid-connected system and avoiding grid disconnection or unstable power output caused by asymmetric faults.

[0061] This invention achieves coordinated control among multiple objectives during fault periods, including supplying reactive power to the grid to support voltage, supplying active power to reduce instantaneous power imbalance, suppressing active power oscillations to reduce DC bus capacitor voltage oscillations, limiting inverter output current, and eliminating negative sequence components, thereby optimizing the overall performance of the photovoltaic power generation grid-connected system.

[0062] This invention addresses the problem that traditional static control methods fail to adequately consider the system's response requirements under different fault scenarios, resulting in insufficient fault ride-through capability. By taking into account the diversity of fault scenarios, the control strategy can be flexibly adjusted to ensure that the system can maintain stable operation under complex fault conditions.

[0063] This invention solves the technical problems existing in the prior art, such as slow response when grid voltage drops occur, resulting in photovoltaic power generation grid-connected systems being disconnected from the grid or responding slowly under asymmetrical fault conditions, being susceptible to negative sequence components, and failing to fully consider the system's response and multi-objective control requirements under different fault scenarios, leading to insufficient fault ride-through capability. Attached Figure Description

[0064] Figure 1 This is a schematic diagram of the basic steps of the fault ride-through control method based on a photovoltaic power generation grid-connected system of the present invention;

[0065] Figure 2 This is a structural diagram of the photovoltaic power generation grid-connected system according to Embodiment 1 of the present invention;

[0066] Figure 3 This is a schematic diagram of the control strategy for the photovoltaic DC / DC conversion circuit placed before the DC bus in Embodiment 1 of the present invention;

[0067] Figure 4 This is a schematic diagram of the control strategy for the energy storage-side DC / DC converter circuit placed in front of the DC bus in Embodiment 1 of the present invention.

[0068] Figure 5 This is a schematic diagram of the grid-connected inverter control strategy placed after the DC bus in Embodiment 1 of the present invention. Detailed Implementation

[0069] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, 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.

[0070] Example 1

[0071] like Figure 1 As shown, the fault ride-through control method based on a photovoltaic power generation grid-connected system provided by this invention includes the following basic steps:

[0072] S1. Establish a system model for the photovoltaic power generation grid-connected system. This system model is set up with an inverter circuit downstream of the DC bus and two DC / DC converter circuits upstream of the DC bus. See details below. Figure 3 and Figure 4 ;

[0073] Figure 3 In, it means, PWM This indicates the pulse width modulation of the DC / DC converter circuit on the photovoltaic side; Figure 4 middle, This represents a reference value for the DC bus voltage. Indicates the DC bus voltage. This represents a reference value for the energy storage output current. Indicates the output current of the energy storage. PI Indicates a proportional-integral controller;

[0074] S2. Using the system model, determine whether a low voltage ride-through occurs on the grid-connected side of the photovoltaic power generation grid-connected system, obtain and detect the voltage drop depth and photovoltaic power level based on the judgment result;

[0075] In this embodiment, in S2, voltage sag detection is performed based on a sliding mode observer and a decoupled dual-synchronous reference system phase-locked loop. The grid-connected point voltage is transformed by Clarke using the decoupled dual-synchronous reference system phase-locked loop. Voltage in a stationary reference frame.

[0076] In this embodiment, different scenarios include, but are not limited to: the first scenario is a deep voltage drop and high photovoltaic power; the second scenario is a deep voltage drop and low photovoltaic power; the third scenario is a moderate voltage drop and high photovoltaic power; the fourth scenario is a moderate voltage drop and low photovoltaic power; the fifth scenario is a low voltage drop and high photovoltaic power; and the sixth scenario is a low voltage drop and low photovoltaic power.

[0077] In this embodiment, a method based on a sliding mode observer and a decoupled dual-synchronous reference system phase-locked loop is used for voltage sag detection. The decoupled dual-synchronous reference system phase-locked loop is used to detect the grid connection point voltage. Transformed by Clarke Voltage in stationary reference frame , These represent the voltages of phases A, B, and C in a balanced three-phase system, respectively. T Indicates transpose. Indicates in In a stationary reference frame shaft and Voltage on the shaft;

[0078] In step S2, the grid connection point voltage on the grid-connected side is obtained by processing with the symmetrical component method, and the positive sequence component is defined in... Expressions in a stationary reference frame, negative order components Expression in a stationary reference frame;

[0079] According to the positive order components in The expression in the stationary reference frame, the negative order component in The expression in the stationary reference frame defines the total voltage. exist Expression in a stationary frame of reference:

[0080] In this embodiment, the symmetric component method is used to obtain... , Let represent the positive-sequence voltage component and the negative-sequence voltage component. Then the positive-sequence component is... The expression in a stationary reference frame is:

[0081]

[0082] In equation (1), Indicates in Positive sequence voltage components in a stationary reference frame They represent in shaft and The positive-sequence component of the voltage on the axis. This represents the amplitude of the positive sequence voltage component. Indicates the synchronization frequency of the power grid. Indicates the positive sequence initial phase angle; This represents a cosine wave rotating in the positive direction. Represents a sine wave rotating in the positive direction;

[0083] Negative order components in The expression in a stationary reference frame is:

[0084]

[0085] In equation (2), Indicates in Negative sequence voltage component in a stationary reference frame They represent in shaft and The negative sequence component of the voltage on the axis. This represents the amplitude of the negative sequence voltage component. Indicates the synchronization frequency of the power grid. Indicates the negative sequence initial phase angle; This represents a cosine wave rotating in the opposite direction. Represents a sine wave rotating in the opposite direction;

[0086] Total voltage exist The expression in a stationary reference frame is:

[0087]

[0088] According to the total voltage exist Expressions in a stationary reference frame are used to design nonlinear sliding surfaces for dynamic response enhancement.

[0089] In this embodiment, a nonlinear sliding surface is designed to enhance the dynamic response, as expressed below:

[0090]

[0091] In equation (4), Represents a nonlinear sliding surface. Indicates estimation Plane voltage components, This represents the sliding surface gain, controlling for error amplification. It is a power-law exponent, used to smooth sliding surfaces. Represents a symbolic function. Indicates estimation Voltage components of the axis, Indicates estimation Voltage components of the axis; It is a power function, used to enhance the system's sensitivity in the small error range. It is a symbolic function that indicates the switching direction of sliding mode control;

[0092] The dynamic adjustment of the sliding mode gain is designed to enhance the system's sensitivity to voltage dips and chatter suppression capability. The expression is as follows:

[0093]

[0094] In equation (5), Represents the sliding mode gain matrix. This is the initial gain. and These are dynamically adjustable parameters used to accelerate the initial response and reduce steady-state chattering.

[0095] Design a sliding mode switching function to reduce high-frequency chattering, as shown in the following expression:

[0096]

[0097] In equation (6), This represents the switching function for sliding mode control. Represents the hyperbolic tangent function. The smoothing parameter controls the smoothness of the sliding surface switching. This represents the improved symbolic function;

[0098] Using a sliding mode observer, the positive-sequence and negative-sequence voltage components are quickly separated, and the state equation of the sliding mode observer is defined.

[0099] In this embodiment, a sliding mode observer is introduced to quickly separate the positive-sequence and negative-sequence voltage components. The state equation of the sliding mode observer is:

[0100]

[0101] In equation (7), It is a state matrix, representing Rotational dynamics of a plane It is the input matrix, usually the identity matrix. Indicates the input signal, i.e. ;

[0102] In S2, the dual synchronous reference frame phase-locked loop is decoupled, and the sliding mode observer estimates are... Plane voltage components Obtained through Park transformation Rotating reference frame Axis voltage components, Axis voltage components;

[0103] In this embodiment, the dual synchronous reference system phase-locked loop is decoupled, and the sliding mode observer estimates... Plane voltage components Obtained through Park transformation The voltage component in the rotating reference frame is expressed as follows:

[0104]

[0105] In equation (8), express Rotating reference frame Axis voltage components, express Rotating reference frame Axis voltage components, The synchronous phase angle representing the grid voltage;

[0106] according to Axis voltage components and The axis voltage components are used to obtain the positive-sequence and negative-sequence components, as expressed below:

[0107]

[0108]

[0109]

[0110]

[0111] In equations (9), (10), (11) and (12), Indicates ascending order Axis voltage components, Indicates ascending order Axis voltage components, Indicates negative order Axis voltage components, Indicates negative order Axial voltage component; j represents the imaginary unit;

[0112] Based on the expressions for the positive-sequence component and the negative-sequence component, the expressions for the positive-sequence voltage amplitude and the negative-sequence voltage amplitude are obtained.

[0113] In this embodiment, according to equations (9), (10), (11) and (12), the formulas for the positive-sequence voltage amplitude and the negative-sequence voltage amplitude are obtained as follows:

[0114]

[0115]

[0116] In equations (13) and (14), Indicates the positive sequence voltage amplitude. Indicates the magnitude of the negative sequence voltage;

[0117] The voltage drop is detected by the dynamic change of the positive sequence voltage amplitude, and the judgment result is obtained.

[0118] In this embodiment, voltage dips are detected by the dynamic change of the positive sequence voltage amplitude, and the judgment condition is:

[0119]

[0120] In equation (15), This represents the fault detection threshold, which is 0.9 times the normal voltage at the grid connection point. When a fault is detected... A voltage drop less than or equal to 0.5 times the normal voltage at the grid connection point is considered a deep voltage dip. A voltage dip exceeding 0.5 times but less than 0.7 times the normal voltage at the grid connection point is considered a moderate voltage drop. A voltage dip is defined as a voltage drop when the voltage is greater than or equal to 0.7 times but less than 0.9 times the normal voltage at the grid connection point.

[0121] In S2, the level of photovoltaic power is detected, and the output current vector of the inverter is divided into positive sequence current vector and negative sequence current vector, as well as active current vector and reactive current vector; the maximum allowable active power of the inverter is injected into the grid.

[0122] In this embodiment, the level of photovoltaic power is detected, and the inverter's output current vector is divided into four vectors, including but not limited to: positive-sequence current component and negative-sequence current vector, as well as active current component and reactive current component vector, the expressions of which are as follows:

[0123]

[0124] In equation (16), This represents the output current vector of the inverter. This represents the positive-sequence active current vector. This represents the positive-sequence reactive current vector. This represents the negative-sequence active current vector. This represents the negative-sequence reactive current vector. Indicates the current time; This represents a cosine wave rotating in the positive direction. This represents a sine wave rotating in the positive direction. This represents a cosine wave rotating in the opposite direction. Represents a sine wave rotating in the opposite direction;

[0125] To fully utilize the capacity of the photovoltaic power generation grid-connected system and mitigate the risk of sudden active power loss during voltage dips, the maximum permissible active power of the inverter is injected into the grid according to the following formula:

[0126]

[0127] In equation (17), The threshold representing the positive-sequence active current. This represents the reference value for the first positive sequence active power. The photovoltaic power at the maximum power point is represented as follows: when the real-time photovoltaic power detected is greater than the photovoltaic power at the maximum power point, it is recorded as high photovoltaic power; when the real-time photovoltaic power detected is less than or equal to the photovoltaic power at the maximum power point, it is recorded as low photovoltaic power.

[0128] S3. Based on the voltage drop depth and the photovoltaic power level, select a different control strategy in at least two different scenarios to perform fault ride-through operation;

[0129] like Figure 2 As shown, in this embodiment, based on the system model of the photovoltaic power generation grid-connected system, the expression for the positive sequence voltage of the power grid is obtained as follows:

[0130]

[0131] In equation (18), Represents the positive-sequence voltage component of the power grid. Indicates the inductive reactance on the grid side. This indicates the resistance value on the power grid side; Figure 2 middle, Indicates the grid voltage. Indicates the DC bus voltage. This represents the DC bus capacitance. Indicates the inductance of the filter circuit. This indicates the inverter output current. This represents the capacitor in the filter circuit. Indicates the inductance on the grid side. PV Indicates photovoltaics, Grid It refers to the power grid.

[0132] To ensure the voltage support capability of the power grid during faults, a method of maximizing positive-sequence voltage support is adopted to obtain a reference value for the positive-sequence voltage, the expression of which is as follows:

[0133]

[0134]

[0135] In equations (19) and (20), The reference value 1 represents the positive sequence voltage. This represents the maximum value among the three cosine values. This indicates the normal voltage value; This represents the cosine value of the initial phase angle difference between the positive-sequence voltage and the negative-sequence voltage of phase A; This represents the cosine value of the initial phase angle difference between the positive-sequence voltage and the negative-sequence voltage of phase B; This represents the cosine value of the initial phase angle difference between the positive-sequence voltage and the negative-sequence voltage of phase C; Indicates the negative sequence active current reference value. This represents the reference value for negative sequence reactive current.

[0136] like Figure 5 As shown, the first positive sequence voltage reference value in the reference values ​​of the positive sequence voltage is... Substituting this into the expression for the positive sequence voltage of the power grid, we obtain the reference value for the first positive sequence reactive current.

[0137] Figure 5 middle, This indicates the reference value for the DC bus voltage. Indicates the DC bus voltage. This represents the reference value for positive-sequence active current. This represents the reference value for negative sequence active current. This represents the reference value for positive sequence reactive current. This represents the reference value for negative sequence reactive current. This represents the positive-sequence active current. Indicates positive sequence reactive current. This represents the negative sequence active current. This represents negative sequence reactive current. Indicates the impedance characteristics of the power grid. This represents the angle required for the inverse Park transformation in the positive direction. This represents the angle required for the inverse Park transformation in the opposite direction. This represents a coordinate system rotating along the positive dq axis. This indicates a coordinate system rotated in the opposite direction along the dq axis. This represents the angle required for the inverse Clarke transform.

[0138] The first positive sequence voltage reference value in equation (19) Substituting into equation (18), we obtain the reference value for the first positive sequence reactive current, as shown in the following expression:

[0139]

[0140] In equation (21), This represents the reference value for the first positive sequence reactive current.

[0141] In S3, the DC bus capacitor voltage oscillation is controlled within the preset normal voltage safety range;

[0142] In this embodiment, during a sudden voltage drop due to unbalance, the output active power oscillates at twice the grid frequency, causing the DC bus capacitor voltage to oscillate at the same frequency. This affects the long-term lifespan of the DC link capacitor and may even endanger its safety. Therefore, the DC bus capacitor voltage oscillation is controlled within a safe range, for example, 10% of the normal voltage. The expression for the amplitude of the active power oscillation is as follows:

[0143]

[0144] In equation (22), This represents the amplitude of active power oscillation. This represents the DC bus capacitance. This represents the DC bus capacitor voltage;

[0145] Based on the expression (22) for the amplitude of the active power oscillation, the second positive sequence reactive current reference value is obtained using the following logic:

[0146]

[0147] In equation (23), This indicates the reference value for the second positive sequence reactive current;

[0148] Under the premise of prioritizing voltage support capability, the second positive sequence active current reference value is obtained, and its expression is as follows:

[0149]

[0150] In equation (24), This represents the reference value for the second positive sequence active current.

[0151] In S3, the maximum amplitude of the phase current in each phase is controlled within a preset multiple of the inverter's rated current to obtain the third reference value of the positive sequence reactive current. ;

[0152] In this embodiment, to avoid overcurrent risk, the maximum amplitude of the phase current in each phase is controlled within 1.1 times the rated current of the inverter, resulting in the third positive sequence reactive current reference value, the expression of which is as follows:

[0153]

[0154] In equation (25), This represents the reference value for the third positive sequence reactive current. Indicates the maximum allowable current of the inverter;

[0155] Under the premise of prioritizing voltage support capability, the third positive sequence active current reference value is obtained, and its expression is as follows:

[0156]

[0157] In equation (26), This represents the reference value for the third positive sequence active current.

[0158] For the first scenario, under conditions of deep voltage dips and high photovoltaic power, the photovoltaic boost converter switches to non-maximum power point tracking control, the energy storage buck-boost converter switches from current loop control to DC bus voltage outer loop and energy storage current inner loop control, and the grid-connected inverter adopts dual-loop control, with the outer loop based on... The conclusion is The selection of the inner loop current reference value is controlled by the inverter current loop, which enables fault ride-through.

[0159] In the second scenario, under conditions of deep voltage dips and low photovoltaic power, the photovoltaic boost converter switches to non-maximum power point tracking control, and the energy storage buck-boost converter switches from current loop control to DC bus voltage outer loop and energy storage current inner loop control. The grid-connected inverter adopts dual-loop control, with the outer loop based on... The conclusion is The selection of the inner loop current reference value is controlled by the inverter current loop, which enables fault ride-through.

[0160] For the third scenario, under moderate voltage dips and high photovoltaic power conditions, the photovoltaic boost converter switches to non-maximum power point tracking control, the energy storage buck-boost converter switches from current loop control to DC bus voltage outer loop and energy storage current inner loop control, and the grid-connected inverter adopts dual-loop control, with the outer loop based on... The conclusion is The selection of the inner loop current reference value is controlled by the inverter current loop, which enables fault ride-through.

[0161] For the fourth scenario, under moderate voltage dips and low photovoltaic power conditions, the photovoltaic boost converter employs maximum power point tracking control. The energy storage buck-boost converter switches from current loop control to DC bus voltage outer loop and energy storage current inner loop control. The grid-connected inverter uses dual-loop control, with the outer loop based on... The conclusion is

[0162] The selection of the inner loop current reference value is controlled by the inverter current loop, which enables fault ride-through.

[0163] For the fifth scenario, under low voltage sag and high photovoltaic power conditions, the photovoltaic boost converter switches to non-maximum power point tracking control, the energy storage buck-boost converter switches from current loop control to DC bus voltage outer loop and energy storage current inner loop control, and the grid-connected inverter adopts dual-loop control. The outer loop is based on... The conclusion is The selection of the inner loop current reference value is controlled by the inverter current loop, which enables fault ride-through.

[0164] For the sixth scenario, under conditions of low voltage drop and low photovoltaic power, the photovoltaic boost converter adopts maximum power point tracking control, the energy storage buck-boost converter switches from current loop control to DC bus voltage outer loop and energy storage current inner loop control, and the grid-connected inverter adopts dual-loop control. The outer loop is based on... The conclusion is The selection of the inner loop current reference value is controlled by the inverter current loop, which enables fault ride-through.

[0165] Compared with existing fault ride-through control strategies for photovoltaic power generation grid-connected systems, the technical advantages achieved by this invention include:

[0166] 1. Eliminate the influence of negative order components

[0167] By combining a sliding mode observer with a decoupled dual synchronous reference system phase-locked loop, negative sequence voltages are quickly separated and suppressed, avoiding interference with power output and DC bus voltage, thus improving the stability of the photovoltaic power generation grid-connected system.

[0168] 2. Enhance fault-tolerant capability

[0169] Six fault scenarios are identified, and an adaptive control strategy is implemented based on the voltage drop depth and photovoltaic power level to ensure the stable operation of the photovoltaic power generation grid-connected system under various fault conditions.

[0170] 3. Fast response to voltage dips

[0171] Sliding mode observers can instantly detect voltage anomalies, shorten response time, and improve the real-time performance of fault ride-through.

[0172] 4. Multi-objective coordinated control

[0173] During a fault, it balances grid voltage support, active power balance, DC bus voltage stability, and inverter current limiting to reduce equipment stress and losses and extend its lifespan.

[0174] In summary, this invention solves the technical problems existing in the prior art, such as slow response when grid voltage drops occur, resulting in photovoltaic power generation grid-connected systems being disconnected from the grid or responding slowly under asymmetrical fault conditions, being susceptible to negative sequence components, and failing to fully consider the system's response and multi-objective control requirements under different fault scenarios, leading to insufficient fault ride-through capability.

[0175] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A fault ride-through control method based on a photovoltaic power generation grid-connected system, characterized in that, The method includes: S1. Establish a system model of the photovoltaic power generation grid-connected system, and set the system model to an inverter circuit after the DC bus and two DC / DC conversion circuits before the DC bus. S2. Using the system model, determine whether a low voltage ride-through has occurred on the grid-connected side of the photovoltaic power generation grid-connected system, obtain and detect the voltage drop depth and photovoltaic power level based on the judgment results; S3. Based on the voltage drop depth and the photovoltaic power level, select different control strategies in no less than two different scenarios to perform fault ride-through operation. For the first scenario, under conditions of deep voltage dips and high photovoltaic power, the photovoltaic boost converter switches to non-maximum power point tracking control, the energy storage buck-boost converter switches from current loop control to DC bus voltage outer loop and energy storage current inner loop control, and the grid-connected inverter adopts dual-loop control, with the outer loop based on... The conclusion is To control the selection of the inner loop current reference value; For the second scenario, under conditions of deep voltage dips and low photovoltaic power, the outer ring... The conclusion is To control the selection of the inner loop current reference value; For the third scenario, under conditions of moderate voltage drop and high photovoltaic power, the outer ring is based on... The conclusion is To control the selection of the inner loop current reference value; For the fourth scenario, under conditions of moderate voltage drop and low photovoltaic power, the photovoltaic boost converter employs maximum power point tracking control, with the outer loop based on... The conclusion is To control the selection of the inner loop current reference value; For the fifth scenario, under conditions of low voltage sag and high photovoltaic power, the photovoltaic boost converter switches to non-maximum power point tracking control, and the energy storage buck-boost converter switches from current loop control to DC bus voltage outer loop and energy storage current inner loop control. The outer loop is based on... The conclusion is To control the selection of the inner loop current reference value; For the sixth scenario, under conditions of low voltage drop and low photovoltaic power, the photovoltaic boost converter employs maximum power point tracking control, with the outer loop based on... The conclusion is The selection of the inner loop current reference value is controlled by the inverter current loop, which enables fault ride-through.

2. The fault ride-through control method based on a photovoltaic power generation grid-connected system according to claim 1, characterized in that, In step S2, voltage dip detection is performed based on a sliding mode observer and a decoupled dual-synchronous reference system phase-locked loop. The grid-connected point voltage is then transformed using Clarke transformation. Voltage in a stationary reference frame.

3. The fault ride-through control method based on a photovoltaic power generation grid-connected system according to claim 1, characterized in that, In step S2, the grid connection point voltage on the grid-connected side is obtained through the symmetrical component method, and the positive sequence component is defined in... Expressions in a stationary reference frame, negative-order components Expression in a stationary reference frame; According to the positive order components in The expression in the stationary reference frame, the negative order component in The expression in the stationary reference frame defines the total voltage. exist Expression in a stationary frame of reference: According to the total voltage exist Expressions in a stationary reference frame; design of a nonlinear sliding surface for dynamic response enhancement: For the nonlinear sliding surface, a dynamic adjustment of the sliding gain is designed to perform system voltage drop sensitivity and chatter suppression operations; a sliding mode switching function is designed to perform high-frequency chatter reduction operations. Using a sliding mode observer, the positive-sequence and negative-sequence voltage components are quickly separated, and the state equation of the sliding mode observer is defined.

4. The fault ride-through control method based on a photovoltaic power generation grid-connected system according to claim 1, characterized in that, In step S2, the dual synchronous reference system phase-locked loop is decoupled, and the sliding mode observer estimates are... Plane voltage components Obtained through Park transformation Rotating reference frame Axis voltage components, Axis voltage components; According to the above The axis voltage component, the The axis voltage components are processed to obtain positive-sequence and negative-sequence components; Based on the expressions for the positive-sequence component and the negative-sequence component, the expressions for the positive-sequence voltage amplitude and the negative-sequence voltage amplitude are obtained. The voltage drop is detected by the dynamic change of the positive sequence voltage amplitude, and the judgment result is obtained.

5. The fault ride-through control method based on a photovoltaic power generation grid-connected system according to claim 1, characterized in that, In step S2, the level of photovoltaic power is detected, and the output current vector of the inverter is divided into positive sequence current vector and negative sequence current vector, as well as active current vector and reactive current vector; the maximum allowable active power of the inverter is injected into the grid.

6. The fault ride-through control method based on a photovoltaic power generation grid-connected system according to claim 1, characterized in that, In step S3, based on the system model, the expression for the positive sequence voltage of the power grid is obtained; By maximizing the positive sequence voltage to support operation, a reference value for the positive sequence voltage is obtained through processing. The first positive sequence voltage reference value in the reference values ​​of the positive sequence voltage. Substituting this into the expression for the positive sequence voltage of the power grid, we obtain the reference value for the first positive sequence reactive current.

7. The fault ride-through control method based on a photovoltaic power generation grid-connected system according to claim 1, characterized in that, In step S3, the DC bus capacitor voltage oscillation is controlled within a preset safe range of normal voltage. The following logic is used to express the expression for the amplitude of the active power oscillation: （22） In equation (22), This represents the amplitude of active power oscillation. This represents the DC bus capacitance. This represents the DC bus capacitor voltage. Indicates the synchronization frequency of the power grid. and Represents the positive-sequence voltage component and the negative-sequence voltage component. This represents the positive-sequence active current vector. This represents the positive-sequence reactive current vector. This represents the negative-sequence active current vector. This represents the negative-sequence reactive current vector. Based on the expression (22) for the amplitude of the active power oscillation, the second positive sequence reactive current reference value is obtained using the following logic: （23） In equation (23), This represents the second positive sequence reactive current reference value.

8. The fault ride-through control method based on a photovoltaic power generation grid-connected system according to claim 1, characterized in that, In step S3, the second positive-sequence active current reference value is obtained using the following logic: （24） In equation (24), This represents the second positive sequence active current reference value. This represents the amplitude of active power oscillation. Represents the negative sequence voltage component. This represents the reference value for the first positive sequence reactive current.

9. The fault ride-through control method based on a photovoltaic power generation grid-connected system according to claim 1, characterized in that, In step S3, the maximum amplitude of the phase current in each phase is controlled to a preset multiple of the inverter's rated current to obtain the third positive sequence reactive current reference value. : （25） In equation (25), This indicates the maximum allowable current of the inverter. This represents the reference value for the third positive sequence reactive current. The following logic is used to obtain the reference value of the positive-sequence active current. : （26）。 10. A fault ride-through control system based on a photovoltaic power generation grid-connected system, used to execute the fault ride-through control method based on a photovoltaic power generation grid-connected system as described in any one of claims 1 to 9, characterized in that, The system includes: The grid-connected system modeling module is used to establish a system model of the photovoltaic power generation grid-connected system. The system model is set up with an inverter circuit after the DC bus and two DC / DC conversion circuits before the DC bus. The voltage and power status detection module is used to determine whether a low voltage ride-through has occurred on the grid-connected side of the photovoltaic power generation grid-connected system using the system model, and to obtain and detect the voltage drop depth and photovoltaic power level based on the judgment result. The voltage and power status detection module is connected to the grid-connected system modeling module. The fault ride-through module is used to select different control strategies in at least two different scenarios based on the voltage drop depth and the photovoltaic power level, and to perform fault ride-through operation. The fault ride-through module is connected to the voltage and power status detection module.

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