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

Abstract

The invention provides a fault ride-through control method and system based on a photovoltaic power generation grid-connected system. The method comprises the steps that a model of the photovoltaic power generation grid-connected system, an inverter circuit arranged at the rear stage of a direct current bus and two DC / DC conversion circuits arranged at the front stage of the direct current bus are established; judging whether low voltage ride through occurs on the grid-connected side of the photovoltaic power generation grid-connected system or not, and detecting the voltage drop depth and the photovoltaic power level; according to the detected voltage drop depth and the photovoltaic power degree, different control strategies are adopted in different scenes, and fault ride-through is achieved. The method solves the problems that a photovoltaic power generation grid-connected system is subjected to off-network operation or slow response under the asymmetric fault condition and is easily influenced by negative sequence components due to slow response when the voltage drop of the power grid occurs, and the response and multi-target control requirements of the system under different fault scenes are not fully considered; and the fault ride-through capability is insufficient.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of power systems and its control technology, in particular to a fault ride-through control method and system based on a photovoltaic power generation grid-connected system. BACKGROUND

[0002] With the rapid development of distributed power and microgrid, photovoltaic power generation grid-connected system has become an important part of renewable energy. However, as a clean and renewable energy solution, the intermittency, randomness and volatility of photovoltaic power output have caused power quality problems such as grid voltage fluctuation, which need to be solved urgently.

[0003] In addition, in the actual operation of the power grid, asymmetric faults often occur, causing the grid-connected point voltage of the photovoltaic power generation grid-connected system to drop asymmetrically. In this case, on the one hand, the tolerance of power electronic devices is poor, which may cause the photovoltaic power generation grid-connected system to be off-grid, thereby affecting the effective use of renewable energy; on the other hand, the existence of negative sequence components will cause the output power of the photovoltaic power generation grid-connected system to fluctuate, thereby causing the DC side voltage of the inverter to appear second harmonic. Therefore, it is urgent to develop a control strategy that can ensure the photovoltaic power generation grid-connected system to operate without off-grid under asymmetric fault conditions, while eliminating the influence of negative sequence components. At present, the research on inverters mainly focuses on: delivering reactive power to the grid to support voltage, delivering active power to reduce instantaneous power imbalance, suppressing active power oscillation to reduce DC bus capacitor voltage oscillation, and limiting the output current of the inverter, but it does not fully consider the real-time changes of photovoltaic power and the simultaneous realization of these goals.

[0004] The existing patent application document with publication number CN116742691A, entitled "Fault ride-through control strategy implementation method and device for photovoltaic and energy storage combined power generation system", the existing method includes: under the condition of normal operation of the photovoltaic and energy storage combined power generation system, the grid-connected inverter adopts the pre-set maximum power point tracking control strategy, and the energy storage converter adopts the constant power control strategy to realize stable operation; in the case of asymmetric fault of the photovoltaic and energy storage combined power generation system grid-connected, the grid-connected inverter outputs active power and reactive power according to the actual reactive current reference value and the actual active current reference value, realizing fault ride-through; the energy storage converter outputs active power and reactive power according to the actual reactive current reference value and the actual active current reference value of the energy storage converter, supporting fault ride-through, and adopts a negative sequence voltage compensation control strategy to reduce the influence of PCC (Point of Common Coupling, common connection point) negative sequence voltage on the grid-connected inverter. However, the influence of the negative sequence component of the existing scheme is not completely eliminated, the fault ride-through capability is limited, the response speed to voltage drop is not fast enough, the dynamic adjustment capability is lacking, and the consideration of equipment life and system loss is insufficient.

[0005] The existing patent application document with publication number CN114784866A, entitled "A whole cooperative low-voltage ride-through control method and its application", the existing method includes: combining the mechanism of reactive power supporting voltage, analyzing the influence mechanism of multi-DC / AC converter station grid-connected reactive power output on the port voltage of near-fault DC / AC converter station; on the basis of the above, a two-stage coordinated control strategy is adopted, and on the basis of refining the coordination mechanism in different scenes, the DC / AC converter station reactive current output is generally set.

[0006] However, the existing scheme has limited scope of application, lacks processing of negative sequence components, has high complexity of control strategy, and does not fully consider the dynamic changes of photovoltaic power and the protection of equipment.

[0007] The existing patent application document with publication number CN117791480A, entitled "New energy equivalent modeling method and system for asymmetric fault short-circuit calculation", the existing method includes: for new energy units adopting negative sequence current suppression control strategy, according to the corresponding relationship between new energy unit positive sequence voltage drop and positive sequence current output, and the positive sequence voltage axis component, a negative sequence current suppression type new energy equivalent model is constructed; for new energy units adopting positive and negative sequence coupling control type strategy, according to the control priority of new energy unit during fault ride-through to positive sequence component and negative sequence component, and the corresponding relationship between sequence voltage and sequence current, a positive and negative sequence coupling control type new energy equivalent model is constructed; for new energy units adopting positive and negative sequence decoupling control strategy, according to the corresponding relationship between new energy unit positive sequence voltage and positive sequence current, and the corresponding relationship between negative sequence voltage and negative sequence current, a positive and negative sequence decoupling control type new energy equivalent model is constructed.

[0008] However, the existing scheme has high model complexity and high calculation cost, lacks dynamic adaptability, does not fully consider the protection of equipment and system economy, and has limited scope of application.

[0009] The method of the existing patent application document with publication number CN117791480A is mainly aimed at asymmetric fault short-circuit calculation and is applicable to the field of relay protection setting calculation. However, it has insufficient applicability to other problems in photovoltaic power generation grid-connected systems, such as voltage fluctuation and power balance.

[0010] The existing patent application document with the publication number CN110071528A discloses a method for constructing a fault model of an inverter grid-connected type distributed power supply, which comprises the following steps: a fault ride-through control model is established according to a single scene or a multi-scene situation set by a fault ride-through control strategy; according to the difference in the operating conditions, the inverter model is divided into independent model stages under each operating condition, and an inverter characteristic model is established to represent the operating characteristics of the inverter; and the fault ride-through control model and the multi-stage inverter characteristic model are combined to obtain a multi-stage inverter grid-connected type distributed power supply symmetrical fault model.

[0011] However, the existing scheme has the following technical defects: the model complexity is high, the calculation cost is high, the dynamic adaptability is poor, and the processing of negative sequence components is not comprehensive.

[0012] In summary, the existing technology has the technical problems that the reaction is slow when the grid voltage drops, the photovoltaic power generation grid-connected system is off-grid operation or response is slow in the case of asymmetric fault, is easily affected by negative sequence components, and the response and multi-objective control requirements of the system under different fault scenarios are not fully considered, resulting in insufficient fault ride-through capability. SUMMARY

[0013] The technical problem to be solved by the present application is how to solve the technical problems that the reaction is slow when the grid voltage drops, the photovoltaic power generation grid-connected system is off-grid operation or response is slow in the case of asymmetric fault, is easily affected by negative sequence components, and the response and multi-objective control requirements of the system under different fault scenarios are not fully considered, resulting in insufficient fault ride-through capability.

[0014] The present application solves the above technical problems by adopting the following technical scheme: a fault ride-through control method based on a photovoltaic power generation grid-connected system comprises: S1, a system model of the photovoltaic power generation grid-connected system is established, and the system model is arranged at an inverter circuit behind a DC bus and two DC / DC (Direct Current) conversion circuits in front of the DC bus; S2, the system model is used to determine whether a low voltage ride-through occurs on the grid-connected side of the photovoltaic power generation grid-connected system, and the voltage drop depth and the photovoltaic power level are detected according to the determination result; S3, according to the voltage drop depth and the photovoltaic power level, a difference control strategy is selected in no less than two difference scenarios for fault ride-through operation.

[0015] The present application dynamically adjusts the control strategy according to the voltage drop depth and the photovoltaic power level combination by dividing six fault scenarios, and realizes the adaptive control of the photovoltaic power generation grid-connected system under various fault conditions.

[0016] The application considers various performance requirements of the photovoltaic power generation grid-connected system under fault conditions through precise multi-target coordinated control. The optimized control strategy not only ensures the safety of the system, but also effectively prolongs the service life of the photovoltaic power generation grid-connected system and related equipment. By avoiding overcurrent, overvoltage and power fluctuation of the equipment, the loss of the photovoltaic power generation grid-connected system and the burden of the equipment are reduced, thereby improving the long-term stability and economy of the photovoltaic power generation grid-connected system.

[0017] In a more specific technical solution, in S2, voltage drop detection is performed based on a sliding mode observer and a decoupled double synchronous reference frame phase-locked loop, and the grid-connected point voltage is converted into a voltage in a stationary reference frame through the decoupled double synchronous reference frame phase-locked loop.

[0018] The application adopts the technology of combining a sliding mode observer with a decoupled double synchronous reference frame phase-locked loop, and can quickly and accurately detect voltage drop and separate positive sequence voltage components and negative sequence voltage components. The traditional fault detection method reacts slowly when voltage drop occurs, which easily leads to disconnection of the photovoltaic power generation grid-connected system or slow response. Through the introduction of the sliding mode observer, the system can immediately capture the change of voltage anomaly and reduce the reaction time of the system to the fault.

[0019] Meanwhile, the decoupled double synchronous reference frame phase-locked loop adopted by the application can accurately extract positive sequence voltage components and negative sequence voltage components, especially for analyzing asymmetric faults. By effectively suppressing negative sequence voltage components, the fluctuation of the power output and the DC bus voltage of the photovoltaic power generation grid-connected system is avoided, thereby improving the stability and reliability of the photovoltaic power generation grid-connected system and avoiding the phenomenon of disconnection or unstable power output caused by asymmetric faults.

[0020] In a more specific technical solution, in S2, the grid-connected point voltage of the grid-connected side is obtained by processing through the symmetrical component method, the expression of the positive sequence component in a stationary reference frame is defined, the expression of the negative sequence component in the stationary reference frame is defined. According to the expression of the positive sequence component in the stationary reference frame, the expression of the negative sequence component in the stationary reference frame, the total voltage In the expression of the total voltage According to the expression of the total voltage In the expression of the total voltage According to the expression of the total voltage ​​​​​​For the nonlinear sliding mode surface, the dynamic adjustment of the sliding mode gain is designed to perform system voltage drop sensitivity and chattering suppression operation; the sliding mode switching function is designed to perform high-frequency chattering reduction operation. The positive sequence and negative sequence voltage components are separated quickly by using a sliding mode observer, and a state equation of the sliding mode observer is defined.

[0021] In a more specific technical solution, in S2, the decoupled double synchronous reference frame phase-locked loop is used to estimate the plane voltage component , and the axis voltage component in the rotating reference frame is obtained through Park transformation, the axis voltage component ; according to the axis voltage component , the positive sequence component and the negative sequence component are obtained through processing; According to the expression of the positive sequence component and the negative sequence component, the expression of the positive sequence voltage amplitude and the negative sequence voltage amplitude is obtained through processing; The voltage drop is detected through the dynamic change of the positive sequence voltage amplitude, and the judgment result is obtained.

[0022] In a more specific technical solution, in S2, the high and low degrees of the photovoltaic power are detected, the output current vector of the inverter is divided into a positive sequence current component and a negative sequence current vector, and an active current component and a reactive current component; the maximum allowed active power of the inverter is injected into the power grid.

[0023] In a more specific technical solution, in S3, according to the system model, the expression of the grid positive sequence voltage is obtained; The reference value of the positive sequence voltage is obtained through the maximum positive sequence voltage support operation; The first positive sequence voltage reference value in the reference value of the positive sequence voltage is brought into the expression of the grid positive sequence voltage to obtain the first positive sequence reactive current reference value.

[0024] The present application realizes the coordinated control among multiple goals such as delivering reactive power to the grid to support voltage, delivering active power to reduce transient power imbalance, suppressing active power oscillation to reduce DC bus capacitor voltage oscillation, limiting the output current of the inverter, and eliminating negative sequence components during faults, and optimizes the comprehensive performance of the photovoltaic power generation grid-connected system.

[0025] In a more specific technical solution, in S3, the DC bus capacitor voltage oscillation is controlled within a preset normal voltage safety range, and the expression of the amplitude of the active power oscillation is expressed by using the following logic:

[0026] in formula (22), denotes the amplitude of the active power oscillation, denotes the DC bus capacitance, denotes the DC bus capacitor voltage, denotes the synchronous frequency of the grid, denotes the positive and negative sequence voltage components, denotes the positive sequence active current vector, denotes the positive sequence reactive current vector, denotes the negative sequence active current vector, denotes the negative sequence reactive current vector; According to the expression (22) of the amplitude of the active power oscillation, the second positive sequence reactive current reference value is obtained by processing with the following logic:

[0027] in formula (23), denotes the second reference value of the positive sequence reactive current.

[0028] In a more specific technical solution, in S3, the reference value of the second positive sequence active current is obtained by processing with the following logic:

[0029] in formula (24), denotes the reference value of the second positive sequence active current, denotes the amplitude of the active power oscillation, denotes the negative sequence voltage component, denotes the reference value of the first positive sequence reactive current.

[0030] In a more specific technical solution, in S3, the maximum amplitude of the phase current of each phase is controlled to a preset multiple of the rated current of the inverter, and the third positive sequence reactive current reference value is obtained :

[0031] in formula (25), denotes the maximum current allowed by the inverter, denotes the third positive sequence reactive current reference value; The third positive sequence active current reference value is obtained by processing with the following logic : .

[0032] The present application aims at the problem that the traditional static control method fails to fully consider the response demand of the system under different fault scenarios, resulting in insufficient fault ride-through capability, and through considering the diversity of fault scenarios, the control strategy can be flexibly adjusted to ensure that the system can still operate stably under complex fault conditions.

[0033] In a more specific technical solution, the fault ride-through control system based on the photovoltaic power generation grid-connected system comprises: The grid-connected system modeling module is used to establish a system model of the photovoltaic power generation grid-connected system, and the system model is arranged at a DC bus post-stage of an inverter circuit and two DC / DC conversion circuits at a DC bus pre-stage; The voltage and power state detection module is used to determine whether a low voltage ride-through occurs at the grid-connected side of the photovoltaic power generation grid-connected system by using the system model, and the voltage drop depth and the photovoltaic power high-low degree are detected, and the voltage and power state detection module is connected with the grid-connected system modeling module; The fault ride-through module is used to select different control strategies according to the voltage drop depth and the photovoltaic power high-low degree according to not less than two different scenarios, and perform fault ride-through operation, and the fault ride-through module is connected with the voltage and power state detection module.

[0034] Compared with the prior art, the present application has the following advantages: The present application divides six fault scenarios, dynamically adjusts the control strategy according to the voltage drop depth and the photovoltaic power high-low combination, and realizes the adaptive control of the photovoltaic power generation grid-connected system under various fault conditions.

[0035] The present application comprehensively considers various performance requirements of the photovoltaic power generation grid-connected system under fault conditions through accurate multi-target coordinated control. The optimized control strategy not only ensures the safety of the system, but also effectively prolongs the service life of the photovoltaic power generation grid-connected system and related equipment. By avoiding overcurrent, overvoltage and power fluctuation, the loss of the photovoltaic power generation grid-connected system and the burden of the equipment are reduced, thereby improving the long-term stability and economy of the photovoltaic power generation grid-connected system.

[0036] The present application adopts the technology of combining the sliding mode observer with the decoupling double synchronous reference frame phase-locked loop, which can quickly and accurately detect voltage drop and separate the positive sequence voltage component and the negative sequence voltage component. The traditional fault detection method reacts slowly when voltage drop occurs, which easily leads to disconnection of the photovoltaic power generation grid-connected system or slow response. Through the introduction of the sliding mode observer, the system can immediately capture the change of voltage anomaly and reduce the response time of the system to the fault.

[0037] Meanwhile, the decoupled double synchronous reference frame phase-locked loop adopted by the application can accurately extract positive sequence voltage components and negative sequence voltage components, especially for analyzing asymmetric faults of the system. By effectively suppressing the negative sequence voltage components, the fluctuation of the power output and the DC bus voltage of the photovoltaic power generation grid-connected system is avoided, thereby improving the stability and reliability of the photovoltaic power generation grid-connected system, and avoiding the phenomenon of disconnection or unstable power output caused by asymmetric faults.

[0038] The application realizes the coordinated control among multiple goals such as delivering reactive power to the power grid to support voltage, delivering active power to reduce transient power imbalance, suppressing active power oscillation to reduce oscillation of the DC bus capacitor voltage, limiting the output current of the inverter, and eliminating negative sequence components during faults, and optimizes the comprehensive performance of the photovoltaic power generation grid-connected system.

[0039] The application can flexibly adjust the control strategy by considering the diversity of fault scenarios, and ensure that the system can still operate stably under complex fault conditions.

[0040] The application solves the technical problems in the prior art that the photovoltaic power generation grid-connected system is prone to disconnection operation or slow response under asymmetric fault conditions, is easily affected by negative sequence components, and the response and multi-target control requirements of the system under different fault scenarios are not fully considered, resulting in insufficient fault ride-through capability. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 The figure is a basic step schematic diagram of the fault ride-through control method based on the photovoltaic power generation grid-connected system of the application. Figure 2 The figure is a structure diagram of the photovoltaic power generation grid-connected system of embodiment 1 of the application. Figure 3 The figure is a control strategy schematic diagram of the photovoltaic DC / DC conversion circuit placed in the front stage of the DC bus of embodiment 1 of the application. Figure 4 The figure is a control strategy schematic diagram of the energy storage side DC / DC conversion circuit placed in the front stage of the DC bus of embodiment 1 of the application. Figure 5 The figure is a control strategy schematic diagram of the grid-connected inverter placed in the rear stage of the DC bus of embodiment 1 of the application. DETAILED DESCRIPTION

[0042] 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.

[0043] Example 1 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: 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 ; 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; 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; 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.

[0044] 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. 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 the stationary reference frame ,voltage in the stationary reference frame represent the voltages of phase A, phase B and phase C in the balanced three-phase system respectively, T denotes the transpose, denotes the voltage of the axis in the stationary reference frame axis in the stationary reference frame axis in the stationary reference frame In step S2, the point-of-connection voltage of the grid-connected side is obtained by processing with the symmetrical component method, and the expression of the positive sequence component in the stationary reference frame, and the expression of the negative sequence component in the stationary reference frame are defined. In step S2, the point-of-connection voltage of the grid-connected side is obtained by processing with the symmetrical component method, and the expression of the positive sequence component in the stationary reference frame, and the expression of the negative sequence component in the stationary reference frame are defined. In step S2, the point-of-connection voltage of the grid-connected side is obtained by processing with the symmetrical component method, and the expression of the positive sequence component in the stationary reference frame, and the expression of the negative sequence component in the stationary reference frame are defined. According to the expression of the positive sequence component in the stationary reference frame, and the expression of the negative sequence component in the stationary reference frame, the total voltage In step S2, the point-of-connection voltage of the grid-connected side is obtained by processing with the symmetrical component method, and the expression of the positive sequence component in the stationary reference frame, and the expression of the negative sequence component in the stationary reference frame are defined. In step S2, the point-of-connection voltage of the grid-connected side is obtained by processing with the symmetrical component method, and the expression of the positive sequence component in the stationary reference frame, and the expression of the negative sequence component in the stationary reference frame are defined. In step S2, the point-of-connection voltage of the grid-connected side is obtained by processing with the symmetrical component method, and the expression of the positive sequence component in the stationary reference frame, and the expression of the negative sequence component in the stationary reference frame are defined. In step S2, the point-of-connection voltage of the grid-connected side is obtained by processing with the symmetrical component method, and the expression of the positive sequence component in the stationary reference frame, and the expression of the negative sequence component in the stationary reference frame are defined. In this embodiment, the positive sequence component and the negative sequence component are obtained by the symmetrical component method. , represent the positive sequence voltage component and the negative sequence voltage component, and the expression of the positive sequence component in the stationary reference frame is:

[0045] In formula (1), represent the voltage positive sequence component in the stationary reference frame, represent the voltage positive sequence component in the stationary reference frame, represent the voltage positive sequence component in the stationary reference frame, represent the voltage positive sequence component in the stationary reference frame, represent the voltage positive sequence component in the stationary reference frame, represent the amplitude of the positive sequence voltage component, represent the synchronous frequency of the grid, represent the initial phase angle of the positive sequence; represent the cosine wave rotating in the positive direction, represent the sine wave rotating in the positive direction. The expression of the negative sequence component in the stationary reference frame is: In formula (2),

[0046] represent the voltage negative sequence component in the stationary reference frame, represent the voltage negative sequence component in the stationary reference frame, represent the voltage negative sequence component in the stationary reference frame, represent the voltage negative sequence component in the stationary reference frame, represent the voltage negative sequence component in the stationary reference frame, represent the voltage negative sequence component in the stationary reference frame, represent the amplitude of the negative sequence voltage component, represent the synchronous frequency of the grid, represents the negative sequence initial phase angle; represents a cosine wave rotating in the opposite direction, represents a sine wave rotating in the opposite direction; total voltage In The expression in the stationary reference frame is:

[0047] According to the total voltage In The expression in the stationary reference frame is designed to design a nonlinear sliding mode surface to enhance dynamic response operation; In this embodiment, a nonlinear sliding mode surface is designed to enhance dynamic response, and the expression is as follows:

[0048] In equation (4), represents a nonlinear sliding mode surface, represents the estimated plane voltage component, represents the sliding mode surface gain, which controls the error amplification effect, is a power index used to smooth the sliding mode surface, represents a sign function, represents the estimated axis voltage component, represents the estimated axis voltage component; is a power function used to enhance the sensitivity of the system in the small error interval, is a sign function representing the switching direction of the sliding mode control; The dynamic adjustment of the sliding mode gain is designed to enhance the sensitivity of the system to voltage drop and the chattering suppression ability, and the expression is as follows:

[0049] In equation (5), represents the sliding mode gain matrix, is the initial gain, and are dynamic adjustment parameters used to speed up the initial response and reduce steady-state chattering; The sliding mode switching function is designed to reduce high-frequency chattering, and the expression is as follows:

[0050] In equation (6), represents the switching function of the sliding mode control, represents a hyperbolic tangent function, is a smoothing parameter that controls the smoothness of the sliding mode surface switching; This represents the improved symbolic function; 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.

[0051] 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:

[0052] 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. ; 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; 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:

[0053] 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; according to Axis voltage components and The axis voltage components are used to obtain the positive-sequence and negative-sequence components, as expressed below:

[0054]

[0055]

[0056]

[0057] In formulas (9), (10), (11) and (12), represents positive sequence axis voltage component, represents positive sequence axis voltage component, represents negative sequence axis voltage component, represents negative sequence axis voltage component; j represents imaginary unit; According to the expression of the positive sequence component and the negative sequence component, the expression of the positive sequence voltage amplitude and the negative sequence voltage amplitude is obtained; In this embodiment, according to formulas (9), (10), (11) and (12), the positive sequence voltage amplitude and the negative sequence voltage amplitude formula are as follows:

[0058]

[0059] In formulas (13) and (14), represents positive sequence voltage amplitude, represents negative sequence voltage amplitude; The voltage drop is detected through the dynamic change of the positive sequence voltage amplitude, and the judgment result is obtained; In this embodiment, the voltage drop is detected through the dynamic change of the positive sequence voltage amplitude, and the judgment condition is:

[0060] In formula (15), represents fault detection threshold, that is, 0.9 times of normal voltage of grid connection point, when detecting less than or equal to 0.5 times of normal voltage of grid connection point, it is recorded as deep voltage drop, when detecting greater than 0.5 times and less than 0.7 times of normal voltage of grid connection point, it is recorded as moderate voltage drop, when detecting greater than or equal to 0.7 times and less than 0.9 times of normal voltage of grid connection point, it is recorded as low voltage drop; In S2, the high and low degree of the photovoltaic power is detected, the output current vector of the inverter is divided into positive sequence current component and negative sequence current vector, and active current component and reactive current component vector; The maximum allowed active power of the inverter is injected into the grid. In this embodiment, the high and low degree of the photovoltaic power is detected, the output current vector of the inverter is divided into four vectors, including but not limited to: positive sequence current component and negative sequence current vector, and active current component and reactive current component vector, and the expression is as follows:

[0061] In formula (16), represents the output current vector of the inverter, represents the positive sequence active current vector, represents the positive sequence reactive current vector, represents the negative sequence active current vector, represents the negative sequence reactive current vector, represents the current time; represents a cosine wave rotating in the positive direction, represents a sine wave rotating in the positive direction, represents a cosine wave rotating in the negative direction, represents a sine wave rotating in the negative direction; In order to fully utilize the capacity of the photovoltaic power generation grid-connected system and reduce the risk of sudden loss of active power during voltage sag, the maximum allowed active power injection of the inverter into the grid is according to the following formula:

[0062] In formula (17), represents the threshold value of the positive sequence active current, represents the first positive sequence active power reference value, represents the photovoltaic power at the maximum power point, when the real-time power of the photovoltaic is detected to be greater than the photovoltaic power at the maximum power point, it is recorded as high photovoltaic power, and when the real-time power of the photovoltaic is detected to be less than or equal to the photovoltaic power at the maximum power point, it is recorded as low photovoltaic power; S3, according to the voltage sag depth and the high and low degree of the photovoltaic power, in not less than 2 different scenarios, the difference control strategy is selected, and the fault ride-through operation is performed; As shown in Figure 2 In this embodiment, according to the system model of the photovoltaic power generation grid-connected system, the expression of the grid positive sequence voltage is as follows:

[0063] In formula (18), represents the positive sequence voltage component of the grid, represents the inductance reactance of the grid side, represents the resistance value of the grid side; Figure 2 In formula (19), represents the grid voltage, represents the DC bus voltage, represents the DC bus capacitance, represents the filter circuit inductance, represents the inverter output current, represents the filter circuit capacitance, represents the inductance of the grid side, PVrepresents a photovoltaic, Grid represents a power grid.

[0064] In order to ensure the voltage support capability of the power grid during a fault, a method of maximizing the positive sequence voltage support is adopted to obtain a reference value of the positive sequence voltage, which is expressed as follows:

[0065]

[0066] In formulas (19) and (20), represents a reference value 1 of the positive sequence voltage, represents the maximum value among the three cosine values, represents a normal voltage value; represents a cosine value of an initial phase angle difference between the positive sequence voltage and the negative sequence voltage of phase A; represents a cosine value of an initial phase angle difference between the positive sequence voltage and the negative sequence voltage of phase B; represents a cosine value of an initial phase angle difference between the positive sequence voltage and the negative sequence voltage of phase C; represents a reference value of the positive sequence active current, represents a reference value of the positive sequence reactive current; As shown in Figure 5 , a first positive sequence voltage reference value among the reference values of the positive sequence voltage is brought into an expression of the power grid positive sequence voltage to obtain a first positive sequence reactive current reference value; Figure 5 In formulas (21) and (22), represents a DC bus voltage reference value, represents a DC bus voltage, represents a reference value of the positive sequence active current, represents a reference value of the negative sequence active current, represents a reference value of the positive sequence reactive current, represents a reference value of the negative sequence reactive current, represents a positive sequence active current, represents a positive sequence reactive current, represents a negative sequence active current, represents a negative sequence reactive current, represents a power grid impedance characteristic, represents an angle required for a forward reverse Park transformation, represents an angle required for a reverse reverse Park transformation, represents a forward dq-axis rotating coordinate system, represents a reverse dq-axis rotating coordinate system, represents an angle required for a reverse Clarke transformation.

[0067] ​The first positive sequence voltage reference value in formula (19) is The first positive sequence reactive current reference value is obtained by substituting into formula (18), and the expression is as follows:

[0068] In formula (21), The first positive sequence reactive current reference value is represented by Iq1; In S3, the DC bus capacitor voltage oscillation is controlled within a preset normal voltage safety range; In this embodiment, during the unbalanced voltage dip, the output active power oscillates at double grid frequency, causing the DC bus capacitor voltage to oscillate at the same frequency, affecting the long-term life of the DC link capacitor, and even endangering the safety of the DC link capacitor. Therefore, the DC bus capacitor voltage oscillation is controlled within a safety range of, for example, 10% of the normal voltage, and the expression of the amplitude of the active power oscillation is as follows:

[0069] In formula (22), The amplitude of the active power oscillation is represented by A, The DC bus capacitor is represented by C, The DC bus capacitor voltage is represented by Vc; According to the expression (22) of the amplitude of the active power oscillation, the second positive sequence reactive current reference value is obtained by using the following logic:

[0070] In formula (23), The second positive sequence reactive current reference value is represented by Iq2; In the case of priority guarantee of voltage support capability, the second positive sequence active current reference value is obtained, and the expression is as follows:

[0071] In formula (24), The second positive sequence active current reference value is represented by Ia2; In S3, the maximum amplitude of the phase current of each phase is controlled within a preset multiple of the inverter rated current, and the third reference value of the positive sequence reactive current is obtained ; In this embodiment, in order to avoid the risk of overcurrent, the maximum amplitude of the phase current of each phase is controlled within 1.1 times of the inverter rated current, and the third positive sequence reactive current reference value is obtained, and the expression is as follows:

[0072] In formula (25), The third positive sequence reactive current reference value is represented by Iq3, represents the maximum current allowed by the inverter; In the case of priority guaranteeing voltage support capability, the third positive sequence active current reference value is obtained, and its expression is as follows:

[0073] In formula (26), represents the third positive sequence active current reference value; For the first scenario, in the case of deep voltage drop 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 double-loop control. The outer loop is controlled according to to obtain to control the selection of the inner loop current reference value, and the inner loop is controlled by the inverter current loop to realize fault ride-through; For the second scenario, in the case of deep voltage drop and low 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 double-loop control. The outer loop is controlled according to to obtain to control the selection of the inner loop current reference value, and the inner loop is controlled by the inverter current loop to realize fault ride-through; For the third scenario, in the case of moderate voltage drop 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 double-loop control. The outer loop is controlled according to to obtain to control the selection of the inner loop current reference value, and the inner loop is controlled by the inverter current loop to realize fault ride-through; For the fourth scenario, in the case of moderate 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 double-loop control. The outer loop is controlled according to to obtain to control the selection of the inner loop current reference value, and the inner loop is controlled by the inverter current loop to realize fault ride-through; For the fifth scenario, in the case of low voltage drop 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 double loop control, and the grid-connected inverter adopts double loop control, and the outer loop is according to Derive To control the selection of the inner loop current reference value, the inner loop is controlled by the inverter current loop, and fault ride-through is realized. For the sixth scenario, in the case 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 double loop control, and the grid-connected inverter adopts double loop control, and the outer loop is according to Derive To control the selection of the inner loop current reference value, the inner loop is controlled by the inverter current loop, and fault ride-through is realized.

[0074] Compared with the existing fault ride-through control strategy of photovoltaic power generation grid-connected system, the technical advantages realized by the present application include: 1. Eliminate the influence of negative sequence component By combining the sliding mode observer with the decoupling double synchronous reference frame phase-locked loop, the negative sequence voltage is quickly separated and suppressed, avoiding interference with power output and DC bus voltage, and improving the stability of the photovoltaic power generation grid-connected system.

[0075] 2. Enhance fault ride-through capability Six fault scenarios are divided, and the strategy is dynamically adjusted according to the voltage drop depth and photovoltaic power level, realizing adaptive control and ensuring stable operation of the photovoltaic power generation grid-connected system under various faults.

[0076] 3. Fast response to voltage drop The sliding mode observer can instantly capture voltage abnormalities, shorten response time, and improve real-time fault ride-through.

[0077] 4. Multi-objective coordinated control During the fault period, the grid voltage support, active power balance, DC bus voltage stability and inverter current limiting are considered, the device stress and loss are reduced, and the service life is prolonged.

[0078] In conclusion, the present application solves the technical problems in the prior art that the response is slow when the grid voltage drops, which leads to the photovoltaic power generation grid-connected system to run off-grid or respond slowly in the case of asymmetric fault, and the system is easily affected by negative sequence components, and the response and multi-objective control requirements of the system under different fault scenarios are not fully considered, resulting in insufficient fault ride-through capability.

[0079] The above examples are only used to illustrate the technical solutions of the present application, but not to limit it; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions recorded in the foregoing examples can still be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for fault ride-through control of a photovoltaic power generation grid-connected system, characterized in that, The method comprises: S1, establishing a system model of a photovoltaic power generation grid-connected system, setting the system model to an inverter circuit at a rear stage of a DC bus, and two DC / DC conversion circuits at a front stage of the DC bus; S2, using the system model to determine whether low voltage ride through occurs on a grid-connected side of the photovoltaic power generation grid-connected system, obtaining and detecting voltage drop depth and photovoltaic power high-low degree according to a determination result; S3, selecting a difference control strategy in no less than two difference scenarios according to the voltage drop depth and the photovoltaic power high-low degree, and performing fault ride through operation. 2.The method of claim 1, wherein, In the S2, based on the sliding mode observer and the decoupled double synchronous reference frame phase-locked loop, voltage sag detection is performed, and the grid-connected point voltage is converted into the voltage in the stationary reference frame through Clarke transformation by using the decoupled double synchronous reference frame phase-locked loop voltage in the stationary reference frame. 3.The method of claim 1, wherein, In S2, the grid-connected point voltage of the grid-connected side is obtained by processing with the symmetric component method, and the positive sequence component is defined as the expression in the stationary reference frame, and the negative sequence component is defined as the expression in the stationary reference frame. in terms of said positive-sequence component in a stationary reference frame, said negative-sequence component in a stationary reference frame, defining the total voltage in terms of said positive-sequence component in a stationary reference frame, said negative-sequence component in a stationary reference frame, defining the total voltage in terms of said positive-sequence component in a stationary reference frame, said negative-sequence component in a stationary reference frame, defining the total voltage in terms of said positive-sequence component in a stationary reference frame, said negative-sequence component in a stationary reference frame, defining the total voltage in terms of said According to the total voltage In Expressions in the stationary reference frame, the nonlinear sliding mode surface is designed to perform dynamic response enhancement operations: For the nonlinear sliding mode surface, dynamic adjustment of a sliding mode gain is designed to perform system voltage drop sensitivity and chattering suppression operation; a sliding mode switching function is designed to perform high-frequency chattering reduction operation; A sliding mode observer is used to quickly separate positive and negative sequence voltage components, and a state equation of the sliding mode observer is defined. 4.The method of claim 1, wherein, In S2, the decoupled dual synchronous reference frame phase-locked loop is used, and the estimated plane voltage component The plane voltage component is obtained by Park transformation axis voltage component in the rotating reference frame axis voltage component axis voltage component According to the above The axis voltage component, the positive sequence component and the negative sequence component are obtained by processing The axis voltage component, the positive sequence component and the negative sequence component are obtained by processing According to expressions of the positive and negative sequence components, expressions of positive sequence voltage amplitude and negative sequence voltage amplitude are processed. Voltage drop is detected through dynamic changes of the positive sequence voltage amplitude to obtain the determination result.

5. The method for fault ride-through control of photovoltaic grid-connected system according to claim 1, wherein, In S2, the high-low degree of the photovoltaic power is detected, output current vectors of the inverter are divided into positive and negative sequence current components and active and reactive current components, and maximum allowed active power of the inverter is injected into a power grid. 6.The method for fault ride-through control of photovoltaic grid-connected system according to claim 1, wherein, In S3, an expression of grid positive sequence voltage is obtained according to the system model; A reference value of the positive sequence voltage is processed through maximum positive sequence voltage support operation; a first positive-sequence voltage reference value of the reference values of the positive-sequence voltage is introduced into an expression of the grid positive-sequence voltage, resulting in a first positive-sequence reactive current reference value. 7.The method for fault ride-through control of photovoltaic grid-connected system according to claim 1, wherein, In S3, DC bus capacitor voltage oscillation is controlled within a preset normal voltage safety range, and an expression of amplitude of active power oscillation is expressed using the following logic: in formula (22), denotes the amplitude of the active power oscillation, denotes the DC bus capacitance, denotes the DC bus capacitor voltage, denotes the synchronous frequency of the power grid, denotes the positive and negative voltage components, denotes the positive active current vector, denotes the positive reactive current vector, denotes the negative active current vector, denotes the negative reactive current vector; According to the expression (22) of the amplitude of the active power oscillation, a second positive sequence reactive current reference value is processed using the following logic: In formula (23), denotes the second positive-sequence reactive current reference value. 8.The method of claim 1, wherein, In S3, a second positive sequence active current reference value is processed using the following logic: In formula (24), denotes the second positive sequence active current reference value, denotes the amplitude of the active power oscillation, denotes the negative sequence voltage component, denotes the first positive sequence reactive current reference value. 9.The method of claim 1, wherein, In the S3, the maximum amplitude in the phase current of each phase is controlled to a preset multiple of the inverter rated current, to obtain a third positive-sequence reactive current reference value : In formula (25), represents the maximum current allowed by the inverter, represents the third positive sequence reactive current reference value; With the following logic, the reference value of the positive sequence active current is processed : 。 10. A fault ride-through control system for a photovoltaic power generation grid-connected system, characterized by, The system comprises: A grid-connected system modeling module is configured to establish a system model of a photovoltaic power generation grid-connected system, set the system model to an inverter circuit at a rear stage of a DC bus, and two DC / DC conversion circuits at a front stage of the DC bus; A voltage and power state detection module is configured to use the system model to determine whether low voltage ride through occurs on a grid-connected side of the photovoltaic power generation grid-connected system, obtain and detect voltage drop depth and photovoltaic power high-low degree according to a determination result, and the voltage and power state detection module is connected with the grid-connected system modeling module; A fault ride through module is configured to select a difference control strategy in no less than two difference scenarios according to the voltage drop depth and the photovoltaic power high-low degree, and perform fault ride through operation, and the fault ride through module is connected with the voltage and power state detection module.

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