阅读说明：本技术 大规模分布式光伏电站的频率调节协同控制方法及系统 (Frequency regulation cooperative control method and system for large-scale distributed photovoltaic power station ) 是由 姚伟 王钦 方家琨 艾小猛 文劲宇 于 2021-07-16 设计创作，主要内容包括：本发明公开了大规模分布式光伏电站的频率调节协同控制方法及系统,属于新能源并网发电技术领域。本发明通过将交流系统频率信号的变化信息转化为并网逆变器直流母线电压的变化信息,从而对单个光伏组件部署分散式的无通讯调频控制。该控制方法分为两个层面,在并网逆变器侧将交流系统频率偏差引入直流母线电压控制环,调整直流母线电容储存的能量实现虚拟惯量控制；在光伏发电侧部署基于直流优化器的分散式f-P下垂控制,从而根据因频率变化而产生的直流优化器输出电压的变化调整各光伏发电单元的有功出力。本发明可以使大规模分布式光伏电站自主响应并网交流系统频率变化,有效地改善系统频率变化率、动态频率最低点以及稳态频率偏差等频率指标。(The invention discloses a frequency regulation cooperative control method and system for a large-scale distributed photovoltaic power station, and belongs to the technical field of new energy grid-connected power generation. According to the invention, the distributed non-communication frequency modulation control is deployed on a single photovoltaic module by converting the change information of the frequency signal of the alternating current system into the change information of the direct current bus voltage of the grid-connected inverter. The control method is divided into two layers, the frequency deviation of an alternating current system is introduced into a direct current bus voltage control loop at the side of a grid-connected inverter, and the energy stored by a direct current bus capacitor is adjusted to realize virtual inertia control; and distributed f-P droop control based on a direct current optimizer is deployed on the side of the photovoltaic power generation, so that the active output of each photovoltaic power generation unit is adjusted according to the change of the output voltage of the direct current optimizer caused by the frequency change. The invention can enable the large-scale distributed photovoltaic power station to autonomously respond to the frequency change of the grid-connected alternating current system, and effectively improve the frequency indexes such as the frequency change rate of the system, the lowest point of dynamic frequency, steady-state frequency deviation and the like.)

1. A frequency regulation cooperative control method for a large-scale distributed photovoltaic power station comprises a plurality of photovoltaic modules, wherein each photovoltaic module is connected with a direct current optimizer, the photovoltaic modules are connected in series to obtain a photovoltaic string, the photovoltaic strings are connected in parallel to form a photovoltaic array, the array is connected into a grid-connected inverter through a direct current transmission line and a direct current bus, and finally power is injected into an alternating current power grid; the photovoltaic power station works in a load shedding mode in a normal operation stage; the method is characterized by comprising the following steps:

s1, before a low-frequency or over-frequency event occurs in the photovoltaic power station, drawing a system frequency-total photovoltaic power f-P linear droop curve, converting the system frequency-total photovoltaic power f-P linear droop curve into a direct current optimizer output voltage-photovoltaic module working voltage nonlinear interpolation curve through equivalent substitution, and generating table lookup data;

s2, after the photovoltaic power station has a low-frequency or over-frequency event, introducing frequency deviation information of an alternating-current/direct-current common coupling point into a direct-current bus voltage control loop of the grid-connected inverter based on virtual inertia control of a direct-current bus capacitor to obtain a dynamic voltage reference value, and controlling the actual value of the direct-current bus voltage to follow the dynamic voltage reference value through a direct-current bus voltage outer loop of the grid-connected inverter;

and S3, after the voltage of the direct current bus changes along with the voltage reference value, determining the variable quantity to be adjusted by the working voltage of each photovoltaic module through the table look-up data of S1 by each direct current optimizer in the photovoltaic array according to the change of the output voltage of the direct current bus, and controlling and adjusting the working voltage value of each photovoltaic module by the constant voltage of the direct current optimizer, so that the active power output of each photovoltaic module is changed and primary frequency support is provided for the alternating current system.

2. The method of claim 1, wherein step S1 includes the sub-steps of:

s11, before a low-frequency or over-frequency event occurs in the photovoltaic power station, obtaining a system frequency-total photovoltaic power f-P linear droop curve according to a frequency deviation threshold and the load shedding standby rate of the photovoltaic power station;

s12, converting the f-P linear droop curve in the step S11 into a linear droop curve of direct-current bus voltage-total photovoltaic power according to the corresponding relation between the allowable fluctuation range of the direct-current bus voltage and the frequency deviation threshold;

s13, selecting a certain number of voltage/power working points on a linear droop curve of direct current bus voltage-total photovoltaic power as sampling data, and solving the output voltage of each direct current optimizer and the working voltage of each photovoltaic module corresponding to the sampling working points according to the quantity relation between the output voltage of each series direct current optimizer and the direct current bus voltage and the quantity relation between the total photovoltaic power station output power and the working voltage of each photovoltaic module;

and S14, converting the linear droop curve of the direct-current bus voltage-total photovoltaic power in the step S13 into a nonlinear interpolation curve of direct-current optimizer output voltage-photovoltaic module working voltage according to the discrete data of each sampling working point calculated in the step S13 through linear interpolation, and generating table look-up data for deploying droop control.

3. The method according to claim 2, wherein the formula for calculating the linear droop curve f-P of the system frequency-total photovoltaic power in step S11 is as follows:

wherein FD_maxIs a frequency deviation threshold, R_pvFor photovoltaic power plantsThe load reduction spare rate, f is the system frequency value, f₀Rated frequency, P, for AC systems_pvallIs the total output power of the photovoltaic power plant,andrespectively the total initial deloading power of the photovoltaic power station and the power upper limit value thereof.

4. The method according to claim 2, wherein the linear droop curve of dc bus voltage-total photovoltaic power in step S12 is calculated as follows:

wherein, U_DCIs a DC bus voltage value, U_DCrefIs an initial reference value, R, of the DC bus voltage_uFor the permitted fluctuation range of the DC bus voltage, P_pvallIs the total output power of the photovoltaic power plant,andrespectively the total initial deloading power of the photovoltaic power station and the power upper limit value thereof.

5. The method of claim 4, wherein in step S13, the number of DC optimizer output voltages and DC bus voltages is as follows:

wherein the content of the first and second substances,for the direct current bus voltage corresponding to the kth sampling operating point,for a single dc optimizer output voltage corresponding to the kth sampling operating point,for the initial value of the output voltage of the direct current optimizer, i and j represent the positions of the photovoltaic power generation units in the array, and are respectively the serial numbers of the photovoltaic strings connected in parallel and the serial numbers of the photovoltaic modules connected in series;

the quantity relation between the total output power of the photovoltaic power station and the working voltage of each photovoltaic module is as follows:

wherein the content of the first and second substances,outputting total power of the whole photovoltaic power station corresponding to the kth sampling working pointFor the working voltage of the single photovoltaic module corresponding to the kth sampling working point,is the initial load shedding value of the operating voltage of the photovoltaic module, I_sci,jAnd U_oci,jShort-circuit current and open-circuit voltage, respectively, of the photovoltaic module, C₁And C₂Is the photovoltaic module coefficient.

6. The method of claim 1, wherein in step S2, the dc bus voltage is dynamically referencedValue ofThe calculation formula of (a) is as follows:

wherein f is the system frequency value, f₀Rated frequency, U, for AC systems_DCrefIs an initial reference value, R, of the DC bus voltage_uFor the permitted fluctuation range of the DC bus voltage, FD_maxIs a frequency deviation threshold.

7. A frequency regulation cooperative control system of a large-scale distributed photovoltaic power station is characterized by comprising: a computer-readable storage medium and a processor;

the computer-readable storage medium is used for storing executable instructions;

the processor is used for reading executable instructions stored in the computer readable storage medium and executing the frequency regulation cooperative control method of the large-scale distributed photovoltaic power plant of any one of claims 1 to 6.

Technical Field

The invention belongs to the technical field of new energy grid-connected power generation, and particularly relates to a frequency regulation cooperative control method and system for a large-scale distributed photovoltaic power station.

Background

In recent years, with the increasing of the photovoltaic power generation proportion in the power system, the distributed maximum power point tracking technology based on the Direct Current Optimizer (DCO) has attracted much attention, and the technology can effectively reduce the energy loss of a large-scale photovoltaic array under the condition of shielding of a local shadow and improve the power generation efficiency of the photovoltaic system. In the face of the development trend that new energy power generation gradually occupies the leading electric power system, a large-scale distributed photovoltaic power station based on DCO is expected to become a main means of future large-scale photovoltaic grid-connected power generation. However, the integration of large-scale photovoltaic power generation capacity into the power system will cause a great reduction in system inertia, and in addition, the fluctuation of photovoltaic output is difficult to cope with low/over-frequency events of the power system only by means of frequency adjustment of the synchronous generator, which aggravates the deterioration of dynamic frequency response, and causes serious faults such as disconnection of the synchronous generator and even disconnection of the power grid. Therefore, in the future, new power systems dominated by new energy sources require photovoltaic power stations to provide active frequency support when the frequency fluctuates, so as to enhance the frequency regulation capability of the power systems.

At present, a control method related to large-scale photovoltaic field stations participating in power system frequency regulation is mostly suitable for a centralized grid-connected single-stage or double-stage photovoltaic power station, the centralized photovoltaic power station adopts a global maximum power point tracking technology, the power generation efficiency is low under the condition that frequent local shadows are shielded, the spare photovoltaic capacity for frequency regulation is reduced, and the frequency modulation capability is limited. Therefore, the corresponding distributed frequency modulation control method and system provided for the DCO-based large-scale distributed photovoltaic power station can effectively improve the grid-connected frequency regulation capability of the large-scale photovoltaic power station.

Disclosure of Invention

The invention provides a communication-free frequency regulation control method and system for a large-scale distributed photovoltaic power station, aiming at solving the problems of low power generation efficiency and limited frequency modulation capability of the existing large-scale centralized photovoltaic power station, and aiming at realizing the purpose of actively providing frequency support for each power generation unit of a photovoltaic array without communication by deploying a photovoltaic module-level direct current optimizer and distributed frequency modulation droop control thereof and combining virtual inertia control of a grid-connected inverter direct current bus capacitor, improving the frequency modulation capability of the large-scale photovoltaic power station and reducing the control deployment cost.

In order to achieve the above object, according to a first aspect of the present invention, a frequency regulation cooperative control method for a large-scale distributed photovoltaic power station is provided, the large-scale distributed photovoltaic power station includes a large number of photovoltaic modules and a dc optimizer, a certain number of photovoltaic modules are connected in series to obtain photovoltaic strings, a certain number of photovoltaic strings are connected in parallel to form a photovoltaic array, each photovoltaic module is connected to one dc optimizer, so as to achieve distributed control of a photovoltaic power generation unit, the whole "photovoltaic-dc optimizer" array is connected to a grid-connected inverter through a dc transmission line and a dc bus, and finally power is injected into an ac power grid. And in the normal operation stage of the photovoltaic power station, each photovoltaic module in the photovoltaic array is controlled in a load shedding mode by a direct current optimizer connected with the photovoltaic module, and certain standby power is reserved.

The control method comprises the following steps:

s1, before a low-frequency or over-frequency event occurs in the photovoltaic power station, drawing a system frequency-total photovoltaic power f-P linear droop curve, converting the system frequency-total photovoltaic power f-P linear droop curve into a direct current optimizer output voltage-photovoltaic module working voltage nonlinear interpolation curve through equivalent substitution, and generating table lookup data;

s2, after the photovoltaic power station has a low-frequency or over-frequency event, introducing frequency deviation information of an alternating-current/direct-current common coupling point into a direct-current bus voltage control loop of the grid-connected inverter based on virtual inertia control of a direct-current bus capacitor to obtain a dynamic voltage reference value, and controlling the actual value of the direct-current bus voltage to follow the dynamic voltage reference value through a direct-current bus voltage outer loop of the grid-connected inverter;

and S3, after the voltage of the direct current bus changes along with the voltage reference value, determining the variable quantity to be adjusted by the working voltage of each photovoltaic module through the table look-up data of S1 by each direct current optimizer in the photovoltaic array according to the change of the output voltage of the direct current bus, and controlling and adjusting the working voltage value of each photovoltaic module by the constant voltage of the direct current optimizer, so that the active power output of each photovoltaic module is changed and primary frequency support is provided for the alternating current system.

Preferably, step S1 includes the following sub-steps:

s11, before a low-frequency or over-frequency event occurs in the photovoltaic power station, obtaining a system frequency-total photovoltaic power f-P linear droop curve according to a frequency deviation threshold and the load shedding standby rate of the photovoltaic power station;

s12, converting the f-P linear droop curve in the step S11 into a linear droop curve of direct-current bus voltage-total photovoltaic power according to the corresponding relation between the allowable fluctuation range of the direct-current bus voltage and the frequency deviation threshold;

s13, selecting a certain number of voltage/power working points on a linear droop curve of direct current bus voltage-total photovoltaic power as sampling data, and solving the output voltage of each direct current optimizer and the working voltage of each photovoltaic module corresponding to the sampling working points according to the quantity relation between the output voltage of each series direct current optimizer and the direct current bus voltage and the quantity relation between the total photovoltaic power station output power and the working voltage of each photovoltaic module;

and S14, converting the linear droop curve of the direct-current bus voltage-total photovoltaic power in the step S13 into a nonlinear interpolation curve of direct-current optimizer output voltage-photovoltaic module working voltage according to the discrete data of each sampling working point calculated in the step S13 through linear interpolation, and generating table look-up data for deploying droop control.

Preferably, in step S11, the calculation formula of the "system frequency-total photovoltaic power" f-P linear droop curve is as follows:

wherein FD_maxIs a frequency deviation threshold, R_pvThe load reduction standby rate of the photovoltaic power station is defined, f is a system frequency value, f₀Rated frequency, P, for AC systems_pvallIs the total output power of the photovoltaic power plant,andrespectively the total initial deloading power of the photovoltaic power station and the power upper limit value thereof.

Preferably, in step S12, the calculation formula of the "dc bus voltage-total photovoltaic power" linear droop curve is as follows:

wherein, U_DCIs a DC bus voltage value, U_DCrefIs an initial reference value, R, of the DC bus voltage_uThe allowable fluctuation range of the direct current bus voltage.

Preferably, in step S13, the number relationship between the output voltage of each dc optimizer and the dc bus voltage is as follows:

wherein the content of the first and second substances,for the direct current bus voltage corresponding to the kth sampling operating point,for a single dc optimizer output voltage corresponding to the kth sampling operating point,for the initial value of the output voltage of the DC optimizer, i and j represent the photovoltaic power generation unitThe positions in the array are respectively the serial numbers of the photovoltaic strings connected in parallel and the serial numbers of the photovoltaic modules connected in series.

The quantity relation between the total output power of the photovoltaic power station and the working voltage of each photovoltaic module is as follows:

wherein the content of the first and second substances,the total power output for the whole photovoltaic power station corresponding to the kth sampling operating point,for the working voltage of the single photovoltaic module corresponding to the kth sampling working point,is the initial load shedding value of the operating voltage of the photovoltaic module, I_sci,jAnd U_oci,jShort-circuit current and open-circuit voltage, respectively, of the photovoltaic module, C₁And C₂Is the photovoltaic module coefficient.

Preferably, in step S2, the dc bus voltage dynamic reference valueThe calculation formula of (a) is as follows:

wherein f is the system frequency value, f₀Rated frequency, U, for AC systems_DCrefIs an initial reference value, R, of the DC bus voltage_uFor the permitted fluctuation range of the DC bus voltage, FD_maxIs a frequency deviation threshold.

To achieve the above object, according to a second aspect of the present invention, there is provided a frequency regulation cooperative control system of a large-scale distributed photovoltaic power plant, the system comprising: a computer-readable storage medium and a processor;

the computer-readable storage medium is used for storing executable instructions;

the processor is configured to read executable instructions stored in the computer-readable storage medium, and execute the frequency regulation cooperative control method for the large-scale distributed photovoltaic power plant according to the first aspect of the present invention.

Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:

(1) according to the invention, through the cooperation of the virtual inertia control of the grid-connected inverter and the distributed f-P droop control of the photovoltaic power generation unit, on one hand, the energy stored by the direct current bus capacitor can be properly adjusted according to the frequency change of the alternating current system, the integral inertia of the system is enhanced, and two indexes of the frequency change rate and the lowest point of the dynamic frequency of the system after a low-frequency/over-frequency event occurs are improved; on the other hand, table look-up data is stored in a direct current optimizer control system connected with the photovoltaic modules, so that each photovoltaic module in a large-scale photovoltaic array can linearly adjust output power according to the change of system frequency, distributed droop control based on the direct current optimizer is realized, and two indexes of a dynamic frequency lowest point and a steady-state frequency deviation of the system after a low-frequency/over-frequency event occurs are improved;

(2) the frequency adjustment cooperative control method provided by the invention converts the frequency deviation information of the alternating current system into the change of the direct current bus voltage, and further influences the output voltage of each series-parallel direct current optimizer in the photovoltaic array, so that each photovoltaic power generation unit does not need any communication equipment to provide active ground frequency support for the grid-connected alternating current system, and the deployment cost of the control method is reduced.

Drawings

Fig. 1 is a schematic view of a photovoltaic module operating in a load shedding mode according to an embodiment of the present invention;

fig. 2 is a graph illustrating a droop curve conversion relationship involved in distributed f-P droop control based on a dc optimizer according to an embodiment of the present invention;

fig. 3 is a table look-up data generation diagram related to distributed f-P droop control based on a dc optimizer according to an embodiment of the present invention;

fig. 4 is a control schematic diagram corresponding to a frequency adjustment cooperative control method and system for a large-scale distributed photovoltaic power station according to an embodiment of the present invention;

fig. 5 is a topology structure diagram of a large-scale distributed photovoltaic power station based on a dc optimizer for simulation test according to an embodiment of the present invention;

fig. 6 is a topology structure diagram of a two-region ac system connected to a large-scale distributed photovoltaic power station for simulation test according to an embodiment of the present invention;

fig. 7 is a simulation result diagram of measurement after a low-frequency event occurs in the ac system according to the embodiment of the present invention, where (a) is system frequency, (b) is frequency change rate, and (c) is output power of the photovoltaic power station;

fig. 8 is a simulation result diagram of measurement after a frequency event occurs in the ac system according to the embodiment of the present invention, where (a) is a system frequency, (b) is a frequency change rate, and (c) is an output power of the photovoltaic power station.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

The invention provides a frequency regulation cooperative control method of a large-scale distributed photovoltaic power station.

The method comprises the following steps:

in order to meet the requirement of providing Active frequency support for a photovoltaic field station, each photovoltaic module in a photovoltaic array works below a maximum power point, a certain standby power is reserved, namely a load shedding operation mode, and two photovoltaic module working voltage values corresponding to the same load shedding power can be known by a photovoltaic external characteristic curve, as shown in fig. 1, a right half plane of the maximum power point is defined as a working area of the photovoltaic module in the embodiment of the invention, so that the larger voltage is selected as a voltage reference value of Active power standby control (APRC) of a direct current optimizer, and the power standby rates of each photovoltaic module in the array are selected to be the same value.

S1, before a low-frequency or over-frequency event occurs in the photovoltaic power station, a system frequency-total photovoltaic power f-P linear droop curve is drawn up and converted into a direct current optimizer output voltage-photovoltaic module working voltage nonlinear interpolation curve through equivalent substitution, and table lookup data are generated.

In order to enable each photovoltaic module to adjust the output power according to the characteristic of a linear droop curve of system frequency-total photovoltaic power, the change of a system frequency signal needs to be converted into the change of a self-measuring signal of a direct current optimizer connected with each photovoltaic module, so that the voltage of the working point of each photovoltaic module is adjusted in a distributed manner, and the working voltage value corresponding to each photovoltaic module is quickly positioned according to the change of the system frequency by adopting a table look-up method in consideration of the nonlinear mathematical relationship between the output power and the working voltage of the photovoltaic module.

S11, before the photovoltaic power station generates a low-frequency or over-frequency event, obtaining a system frequency-total photovoltaic power f-P linear droop curve according to a frequency deviation threshold and the load shedding standby rate of the photovoltaic power station.

The calculation formula of the f-P linear droop curve of the system frequency-total photovoltaic power is as follows:

wherein FD_maxIs a frequency deviation threshold, R_pvThe load reduction standby rate of the photovoltaic power station is defined, f is a system frequency value, f₀Rated frequency, P, for AC systems_pvallIs the total output power of the photovoltaic power plant,andrespectively the total initial deloading power of the photovoltaic power station and the power upper limit value thereof.

The present invention provides for the photovoltaic system to linearly adjust the total power output according to the change in the ac system frequency to cope with different degrees of frequency droop or rise, via step S11.

And S12, after the droop curve of the system frequency-total photovoltaic power is obtained, converting the f-P linear droop curve in the step S11 into a linear droop curve of the direct-current bus voltage-total photovoltaic power according to the corresponding relation between the allowable fluctuation range of the direct-current bus voltage and the frequency deviation threshold, as shown in FIG. 2.

The calculation formula of the linear droop curve of the direct current bus voltage-total photovoltaic power is as follows:

wherein, U_DCIs a DC bus voltage value, U_DCrefIs an initial reference value, R, of the DC bus voltage_uThe allowable fluctuation range of the direct current bus voltage.

According to the invention, through the step S12, the fluctuation range of the direct current bus voltage corresponding to the system frequency deviation is defined, and the fluctuation range corresponds to the total output power of the photovoltaic system one by one, so that the linear droop relation is satisfied.

S13, selecting a certain number of voltage/power working points on a linear droop curve of direct current bus voltage-total photovoltaic power as sampling data, and solving the output voltage of each direct current optimizer and the working voltage of each photovoltaic module corresponding to the sampling working points according to the quantity relation between the output voltage of each series direct current optimizer and the direct current bus voltage and the quantity relation between the total photovoltaic power station output power and the working voltage of each photovoltaic module.

The quantity relationship between the output voltage of each direct current optimizer and the direct current bus voltage is as follows:

wherein the content of the first and second substances,for the direct current bus voltage corresponding to the kth sampling operating point,for a single dc optimizer output voltage corresponding to the kth sampling operating point,for the initial value of the output voltage of the direct current optimizer, i and j represent the positions of the photovoltaic power generation units in the array, and are respectively the serial numbers of the photovoltaic strings connected in parallel and the serial numbers of the photovoltaic modules connected in series;

the quantity relation between the total output power of the photovoltaic power station and the working voltage of each photovoltaic module is as follows:

wherein the content of the first and second substances,the total power output for the whole photovoltaic power station corresponding to the kth sampling operating point,for the working voltage of the single photovoltaic module corresponding to the kth sampling working point,is the initial load shedding value of the operating voltage of the photovoltaic module, I_sci,jAnd U_oci,jShort-circuit current and open-circuit voltage, respectively, of the photovoltaic module, C₁And C₂Is the photovoltaic module coefficient.

S14, according to the discrete data of each sampling working point calculated in the step S13, converting the linear droop curve of the direct current bus voltage-total photovoltaic power in the step S13 into a nonlinear interpolation curve of the direct current optimizer output voltage-photovoltaic module working voltage through linear interpolation, wherein the nonlinear interpolation curve is shown in figure 3, and therefore table lookup data are generated to be used for deploying droop control.

According to the invention, through the steps S13-S14, the direct current bus voltage/power relation curve of the system layer is converted into the input/output voltage relation curve of the direct current optimizer at the photovoltaic module level, so that each module in the photovoltaic array can independently and dispersedly adjust the power output of each module according to the frequency change of the system, and the distributed frequency control effect is achieved.

S2, after the photovoltaic power station has a low-frequency or over-frequency event, introducing frequency deviation information of an alternating-current/direct-current common coupling point into a direct-current bus voltage control loop of the grid-connected inverter based on virtual inertia control of a direct-current bus capacitor to obtain a dynamic voltage reference value, and controlling the actual value of the direct-current bus voltage to follow the dynamic voltage reference value through a direct-current bus voltage outer loop of the grid-connected inverter;

dynamic reference value of DC bus voltageThe calculation formula of (a) is as follows:

wherein f is the system frequency value, f₀Rated frequency, U, for AC systems_DCrefIs an initial reference value, R, of the DC bus voltage_uFor the permitted fluctuation range of the DC bus voltage, FD_maxIs a frequency deviation threshold.

Because the energy stored by the direct current bus capacitor is in direct proportion to the square of the direct current bus voltage, the derivative of the capacitor power and the direct current bus voltage is derived to be in direct proportion, a dynamic voltage reference value is calculated according to the system frequency deviation, and the method is equivalent to determining the size of the capacitor power according to the change rate of the system frequency, so that the bus capacitor is determined to release or absorb the energy, the virtual inertia is added to the system, meanwhile, the system frequency deviation information is converted into direct current bus voltage information, and the frequency modulation control without communication is realized.

S43, after the voltage of the direct current bus changes along with the voltage reference value, each direct current optimizer in the photovoltaic array determines the variable quantity to be adjusted of the working voltage of each photovoltaic module according to the change of the output voltage of each direct current optimizer through table look-up data of S1, and the working voltage value of each photovoltaic module is controlled and adjusted through the fixed voltage of the direct current optimizer, so that the active power output of each photovoltaic module is changed, and primary frequency support is provided for an alternating current system.

In steps S1 to S3, the targeted large-scale distributed photovoltaic power station includes a large number of photovoltaic modules and a dc optimizer, a certain number of photovoltaic modules are connected in series to obtain a photovoltaic string, and then a certain number of photovoltaic strings are connected in parallel to form a photovoltaic array, each photovoltaic module is connected to one dc optimizer, so as to implement distributed frequency modulation control of the photovoltaic power generation units, the whole array of the "photovoltaic-dc optimizer" is connected to a grid-connected inverter through a dc transmission line and a dc bus, and finally power is injected into an ac power grid.

The invention also provides a frequency regulation cooperative control system of the large-scale distributed photovoltaic power station, which comprises the following components: a computer-readable storage medium and a processor;

the computer-readable storage medium is used for storing executable instructions;

the processor is used for reading the executable instructions stored in the computer readable storage medium and executing the frequency regulation cooperative control method of the large-scale distributed photovoltaic power station.

Specifically, the method is divided into:

the grid-connected inverter control system has the functions of 2 aspects: 1) converting the direct current power of the photovoltaic system into alternating current power and injecting the alternating current power into a power grid; 2) according to the deviation information of the system frequency, the reference value of the direct current bus voltage is dynamically adjusted, so that the energy stored in the direct current bus capacitor is changed, the virtual inertia control of the direct current bus capacitor is realized, the system inertia is enhanced, meanwhile, the frequency deviation information can be converted into the change of the direct current bus voltage, and the communication-free frequency modulation control is realized. The control system realizes P/Q decoupling control based on the directional vector control principle of the power grid voltage, and is divided into outer loop voltage control and inner loop current control, wherein the frequency deviation information of the alternating current system side is superposed to the reference value of the outer loop direct current voltage control to realize virtual inertia control based on the bus capacitor;

the phase-locked loop control system is used for tracking the voltage phase angle of the AC-DC coupling point, realizing equivalent conversion of the voltage and current values of the AC coupling point from an abc coordinate system to a dq coordinate system and ensuring the normal operation of P/Q decoupling control of the grid-connected inverter;

the direct current optimizer control system is used for realizing a load shedding operation mode of the photovoltaic system before a low-frequency or over-frequency event, storing upper/lower standby power, deploying f-P distributed droop control based on a table look-up method, and dynamically adjusting the output power of each photovoltaic module according to the frequency change of the alternating current system, so that frequency support is provided for the alternating current system.

The working principle of the proposed control system is shown in fig. 4, where the meanings of the main variables involved are listed in table 1 below.

TABLE 1

The specific control process of the system is as follows:

firstly, in a normal operation stage, each photovoltaic module measures the temperature and the illumination intensity under the environment, a working voltage value under a load shedding mode is solved through a photovoltaic module power-voltage external characteristic equation and a power standby coefficient, the photovoltaic module operates in the load shedding mode through active power standby control of a direct current optimizer, at the moment, f-P droop control does not act, and the correction quantity delta U of the output photovoltaic module working voltage reference value is output_pvIs 0;

after a low-frequency or over-frequency event occurs, the outer-ring direct-current voltage control of the grid-connected inverter responds to the frequency change of an alternating-current system, and the system frequency deviation information is introduced to modify the direct-current bus voltage reference value so as to realize the virtual inertia control of the direct-current bus capacitor;

thirdly, when the reference value of the direct current bus voltage is changed along with the tracking of the direct current bus voltage, the output voltage value of each series direct current optimizer also synchronously changes to serve as input information for f-P droop control of each direct current optimizer, and the change quantity delta U of the working voltage reference value of each photovoltaic module is obtained through a table look-up method_pvAnd the power output of each photovoltaic module is dynamically adjusted to provide frequency support for an alternating current system.

The "PWM modulator", "PI controller", and "photovoltaic module power-voltage external characteristic equation" related in fig. 4 are all known technologies, and the technical details thereof are not repeated.

The large-scale distributed photovoltaic power station topological structure based on the direct current optimizer in the embodiment of the invention is shown in fig. 5. The photovoltaic array is composed of M photovoltaic strings connected in parallel, each photovoltaic string comprises N photovoltaic modules connected in series, output ports of the photovoltaic modules in the array are connected with a direct current optimizer, the whole photovoltaic-direct current optimizer array is connected to a grid-connected inverter through a direct current transmission line and a direct current bus, and is connected with an alternating current-direct current common coupling point after filtering, and finally photovoltaic power is injected into an alternating current power grid.

The simulation test system in the embodiment of the invention is shown in fig. 6. The four-machine two-area alternating current system is used for simulation test of effectiveness of the communication-free frequency regulation cooperative control method and system of the large-scale distributed photovoltaic power station, the whole system comprises 4 synchronous generators, 5 transformers, 11 bus nodes and an alternating current transmission line, the large-scale distributed photovoltaic power station shown in the figure 5 is connected to the bus 5, and loads are connected to the bus 7 and the bus 9. Rated power of 4 synchronous generators is 263MW, 422MW, 424MW, 431MW respectively, and photovoltaic power plant's initial deloading operating power is 105MW, and the deloading standby coefficient is 0.4, and the active load that 7 and generating line 9 of generating line insert is 900MW, 800MW respectively, and reactive load is 100 MVar. The parallel number M of photovoltaic strings in the photovoltaic array is set to be 20, the serial number N of photovoltaic modules is set to be 20, and the number of each photovoltaic module isThe temperature parameters were all set to 30 ℃. In order to simulate the local shadow condition of multiple running of an actual photovoltaic power station, every 4 of 20 series photovoltaic modules in each photovoltaic string are taken as 1 group, and the illumination intensity of each group is respectively set to be 1000W/m²，900W/m²，800W/m²，700W/m²，600W/m²。

At the time when t is 2.0s, 150MW load sudden increase disturbance is set at the bus 7, a system low-frequency event is triggered, system frequency deviation information at the common coupling point is measured and transmitted to the grid-connected inverter control system, frequency regulation control according to the present invention is performed, and a simulation curve of the system frequency, the frequency change rate, and the total output power of the photovoltaic power station is measured by using no frequency regulation control as a control group, as shown in fig. 7. As can be seen from fig. 7, after the load suddenly increases, the system frequency drops immediately, and when the frequency adjustment control method provided by the present invention is not adopted, the frequency dropping speed is obviously faster than that when the corresponding frequency adjustment control method is adopted, the frequency change rate is larger, and the lowest point of the dynamic frequency is lower. After the frequency regulation control provided by the invention is deployed, the lowest point of the dynamic frequency is improved to 49.72Hz from 49.64Hz, the deviation of the steady-state frequency is reduced to 0.13Hz from 0.16Hz, and meanwhile, the output power of the photovoltaic power station is correspondingly increased and adjusted according to the magnitude of the frequency deviation in the frequency modulation process, so that the effectiveness of the frequency regulation cooperative control method provided by the invention when a low-frequency event occurs in a system is verified.

At the time when t is 2.0s, 150MW of load sudden reduction disturbance is set at the bus 7, and similarly, the control without frequency adjustment is taken as a control group, and a simulation curve of the measured system frequency, the measured frequency change rate, and the measured total output power of the photovoltaic power station is shown in fig. 8. As can be seen from fig. 8, after the load suddenly decreases, the system frequency increases, the frequency regulation control method provided by the present invention can significantly reduce the system frequency change rate, the peak frequency is reduced from 50.37Hz to 50.29Hz, and the output power of the photovoltaic power station is reduced from 104MW to 85MW, so that the steady-state frequency deviation of the system is reduced, and the effectiveness of the frequency regulation cooperative control method provided by the present invention when the system has an over-frequency event is verified.

The control method is suitable for the large-scale distributed photovoltaic power station based on the direct current optimizer to participate in the frequency adjustment of the alternating current system, but the application of the control method is not limited to the control method, and the control method is also suitable for the large-scale photovoltaic power station of a two-stage centralized grid-connected type.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.