A method and device for coordinating control of power plant excitation and low-penetration photovoltaic power

Abstract

This invention provides a method and device for coordinated control of thermal power plant excitation and photovoltaic low-voltage transmission in a power plant, belonging to the field of power system technology. The method dynamically adjusts the quadrature-axis current output of the photovoltaic inverter by real-time acquisition of the grid-connected voltage of the photovoltaic power plant and the terminal voltage of the thermal power unit, using the stable voltage at the thermal power unit terminal as a reference. Based on whether the thermal power unit terminal voltage is within the strong excitation range of the excitation system, this invention adaptively adjusts the maximum output current limit of the inverter: when the fault is severe and the strong excitation does not operate, the system increases the maximum output quadrature-axis current of the inverter; when the fault is minor and the strong excitation operates, reactive power support is jointly provided by thermal power and photovoltaic power. In this case, the photovoltaic inverter only needs to output a small current to achieve effective voltage support, thereby significantly improving the system's stability and support duration. The corresponding device integrates data acquisition, mode judgment, coordination calculation, and signal output units to achieve coordinated control.

Description

Technical Field

[0001] This invention belongs to the field of power system and new energy power generation control technology, specifically relating to a method and device for coordinated control of thermal power excitation and low voltage transmission of photovoltaic power plants. Background Technology

[0002] With the transformation of energy structure and the demand for ecological restoration, the construction of large-scale photovoltaic power plants in abandoned factory areas has become an important development model. This model not only realizes land reuse but also provides clean energy for the region. However, factory areas are often located in remote areas with relatively weak power grid architecture, representing typical "weak grid" scenarios. When such photovoltaic power plants are connected to the grid via power electronic inverters, their inherent low inertia and low short-circuit capacity characteristics cannot guarantee the stability of the power grid. Especially when the grid fails, the "low-voltage ride-through" capability of the photovoltaic power plant is directly related to the safe and stable operation of the entire system.

[0003] Low-voltage ride-through (LVRT) refers to the ability of a power plant to maintain grid connection and provide reactive power to the grid to help restore voltage when a grid fault causes a voltage drop at the grid connection point. Grid companies in various countries have established strict LVRT standards, specifying the depth and corresponding duration requirements of voltage drops. Currently, the LVRT control strategy for photovoltaic power plants mainly relies on locally measured grid connection point voltage signals. This voltage signal, after being processed by a phase-locked loop (PLL), is used to generate a reference value for the inverter's quadrature-axis current, which is then used to raise the grid voltage by injecting capacitive reactive current.

[0004] However, this conventional control method, which relies on local voltage signals, has inherent limitations in weak grid scenarios:

[0005] 1. Poor signal quality and risk of control instability: During a fault, the voltage at the grid connection point of a weak power grid will fluctuate drastically, become distorted, or even become asymmetrical. Using this unstable and poor-quality signal as a control reference can easily lead to problems such as phase-locked loop (PLL) loss of lockout and controller response oscillation, which may exacerbate system instability and prevent the provision of accurate and effective reactive power support.

[0006] 2. Lack of inertial support and damping: As a power device lacking inertia, a photovoltaic inverter cannot support the system frequency and voltage by releasing rotor kinetic energy during a fault, unlike a synchronous generator. Its control response relies entirely on a fast current loop. When the voltage reference signal itself changes drastically, the control system lacks necessary damping, resulting in large output current overshoot and oscillations during the recovery process, which is not conducive to rapid and stable recovery after a fault.

[0007] 3. Lack of coordination with traditional power sources: Traditional thermal power plants often exist in and around the plant area, serving as regional backup power sources. The automatic voltage regulators (AVRs) and forced excitation functions of these thermal power units provide rapid, stable, and inertial support for their terminal voltage. However, under the current operating model, photovoltaic power plants and thermal power plants are controlled independently, lacking information exchange and coordination during faults. This prevents the photovoltaic power plant from utilizing the stable voltage reference provided by the thermal power units, resulting in a waste of control resources.

[0008] To address the aforementioned issues, some improvements exist in the existing technology, such as adding additional reactive power compensation devices or optimizing inverter control algorithms. However, these solutions are either costly or fail to fundamentally solve the core problem of unreliable local voltage signals. Therefore, there is an urgent need in the field for a novel control method that can adapt to the characteristics of weak power grids, fully utilize existing power grid resources, and achieve stable and reliable low-voltage ride-through. Summary of the Invention

[0009] The purpose of this invention is to overcome the shortcomings of the prior art and propose a method and device for coordinated control of thermal power excitation and low-voltage photovoltaic power generation in the plant area.

[0010] The present invention adopts the following technical solution:

[0011] A method for coordinated control of thermal power excitation and low-voltage power transmission in plant photovoltaic systems includes the following steps:

[0012] S1: Collect the grid connection point voltage of the photovoltaic power station in the plant area, as well as the terminal voltage and excitation system switching signals of the thermal power plant, and send the terminal voltage data to the low voltage ride-through coordination control device of the photovoltaic power station.

[0013] S2: The low-voltage ride-through coordination control device compares the grid connection point voltage with the preset low-voltage ride-through trigger threshold. When the grid connection point voltage drops below the trigger threshold, the low-voltage ride-through control mode is activated.

[0014] S3: In the low-voltage ride-through control mode, the terminal voltage of the thermal power plant is used as the stable reference voltage to calculate the reference value of the quadrature axis current of the photovoltaic inverter, and the reactive power injected into the grid by the photovoltaic power station is changed by adjusting the quadrature axis current output of the photovoltaic inverter.

[0015] S4: Adjust the maximum allowable output quadrature-axis current limit of the photovoltaic inverter according to the value of the terminal voltage.

[0016] Furthermore, in S2, after activating the low-voltage ride-through control mode, the control strategy of the photovoltaic power station is switched from the maximum power point tracking mode to the constant voltage mode, and its DC voltage reference value is generated according to the ratio of the generator terminal voltage of the thermal power plant.

[0017] Furthermore, in the constant voltage mode, the DC voltage reference value of the photovoltaic inverter is generated according to the ratio of the terminal voltage of the thermal power plant.

[0018] Furthermore, in S3, the quadrature-axis current reference value Calculated using the following formula:

[0019] ;

[0020] in, This is the gain coefficient. This is the grid connection point voltage value after normalization based on the terminal voltage of thermal power plants. This is the voltage at the grid connection point.

[0021] Furthermore, in S4, when the generator terminal voltage is in the strong excitation range of the excitation system of the thermal power plant (when the grid voltage collected on the generator side drops to between 80% and 85%, it will be triggered), the photovoltaic power station determines whether it is in a strong excitation state through the excitation system switch signal, and maintains or reduces the maximum allowable output quadrature axis current limit of the photovoltaic inverter.

[0022] When the terminal voltage exceeds the forced excitation range, there is no forced excitation signal in the excitation system's switching signal, so the maximum output quadrature axis current limit of the photovoltaic inverter is increased.

[0023] A low-voltage ride-through coordination control device, integrated within the inverter control system of a photovoltaic power plant, comprising:

[0024] The data acquisition and communication unit is used to acquire the grid connection voltage of photovoltaic power plants and the terminal voltage of thermal power plants in real time.

[0025] The judgment and triggering unit is used to compare the grid connection point voltage with the triggering threshold to determine whether to trigger the low-voltage ride-through mode.

[0026] The coordinated control calculation unit is used to calculate the quadrature axis current reference signal based on the generator terminal voltage after the low-voltage ride-through mode is triggered, and to generate the current limit adjustment signal.

[0027] The signal output unit is used to output the quadrature-axis current reference value and current limit adjustment signal to the pulse width modulation driver of the photovoltaic inverter to adjust its quadrature-axis current output.

[0028] Furthermore, the data acquisition and communication unit establishes a communication connection with the data source of the thermal power plant using one of the following methods: power line carrier communication, optical fiber communication, or wireless communication module.

[0029] The method of this invention utilizes the stable voltage reference provided by the excitation system of thermal power plants to improve the reactive power support capability and operational stability of photovoltaic power plants during grid faults, especially under weak grid conditions, to ensure that photovoltaic power plants meet the stringent LVRT standard requirements.

[0030] This method can be abstracted into a logical architecture model consisting of a perception layer, a decision layer, and an execution layer, and is specifically constructed as follows:

[0031] 1) Perception layer:

[0032] This is responsible for acquiring key data on the system's operational status in real time, providing information input for decision-making, and is implemented by the data acquisition and communication unit. It includes:

[0033] Local sensing: Continuously monitors the grid connection point voltage of the photovoltaic power station itself to determine whether a fault has occurred;

[0034] Remote sensing: Real-time acquisition of generator terminal voltage and excitation system switching signals from a remote thermal power plant via communication links.

[0035] 2) Decision-making layer: Based on information from the perception layer, it makes judgments and calculations and generates control commands; it includes two levels of decision-making logic:

[0036] Mode switching decision: Based on local sensing signals, determine whether to enter LVRT emergency state;

[0037] Adaptive collaborative decision-making: The severity of a global fault is determined by the terminal voltage of the thermal power unit.

[0038] 3) Execution layer: By changing the PWM driver of the photovoltaic power station inverter through instructions, it can coordinate with the thermal power plant to achieve fast and accurate support for the grid connection point voltage.

[0039] The quadrature-axis current reference value and current limit command generated by the decision layer are received by the signal output unit and converted into a specific pulse width modulation signal to drive the power devices of the photovoltaic inverter. Ultimately, the quadrature-axis current output is precisely adjusted to change the reactive power injected into the grid and complete the voltage support task.

[0040] The beneficial effects of this invention are as follows:

[0041] 1. Significantly improves control stability and accuracy: In this invention, by using the terminal voltage of a thermal power plant far from the fault point as a stable reference, the problems of phase-locked loop loss and controller response oscillation caused by drastic fluctuations or distortions in the grid connection point voltage are fundamentally avoided in weak grid scenarios. This provides a reliable benchmark for reactive current calculation, thereby achieving more accurate and stable reactive power control.

[0042] 2. Adaptive optimization of fault ride-through capability is achieved. By analyzing the relationship between the terminal voltage of the thermal power unit and the excitation threshold of the excitation system: In this invention, the severity of the fault across the entire network is intelligently judged, and the maximum output current limit of the photovoltaic inverter is dynamically adjusted accordingly. This enables the system to smoothly support the system under minor faults and to output at full capacity under severe faults. This achieves adaptive optimization of the LVRT strategy under different fault scenarios and improves the robustness of the system.

[0043] 3. Enhancing the synergistic support effect of traditional and new energy sources: This invention breaks down the barriers between traditional and new energy sources in terms of control, combining the inertia and excitation capability of thermal power units with the rapid response capability of photovoltaic inverters; during faults, the system can integrate the stability of thermal power generation with the speed of photovoltaic power generation to form a synergistic support effect, thereby providing voltage recovery capability superior to that of a single power source. Attached Figure Description

[0044] Figure 1 : Flowchart of the method of the present invention;

[0045] Figure 2 : A schematic diagram of the decision-making layer in this embodiment of the invention;

[0046] Figure 3 : A schematic diagram of the execution layer in this embodiment of the invention;

[0047] Figure 4 : A block diagram of the device architecture in an embodiment of the present invention;

[0048] Figure 5 In this invention, a comparison diagram of the equipment's operating status and parameter waveforms during a severe fault is provided (compared to traditional methods).

[0049] Figure 6 In this invention, a comparison chart of the equipment operating status and parameter waveforms under non-serious fault conditions is shown (compared to traditional methods).

[0050] Figure 7 : Output waveform diagram of the decision layer of this invention. Detailed Implementation

[0051] To facilitate understanding of this application, a more detailed description is provided below with reference to the accompanying drawings and specific embodiments. Preferred embodiments of this application are shown in the drawings. However, this application can be implemented in many different forms and is not limited to the embodiments described herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the disclosure of this application.

[0052] It should be noted that, unless otherwise defined, all technical and scientific terms used in this specification have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used in this specification is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. The term "and / or" as used in this specification includes any and all combinations of one or more of the associated listed items.

[0053] Example:

[0054] A coordinated control method integrating thermal power plant excitation and photovoltaic (PV) low-voltage transmission rate (LVRT) in a power plant area is proposed. This method aims to improve the reactive power support capability and operational stability of PV power plants during grid faults by utilizing the stable voltage reference provided by the excitation system of thermal power plants, especially under weak grid conditions, to ensure that PV power plants meet the stringent LVRT standards. This method constructs an integrated cross-energy collaborative control system encompassing the perception layer, decision-making layer, and execution layer. It leverages the "stability" and "strong inertia" of traditional thermal power generation systems as a callable service to empower renewable energy PV systems, thereby addressing issues such as unstable control signals and insufficient support capabilities when integrating renewable energy into the grid, particularly in weak grid environments.

[0055] In this embodiment, the perception layer, decision-making layer, and execution layer are constructed as follows:

[0056] 1) Perception layer:

[0057] This is responsible for acquiring key data on the system's operational status in real time, providing information input for decision-making, and is implemented by the data acquisition and communication unit. It includes:

[0058] Local sensing: Continuously monitors the grid connection point voltage of the photovoltaic power station itself to determine whether a fault has occurred;

[0059] Remote sensing: Real-time acquisition of generator terminal voltage and excitation system switching signals from a remote thermal power plant via communication links.

[0060] 2) Decision-making layer: Based on information from the perception layer, it makes judgments and calculations and generates control commands; it includes two levels of decision-making logic:

[0061] Mode switching decision: Based on local sensing signals, determine whether to enter LVRT emergency state;

[0062] Adaptive collaborative decision-making: The severity of a global fault is determined by the terminal voltage of the thermal power unit.

[0063] 3) Execution layer: By changing the PWM driver of the photovoltaic power station inverter through instructions, it can coordinate with the thermal power plant to achieve fast and accurate support for the grid connection point voltage.

[0064] The quadrature-axis current reference value and current limit command generated by the decision layer are received by the signal output unit and converted into a specific pulse width modulation signal to drive the power devices of the photovoltaic inverter. Ultimately, the quadrature-axis current output is precisely adjusted to change the reactive power injected into the grid and complete the voltage support task.

[0065] The specific process of the method is as follows:

[0066] Construct a perception layer architecture that integrates data acquisition points from near-end photovoltaic grid-connected points and data acquisition points from far-end thermal power plants to realize the coordination of low-voltage ride-through of photovoltaic power plants and the strong excitation function and voltage control system of power plants.

[0067] The reference voltage output and limiting parameters of the low-voltage ride-through function at the decision-making level and the signal modulation method at the execution level are optimized to enhance the voltage support capability of the photovoltaic power station grid connection point in the event of severe faults, thereby effectively improving the maintenance time and stability of the low-voltage ride-through function.

[0068] Based on actual engineering needs, this application constructs a coordinated control mechanism for low-voltage ride-through (LVRT) and forced excitation in thermal power plants. It also utilizes the stable voltage output capability (i.e., inertia) of thermal power plants to improve the stability of the quadrature-axis current output by the LVRT. LVRT typically occurs near the grid connection of photovoltaic power plants when a fault occurs, resulting in a significant drop in grid connection voltage. The severity of the fault leads to inconsistent voltage drop magnitudes. Engineering experience shows that the more severe the fault near the grid connection point, the more severe the voltage drop, and the greater the difficulty in supporting LVRT. This embodiment designs a coordinated control method integrating the excitation system control system of the thermal power plant and the LVRT of the photovoltaic power station within the plant area. When the thermal power plant faces a sudden voltage drop, the forced excitation function can quickly increase the generator excitation voltage and current to improve the power plant's output.

[0069] This embodiment uses a real-time simulation model built in a laboratory based on a practical application scenario to simulate a photovoltaic project located in an abandoned factory area with a total capacity of 20MW. Multiple 0.5MW photovoltaic modules are connected in parallel and connected to the simulated power system via a rectifier booster and a centralized three-level inverter. A small steam turbine unit is also simulated, using an excitation regulator from a certain manufacturer to simulate the forced excitation function and automatic voltage regulator function of a thermal power generator unit. Finally, it is connected to the photovoltaic power station as a small-scale terminal power system unit, connected to an infinitely large power grid 50 kilometers away, forming a weak grid state.

[0070] The integrated control system for the excitation system of a thermal power plant and the low-voltage ride-through coordination control method and device for the photovoltaic power station in this embodiment are embedded in the digital signal processor of the photovoltaic inverter. In this embodiment, it can be directly embedded into the digital signal processor in software form, or it can be used as a hardware module to upgrade existing photovoltaic inverters. In this embodiment, the method of embedding it into the photovoltaic inverter in software form is adopted. Communication is conducted via a dedicated fiber optic network to form a low-latency communication link. The field model and parameters are shown in the table below:

[0071] Table 1 Power Generation Equipment Setup

[0072]

[0073] Table 2 Control Parameter Settings

[0074]

[0075] Table 3 Rectifier Filter Circuit Parameters

[0076]

[0077] Table 4 Inverter Filter Circuit Parameters

[0078] .

[0079] In the control parameters, Ka represents the regulator gain, Ta represents the time constant, Te represents the excitation system time constant, and Ke represents the excitation system gain.

[0080] In the event of a severe fault, the forced excitation function of a thermal power plant will not be triggered due to the limitation of protecting the generator. However, the improved low-voltage ride-through method can identify the severity of the fault and raise the maximum value of the reference quadrature axis current to ensure that the low-voltage ride-through function can be output when triggered.

[0081] In the event of a less severe fault, the forced excitation function of the thermal power plant is activated, rapidly raising the generator's excitation voltage to approximately twice its normal value. Since the forced excitation function is maintained for a longer period than the low-voltage ride-through duration required by the standard, the thermal power plant can work with the photovoltaic power station to maintain reactive power output, achieving stronger grid connection point voltage support capability. Compared to the traditional low-voltage ride-through method, it places lower requirements on the reactive current of the photovoltaic power station.

[0082] Figure 1 This paper briefly demonstrates the basic operational logic of this application. Figure 2 This is the core innovation of this application. Figure 3 This explains the basic principle by which the grid connection point voltage is supported in this application.

[0083] First, a very serious three-phase ground fault is simulated near the grid connection point. Based on... Figure 1 The process involves a rapid drop in voltage at the grid connection point. Due to the excessively low voltage, the forced excitation function at the thermal power plant level fails to activate. Both signals are sent to the data acquisition and communication unit and then to the judgment and triggering unit. Based on... Figure 2 At this point, the decision-making process is initiated. Since the grid connection point voltage is lower than the low-voltage ride-through setting value, the process enters the coordinated control calculation unit. Simultaneously, a low-voltage ride-through switching constant voltage mode command is issued. The system determines whether the thermal power plant is in a severe fault state based on whether it is in the forced excitation operation range. According to the above fault conditions, the forced excitation is not activated at this time, indicating a severe fault state. A full support command is executed, increasing the quadrature-axis current limit. Due to the relaxed limit and the fact that the voltage to be compared is changed to the generator terminal voltage setting value, the calculated final quadrature-axis current reference value is larger. The quadrature-axis current limit and the reference value are sent together to the execution layer.

[0084] The execution layer first calculates the DC voltage error on the inverter input side, which is derived from the following formula:

[0085] ;

[0086] The DC voltage error is fed into the outer loop of the voltage control system, and the initial direct-axis reference current is obtained through a proportional-integral circuit to maintain the stable output of the inverter.

[0087] ;

[0088] in This represents the actual input DC voltage. This represents the integrator output voltage, which is derived from the following formula:

[0089] ;

[0090] ;

[0091] in This represents the voltage increment of the integrator.

[0092] The following are the core modifications of this innovative model: The setpoint of the generator terminal voltage of the thermal power plant is introduced as a reference value for comparison with the grid connection point voltage to calculate the reactive current to be injected, i.e., the quadrature-axis current reference value is used as one of the input parameters for the current inner loop control. The traditional model uses the rated voltage and the grid connection point voltage to calculate the voltage sag depth, as shown below:

[0093] ;

[0094] in This represents the rated grid connection point voltage. This represents the actual voltage value at the photovoltaic grid connection point.

[0095] The improved calculation method is as follows:

[0096] ;

[0097] in This represents the terminal voltage of a thermal power plant, and the low-voltage through-axis current is obtained through the controller. Reference values, using simple proportional control as an example:

[0098] ;

[0099] During low-voltage ride-through, this embodiment employs an algorithm that prioritizes reactive current injection for optimization. During low-voltage ride-through, the total current reference value is highly likely to exceed the set maximum current threshold. In this case, this embodiment uses the following algorithm:

[0100] ;

[0101] When the total current reference value is greater than the set maximum current threshold:

[0102] ;

[0103] To ensure that the inverter output is synchronized with the grid, this embodiment uses Clarke transformation and Park transformation to process the data.

[0104] The Clarke and Park transformations for the grid connection point voltage and current are shown in the following equations:

[0105] ;

[0106] ;

[0107] After Clarke transform and Park transform, the quadrature-axis and direct-axis voltages at the grid connection point are obtained. and and current and To ensure that the inverter of the improved method is in the same phase as the power grid, the phase-locked loop is a mature application, so the introduction is omitted here. This represents the grid connection point phase output by the phase-locked loop. The final output signal processing method of the decision layer is as follows:

[0108] ;

[0109] ;

[0110] ;

[0111] After passing through the proportional-integral control stage in the inner current loop, a specific signal is output to the modulation signal output stage through signal transformation. The signal processing procedure is as follows:

[0112] ;

[0113] ;

[0114] Figure 7 The final waveform sent to the PWM signal generator is shown. Furthermore, this embodiment also simulates various fault types in the laboratory, including three-phase grounding with minor severity, two-phase grounding with severe severity, and two-phase grounding with minor severity. The data transmission process and signal processing method are similar to those shown for the three-phase severe fault, with the main difference being that the generator's forced excitation function can function under the three-phase grounding with minor severity condition. Figure 2 This is reflected in the decision-making process entering the collaborative support process; the situation is similar to that of a three-phase grounding non-serious condition under severe two-phase grounding.

[0115] Figure 5 and Figure 6 As a demonstration of the experimental results in this embodiment, Figure 5 and Figure 6 The diagram shows the quadrature-axis current, grid connection point voltage, and excitation voltage in sequence. According to... Figure 5 and Figure 6 As can be seen from the improved strategy, it can output a larger quadrature-axis current under severe three-phase fault conditions. This is because the automatic voltage regulator will forgo outputting a higher excitation voltage, assuming the fault is unrecoverable. Therefore, the photovoltaic power station can only support the voltage itself. However, due to the algorithm improvement, the low-voltage ride-through function can output a larger quadrature-axis current reference value as the input signal for the control loop, ensuring that the grid connection point voltage is higher than that of traditional methods. Figure 5 As can be seen, the forced excitation function was not triggered at this time. For other fault conditions, the forced excitation function works together with the photovoltaic power station to regulate reactive power, which can be seen from... Figure 5 and Figure 6 It can be seen that the grid-connected voltage can reach a higher value, but only a smaller quadrature-axis current is needed to achieve a stronger support effect.

[0116] The two figures also show that the grid connection point voltage is improved with smaller quadrature-axis currents, meaning that the enhanced low-voltage ride-through can be activated for a longer time compared to the traditional strategy. Furthermore, during fault clearing, the improved strategy outperforms the traditional strategy in terms of both quadrature-axis current and grid connection point voltage overshoot or rise time, except in cases of severe three-phase ground faults.

[0117] The above are merely embodiments of this application and do not limit the scope of this patent application. Any equivalent structural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the scope of patent protection of this application.

Claims

1. A method for coordinated control of thermal power excitation and low-voltage power transmission in plant photovoltaic systems, characterized in that... include: S1: Collect the grid connection point voltage of the photovoltaic power station in the plant area, as well as the terminal voltage and excitation system switching signals of the thermal power plant, and send the terminal voltage data to the low voltage ride-through coordination control device of the photovoltaic power station. S2: The low-voltage ride-through coordination control device compares the grid connection point voltage with the preset low-voltage ride-through trigger threshold. When the grid connection point voltage drops below the trigger threshold, the low-voltage ride-through control mode is activated. S3: In the low-voltage ride-through control mode, the terminal voltage of the thermal power plant is used as the stable reference voltage to calculate the reference value of the quadrature axis current of the photovoltaic inverter, and the reactive power injected into the grid by the photovoltaic power station is changed by adjusting the quadrature axis current output of the photovoltaic inverter. S4: Adjust the maximum allowable output quadrature-axis current limit of the photovoltaic inverter according to the value of the terminal voltage.

2. The method according to claim 1, characterized in that, In S2, after the low-voltage ride-through control mode is activated, the control strategy of the photovoltaic power station is switched from the maximum power point tracking mode to the constant voltage mode.

3. The method according to claim 2, characterized in that, In the constant voltage mode, the DC voltage reference value of the photovoltaic inverter is generated according to the ratio of the terminal voltage of the thermal power plant.

4. The method according to claim 1, characterized in that, In S3, the quadrature axis current reference value Calculated using the following formula: ； in, This is the gain coefficient. This is the grid connection point voltage value after normalization based on the terminal voltage of thermal power plants. This is the voltage at the grid connection point.

5. The method according to claim 1, characterized in that, In S4, when the terminal voltage is in the strong excitation range of the excitation system of the thermal power plant, the photovoltaic power station determines whether it is in a strong excitation state through the excitation system switch signal, and maintains or reduces the maximum allowable output quadrature axis current limit of the photovoltaic inverter. When the terminal voltage exceeds the forced excitation range, there is no forced excitation signal in the excitation system's switching signal, so the maximum output quadrature axis current limit of the photovoltaic inverter is increased.

6. A low-voltage ride-through coordinated control device for implementing the method as described in any one of claims 1-5, characterized in that, The device is integrated into the inverter control system of the photovoltaic power station and includes: The data acquisition and communication unit is used to acquire the grid connection voltage of photovoltaic power plants and the terminal voltage of thermal power plants in real time. The judgment and triggering unit is used to compare the grid connection point voltage with the triggering threshold to determine whether to trigger the low-voltage ride-through mode. The coordinated control calculation unit is used to calculate the quadrature axis current reference signal based on the generator terminal voltage after the low-voltage ride-through mode is triggered, and to generate the current limit adjustment signal. The signal output unit is used to output the quadrature-axis current reference value and current limit adjustment signal to the pulse width modulation driver of the photovoltaic inverter to adjust its quadrature-axis current output.

7. The apparatus according to claim 6, characterized in that, The data acquisition and communication unit establishes a communication connection with the data source of the thermal power plant using one of the following methods: power line carrier communication, optical fiber communication, or wireless communication module.

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