Energy storage grading charging and discharging regulation and control method for stabilizing photovoltaic power fluctuation

Abstract

The invention discloses an energy storage grading charging and discharging regulation and control method for stabilizing photovoltaic power fluctuation, and relates to the technical field of photovoltaic power generation and energy storage cooperative control, and the method is applied to a photovoltaic power station system comprising a group cascade self-balancing networking unit, a station-level central networking energy storage system and a station-level cooperative networking controller. Power deviation caused by local shadow is monitored and compensated in real time through a group cascade unit, power cooperative scheduling and reactive power dynamic distribution are carried out on a station level layer, cooperative optimization of photovoltaic array internal power balance and grid-connected point power grid support is achieved, and meanwhile, through virtual synchronous machine cluster control and a protective off-grid / re-grid-connected mechanism, the photovoltaic array internal power balance and the grid-connected point power grid support are optimized. According to the method, power fluctuation can be effectively stabilized in a complex shielding environment, the power generation efficiency of the photovoltaic power station and the interaction capability of a power grid are improved, and the stability and reliability of a power system are enhanced.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photovoltaic power generation and energy storage collaborative control, in particular to a kind of energy storage hierarchical charge-discharge regulation method for photovoltaic power fluctuation suppression. BACKGROUND

[0002] With the transformation of global energy structure to clean, the installed capacity and penetration rate of photovoltaic power generation continue to increase rapidly. Large-scale centralized photovoltaic power stations are usually deployed in open areas, and their power generation is easily affected by environmental factors such as cloud drift, local dust accumulation or object shading, resulting in significant randomness and volatility. This fluctuation not only affects the economic output of the power station itself, but also transmits power disturbances to the grid through power electronic devices when it is connected to the grid, especially in areas where the grid structure is relatively weak, which may cause grid frequency deviation, voltage flicker and even stability problems, restricting the safe consumption of high proportion of photovoltaic power generation. Therefore, how to effectively suppress photovoltaic power fluctuation and improve the active support capability of photovoltaic power station to the grid has become a technical problem to be solved in this field.

[0003] To solve the above problems, the existing technology proposes a variety of power smoothing and grid support schemes combined with energy storage systems. One type of scheme focuses on power optimization within the photovoltaic array, which compensates for the current of shaded components to make the current of series branches consistent, thereby improving the maximum output power of the array under shadow conditions. Although this type of scheme can improve local power generation efficiency, the energy storage unit only serves as a current compensation source, has a single function, does not interact with the grid, and does not have the ability to suppress grid-connected power fluctuations or provide grid support. Another type of scheme focuses on the overall control of the grid-connected point of the power station, which simulates the external characteristics of a synchronous generator to provide voltage, frequency and inertia support for the grid. However, this type of scheme regards the photovoltaic power station as a whole power source and lacks the ability to perceive and finely manage the serious power imbalance within the array caused by local shading. When there is a serious power imbalance within the array, the overall output characteristics of the power station deteriorate, which significantly increases the burden of station-level energy storage for smoothing and may even affect the effectiveness and reliability of its grid support, failing to solve the problem from the root.

[0004] In summary, the existing technical solutions either focus on power optimization within the array and fail to extend to the grid support level, or start from the overall power station and ignore the fine treatment of the root problem of internal power imbalance. There is a gap in the technical path between the two, making it difficult to simultaneously achieve power optimization of the photovoltaic array and grid strength enhancement of the grid-connected point under complex shading conditions. Therefore, an innovative regulation method is needed to deeply integrate and collaborate the fine balance compensation within the photovoltaic array and the active grid support of the power station.

[0005] The present application aims to overcome the limitations of the prior art, and provides a storage hierarchical charge-discharge regulation method capable of simultaneously solving the problems of internal power mismatch and external grid support. SUMMARY

[0006] The present application aims to overcome the limitations of the prior art, and provides a storage hierarchical charge-discharge regulation method capable of simultaneously solving the problems of internal power mismatch and external grid support.

[0007] The present application aims to overcome the limitations of the prior art, and provides a storage hierarchical charge-discharge regulation method capable of simultaneously solving the problems of internal power mismatch and external grid support.

[0008] Step one, each self-balancing network construction unit of the group string level real-time monitors the actual output power of the corresponding photovoltaic group string, and calculates the theoretical output power of the photovoltaic group string under the current environment based on the operating environment parameters of the photovoltaic group string. The group string level self-balancing network construction unit includes an energy storage battery and a bidirectional converter with independent network construction control capability.

[0009] Step two, each self-balancing network construction unit of the group string level calculates the power difference between the actual output power and the theoretical output power, and compares the power difference with a preset first power threshold. When the power difference is greater than the first power threshold, the photovoltaic group string is determined to be a weak group string affected by local shadow, and the local compensation mode is started.

[0010] Step three, the self-balancing network construction unit in the local compensation mode controls the energy storage battery to discharge through the bidirectional converter, so that the total active power output by the self-balancing network construction unit of the group string level tends to approach the theoretical output power. At the same time, the bidirectional converter of the self-balancing network construction unit of the group string level operates in the network construction mode to maintain the voltage and frequency stability of the AC bus of the group string connected to the self-balancing network construction unit of the group string level.

[0011] Step four, the station level cooperative network controller collects the operating state, power difference and state of charge information of the energy storage battery of all self-balancing network construction units of the group string level through a communication network, and monitors the voltage and frequency of the photovoltaic power station grid connection point.

[0012] Step five, when the state of charge of the energy storage battery of any one string-level self-balancing grid-connected unit in local compensation mode is lower than the preset state of charge threshold, the station-level cooperative grid controller initiates power cooperative scheduling, which includes instructing the station-level central grid-connected energy storage system to inject power into the string AC bus, or instructing other string-level self-balancing grid-connected units with a state of charge higher than the state of charge threshold to adjust the output power of the other string-level self-balancing grid-connected units to provide power support for the string-level self-balancing grid-connected unit with a low state of charge of the energy storage battery through the string AC bus;

[0013] Step six, when the grid-connected point voltage deviates from the reference voltage value set for the grid-connected point, the station-level cooperative grid controller calculates the total reactive power required by the entire station according to the voltage deviation, and dynamically allocates reactive power adjustment instructions to all string-level self-balancing grid-connected units and the station-level central grid-connected energy storage system, so that each string-level self-balancing grid-connected unit cooperatively issues or absorbs reactive power while completing its own active power adjustment task, to support grid-connected point voltage stability.

[0014] Further, in step two, the algorithm for calculating the power difference and determining the weak string is executed by the local controller embedded in each string-level self-balancing grid-connected unit. The theoretical output power of the string is calculated according to the following formula: ;

[0015] Wherein, Pstring represents the theoretical output power of the photovoltaic string under the current operating environment, η represents the photoelectric conversion efficiency of the photovoltaic module under standard test conditions, A represents the total area of all photovoltaic modules in the string, I represents the real-time solar irradiance collected by the irradiance sensor installed on the string, T represents the real-time temperature of the photovoltaic module backplane collected by the temperature sensor, Tref represents the reference temperature under standard test conditions, α represents the power temperature coefficient of the photovoltaic module.

[0016] Further, in step three, the string-level self-balancing grid-connected unit operates in grid-connected mode, specifically: the bidirectional converter adopts a voltage source control strategy, and the bidirectional converter adjusts the amplitude and phase of the modulation wave through its own controller to autonomously establish and maintain the voltage amplitude and frequency reference of the string AC bus, providing voltage and frequency support for other power electronic devices connected to the same bus, without relying on external grid voltage and frequency signals.

[0017] Further, the power coordinated scheduling in step five specifically includes: the station-level coordinated grid construction controller establishes a scheduling model with the optimization target of full-station power balance and each energy storage unit state of charge balance, and solves the optimal power instruction of each controllable unit, and the objective function of the scheduling model is based on minimizing the output power change rate of the station-level central energy storage and maximizing the consistency of the state of charge of each energy storage unit.

[0018] Further, in step six, the specific method of the station-level coordinated grid construction controller dynamically allocating reactive power regulation instructions is: according to the current apparent power capacity margin of each grid construction unit, i.e., each group of string-level self-balancing grid construction units and the station-level central grid construction energy storage system, and the response priority coefficient, the total reactive power demand is allocated in proportion, wherein the unit with larger capacity margin is allocated larger reactive power regulation amount, and the unit in idle or light load state has higher response priority.

[0019] Further, the method further comprises:

[0020] Step seven, virtual synchronous machine cluster control: the station-level coordinated grid construction controller uniformly configures virtual moment of inertia parameters and virtual damping parameters for bidirectional converters of all group string-level self-balancing grid construction units and the station-level central grid construction energy storage system, so that when the bidirectional converters respond to grid frequency changes, the bidirectional converters simulate the rotor motion equation of a synchronous generator, the rotor motion equation of the synchronous generator provides inertia support and damping effect for the grid, and the frequency-active power droop characteristic equation realized by the virtual synchronous machine control algorithm is: ;

[0021] wherein, represents the active power output value that needs to be adjusted by the grid construction unit according to the frequency deviation, represents the frequency droop coefficient, represents the rated frequency reference value, represents the actual grid frequency collected in real time, represents the set virtual moment of inertia.

[0022] Further, in step seven, all grid construction units adopt consistent virtual moment of inertia parameters and virtual damping parameters under the unified coordination of the station-level coordinated grid construction controller, so that the entire photovoltaic power station appears as an equivalent virtual synchronous generator with unified inertia and damping characteristics to the outside, participating in the primary frequency modulation of the grid.

[0023] Further, the group string level self-balancing network construction units and the station level cooperative network controller, and each group string level self-balancing network construction unit, communicate through a high-speed industrial Ethernet based on time-sensitive network technology, to ensure that power difference information, scheduling instructions and state feedback information are transmitted within a millisecond level time, to meet the strict requirements of fast cooperative control on communication delay.

[0024] Further, the energy storage battery is a lithium iron phosphate battery or a lithium titanate battery, the bidirectional converter is a three-phase voltage source converter with a full-bridge or three-level topology, and the switching device used by the bidirectional converter is an insulated gate bipolar transistor or a silicon carbide metal oxide semiconductor field effect transistor.

[0025] Further, the method further comprises:

[0026] Step eight, protective off-grid and seamless re-grid: when serious faults occur in the group string AC bus or the power station main grid, the station level cooperative network controller instructs all network construction units in the affected area to switch to island operation mode, and the network construction units in the area continue to cooperatively maintain the voltage and frequency stability of the local power grid, and after the fault is cleared and the grid voltage and frequency return to normal, the station level cooperative network controller adjusts the output voltage phase and amplitude of each network construction unit to realize synchronization of these voltages with the main grid, and realizes smooth and impact-free grid connection of all units.

[0027] Compared with the prior art, the energy storage grading charge and discharge regulation method for photovoltaic power fluctuation suppression has the following beneficial effects:

[0028] First, the group string level self-balancing network construction unit detects and compensates for power loss caused by local shadows in real time, and the station level cooperative network controller cooperatively schedules power and dynamically allocates reactive power, which can simultaneously realize fine power balancing within the photovoltaic array and active support of the grid at the grid connection point, thereby effectively suppressing photovoltaic power fluctuations and improving the overall power generation efficiency and grid stability of the photovoltaic power station under complex shading conditions.

[0029] Second, by uniformly configuring virtual synchronous machine control parameters for all network construction units and quickly switching to island operation mode and realizing seamless re-grid under fault conditions, the photovoltaic power station as a whole has inertia, damping and autonomous networking capabilities similar to synchronous generators, thereby enhancing the frequency regulation and fault ride-through capability of the grid and improving the operational reliability and safety of the power system in high-proportion photovoltaic access scenarios.

[0030] Other advantages, objectives and features of the invention will be set forth in part in the description which follows, and in part will be apparent to those skilled in the art from the following examination or study, or may be learned from the practice of the invention. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are merely some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without any creative effort.

[0032] Fig. 1 This is a diagram illustrating the method steps of the present invention;

[0033] Fig. 2 This is a schematic diagram of the system architecture and energy / information flow of the present invention;

[0034] Fig. 3 This is a flowchart of the core logic of hierarchical regulation in this invention. Detailed Implementation

[0035] To further illustrate the technical means and effects of the present invention in achieving its intended purpose, the following detailed description of the specific implementation methods, structures, features, and effects of the present invention, in conjunction with the accompanying drawings and preferred embodiments, is provided below.

[0036] Example

[0037] like Figs. 1 to 3 As shown in this embodiment, the photovoltaic power fluctuation smoothing energy storage hierarchical charging and discharging regulation method disclosed is applied to a large-scale centralized photovoltaic power station system. The system specifically includes 30 photovoltaic strings, 30 string-level self-balancing grid-building units corresponding to each photovoltaic string, a station-level central grid-building energy storage system, and a station-level collaborative grid-building controller. All devices are connected via high-speed industrial Ethernet based on time-sensitive networking technology to ensure rapid transmission of control commands and status information.

[0038] Photovoltaic string: Each photovoltaic string consists of 22 monocrystalline silicon photovoltaic modules connected in series. The rated power of a single photovoltaic module is 550W, the open-circuit voltage is 45V, and the short-circuit current is 13.5A. The photovoltaic string is installed at a tilt angle of 30° with the azimuth facing due south, and is used to convert solar energy into electrical energy output.

[0039] String-level self-balancing networking unit: Each string-level self-balancing networking unit integrates an energy storage battery and a bidirectional converter. The energy storage battery is a lithium iron phosphate battery with a rated capacity of 10 kWh, a rated voltage of 51.2 V, a charge-discharge rate of 1C, and a state of charge operating range of 10% to 90%. The bidirectional converter is a three-phase voltage source converter with a three-level topology, a rated power of 12 kW, an input voltage range of 45 V to 60 V, an output voltage of 380 V, an output frequency of 50 Hz, and insulated gate bipolar transistors as switching devices with a switching frequency of 10 kHz. It has independent networking control capability and bidirectional power transmission function. In addition, each string-level self-balancing networking unit also embeds a local controller with an STM32H743 high-performance microprocessor for power monitoring, theoretical power calculation, weak string determination, and local compensation control functions.

[0040] Station-level central networking energy storage system: The station-level central networking energy storage system is composed of a lithium iron phosphate battery cluster, a bidirectional converter cabinet, and a local monitoring unit. The total rated capacity is 500 kWh, the rated power is 250 kW, the rated voltage of the energy storage battery cluster is 600 V, the charge-discharge rate is 1C, and the state of charge operating range is 15% to 85%. The bidirectional converter cabinet uses a three-phase voltage source converter with a full-bridge topology, an output voltage of 10 kV, and a transformer connection with the photovoltaic power station main bus. It has independent adjustment capability for active power and reactive power.

[0041] Station-level cooperative networking controller: The station-level cooperative networking controller uses an industrial-grade server with an Intel Core i7 processor, 32 GB of memory, and 1 TB of storage capacity running a real-time operating system. It communicates with all string-level self-balancing networking units and station-level central networking energy storage systems through high-speed industrial Ethernet with a communication rate of 1 Gbps and a transmission delay of ≤10 ms. It collects operating state information, performs power cooperative scheduling, reactive power distribution, and virtual synchronous machine cluster control.

[0042] Auxiliary detection equipment: Each photovoltaic string is equipped with an irradiance sensor and a temperature sensor. The irradiance sensor is a TBQ-2 model with a measurement range of 0 to 2000 W / m² and a measurement accuracy of ±5%. The temperature sensor is a PT100 platinum resistance with a measurement range of -40°C to 120°C and a measurement accuracy of ±0.5°C. They are used to collect solar irradiance and photovoltaic module backboard temperature in real time. Voltage and current transformers are installed at the grid-connected point of the photovoltaic power station. The voltage transformer has a ratio of 10 kV / 100 V, and the current transformer has a ratio of 200 A / 5 A. They are used to monitor voltage and current signals at the grid-connected point.

[0043] In this embodiment, the method steps are implemented as follows:

[0044] Step one, actual output power monitoring and theoretical output power calculation:

[0045] The local controller of the string-level self-balancing network unit corresponding to each photovoltaic string, through its internal voltage sampling module and current sampling module, collects the output voltage and output current of the photovoltaic string in real time, with a sampling frequency of 1 kHz. According to the active power calculation formula , the actual output power of the photovoltaic string is calculated, where is the effective value of the output line voltage of the photovoltaic string obtained by sampling, is the effective value of the output line current of the photovoltaic string obtained by sampling, is the power factor, which is detected by phase-locked loop technology, and is 0.98 to 1.0 when operating normally.

[0046] At the same time, the local controller calculates the theoretical output power under the current environment based on the operating environment parameters of the photovoltaic string through the theoretical output power formula, as follows:

[0047] The parameters are explained as follows:

[0048] : The photoelectric conversion efficiency of the photovoltaic module under standard test conditions, the standard test conditions are defined as irradiance 1000 W / m², module temperature 25℃, and atmospheric mass AM1.5. The of the monocrystalline silicon photovoltaic module used in this embodiment is 23.5%, which is the calibration value of the photovoltaic module at the factory, and can be obtained by querying the datasheet of the module.

[0049] : The total area of all photovoltaic modules of the photovoltaic string, the size of each photovoltaic module in this embodiment is 1.6m x 1.0m, the area of a single module is 1.6m², and each photovoltaic string contains 22 modules, so , the unit is square meter.

[0050] : The solar irradiance collected in real time by the irradiance sensor installed on the string, the unit is watt per square meter, the sensor outputs collected data every 100ms, the local controller performs sliding average filtering on the collected data to remove random interference, and the filtering window length is 10 sampling points.

[0051] ​The temperature of the photovoltaic module backsheet is collected in real time by a temperature sensor, in degrees Celsius. The sensor outputs the collected data every 100ms. The local controller performs first-order low-pass filtering on the collected data, with a filtering time constant of 0.1s.

[0052] The reference temperature under standard test conditions is 25℃. This parameter is a recognized standard test condition parameter in the photovoltaic industry and is used to unify the temperature benchmark for photovoltaic module performance calibration.

[0053] The power temperature coefficient of a photovoltaic module, expressed as % / ℃, represents the percentage decrease in output power for every 1℃ increase in module temperature. In this embodiment, a monocrystalline silicon photovoltaic module is used. The value is -0.38% / ℃. This parameter is the factory calibration value of the component and can be obtained by looking up the component datasheet. The negative sign indicates that the output power decreases when the temperature increases.

[0054] Example calculation: Suppose that at a certain moment, the irradiance sensor collects... Temperature sensor collected , Then substitute into the formula to calculate: ;

[0055] Step 2, Power Difference Calculation and Weak String Identification:

[0056] The local controller of each cascaded self-balancing network unit determines the actual output power obtained in step one. and theoretical output power Calculate the power difference The unit is watts.

[0057] The local controller presets a first power threshold. The threshold is set based on the rated power of the photovoltaic string and the allowable range of power fluctuations during actual operation. In this embodiment, the rated power of the photovoltaic string is 12.1kW, with 22 strings × 550W. Considering the power deviation caused by normal environmental fluctuations, the threshold is set... That is, 10% of the rated power.

[0058] The local controller will calculate the power difference. With the first power threshold Comparison:

[0059] when At that time, it was determined that the photovoltaic string was operating normally, with no obvious local shading effect, the string-level self-balancing grid unit maintained the normal operating mode, and the energy storage battery did not perform charging and discharging compensation.

[0060] when When the photovoltaic string is determined to be a weak string affected by local shading, the local controller activates the local compensation mode and uploads the activation status information to the station-level collaborative network controller via the communication network.

[0061] Example: Suppose a photovoltaic string has... The actual data collected and calculated ,but The string is determined to be a weak string, and the local compensation mode is activated.

[0062] Step 3: Local compensation mode operation and string AC bus stability control:

[0063] In string-level self-balancing grid units operating in local compensation mode, the local controller adjusts the power difference based on the local compensation mode. Generate discharge power command for energy storage battery Ignoring converter losses, in practical applications, the losses can be corrected using a loss compensation coefficient. This allows the energy storage battery to discharge to the string AC bus via a bidirectional converter, thereby increasing the total active power output of the string-level self-balancing grid unit. Approaching the theoretical output power .

[0064] Meanwhile, the bidirectional converter of the string-level self-balancing grid unit operates in grid mode, specifically adopting a voltage source control strategy. Its core is to adjust the amplitude and phase of the modulation wave through the controller of the bidirectional converter, and autonomously establish and maintain the voltage amplitude and frequency reference of the string AC bus.

[0065] Specific implementation of the voltage source control strategy: The controller of the bidirectional converter adopts a droop control algorithm, and the voltage amplitude droop characteristic is as follows: The frequency droop characteristic is ,in:

[0066] The rated voltage amplitude of the string AC bus is set to 380V in this embodiment, which is the effective value of the line voltage.

[0067] This is the voltage droop factor, with a value of 0.001V / Var, in volts per var, used to adjust the voltage amplitude according to the reactive power output.

[0068] This represents the reactive power output value of the bidirectional converter, expressed in volt-amperes (VA).

[0069] This is the reference value for the rated frequency, which is 50Hz.

[0070] This is the frequency droop factor, with a value of 0.002 Hz / kW, in Hertz per kilowatt, used to adjust the frequency based on active power output.

[0071] The bidirectional converter acquires the voltage and frequency signals of the string AC bus in real time through its own controller, compares them with a reference value, and generates amplitude and phase control signals for the modulation wave through a PI regulator. This drives the insulated-gate bipolar transistor (IGBT) to switch, achieving closed-loop control of the output voltage and frequency. This grid configuration does not rely on the voltage and frequency signals of the external power grid and can autonomously establish a stable string AC bus voltage and frequency reference. It provides reliable voltage and frequency support for other power electronic devices connected to the same bus, ensuring the stable operation of the power system at the string level.

[0072] Example: A weak string The local controller instructs the energy storage battery to discharge at a power of 1.78kW. After conversion by the bidirectional converter, the output is sent to the string AC bus. At this time, the total active power of the string-level self-balancing grid unit is... The output power is consistent with the theoretical output power. The bidirectional converter stabilizes the string AC bus voltage within the range of 380V±5V and the frequency within the range of 50Hz±0.2Hz through a voltage source control strategy.

[0073] Step 4: Collection of operational status information and monitoring of grid connection points:

[0074] The station-level collaborative network controller periodically collects the operating status information of all string-level self-balancing network units through a high-speed industrial Ethernet based on time-sensitive networking technology, according to a preset communication protocol, with a collection period of 100ms.

[0075] The collected information includes the following:

[0076] Operating status: Normal operating mode or local compensation mode.

[0077] Power parameters: actual output power Theoretical output power Power difference .

[0078] Energy storage battery status includes: State of Charge (SOC), battery voltage, and battery temperature. The SOC is calculated using the ampere-hour integration method, with the following formula: , The initial state of charge, For battery charging and discharging current, In this embodiment, the rated capacity of the battery is... .

[0079] Meanwhile, the station-level collaborative grid controller collects voltage and current signals through voltage and current transformers installed at the grid connection point, with a sampling frequency of 1kHz, and calculates the grid connection point voltage. and frequency And monitor its changes in real time.

[0080] Step 5, Power Co-scheduling Execution:

[0081] The station-level collaborative grid controller monitors the state of charge (SOC) of the energy storage batteries of each string-level self-balancing grid unit in local compensation mode in real time, and compares it with a preset SOC threshold. A comparison is made. In this embodiment, considering the cycle life and depth of discharge limitations of the energy storage battery, the following settings are made: .

[0082] When the SOC of the energy storage battery of any string-level self-balancing grid unit in local compensation mode is lower than At this time, the station-level collaborative network controller initiates power collaborative scheduling, and the specific process is as follows:

[0083] Scheduling model establishment: The station-level collaborative network controller establishes a scheduling model with the optimization objectives of overall station power balance and balanced state of charge of each energy storage unit.

[0084] The objective function is: ;

[0085] in, , The weighting coefficient is set according to actual operational requirements. In this embodiment... , , dimensionless.

[0086] This represents the change in output power of the station-level central grid energy storage system at time k, expressed in kW.

[0087] In this embodiment, the number of string-level self-balancing network units is [number to be specified]. .

[0088] Let represent the state of charge of the i-th string-level self-balancing grid unit energy storage battery.

[0089] This represents the average state of charge (SOC) of all self-balancing grid-connected energy storage cells in the string-level unit, expressed in units of %.

[0090] The constraints include:

[0091] Energy storage battery charge and discharge power constraints: In this embodiment .

[0092] Energy storage battery state of charge constraints: In this embodiment , .

[0093] Power constraints of station-level central grid-connected energy storage systems: In this embodiment .

[0094] Optimal power command solution: The scheduling model is solved using a particle swarm optimization algorithm to obtain the optimal power command for each controllable unit. The parameters of the particle swarm optimization algorithm are set as follows: number of particles 50, number of iterations 30, inertia weight 0.7, cognitive factor 1.5, social factor 1.5, and solution accuracy 0.1kW.

[0095] Power support execution: When the station-level central grid-connected energy storage system has power support capability, the station-level collaborative grid-connected controller issues a power injection command to it. It controls the injection of active power into the string AC bus through the bidirectional converter. This power is then transmitted through the string AC bus to the string-level self-balancing grid unit where the energy storage battery SOC is low, to charge the energy storage battery or directly supplement its output power gap.

[0096] When the state of charge is higher than When constructing other string-level self-balancing network units, the station-level collaborative network controller issues power adjustment commands to these units. Adjust its output power to provide power support to units with low SOC through the string AC bus.

[0097] Example: Suppose that the SOC of the energy storage battery in a string-level self-balancing grid unit in local compensation mode drops to 18%, and the power difference... The station-level collaborative grid controller, through a scheduling model, obtains an injection power command of 1.8kW for the station-level central grid-connected energy storage system. Simultaneously, it instructs two adjacent string-level self-balancing grid-connected units with a state of charge (SOC) of 60% to each adjust their output power by 0.1kW. The station-level central grid-connected energy storage system injects 1.8kW of active power into the string's AC bus according to the command, supplementing the power gap of the weak string. At this point, the discharge power of the energy storage battery in that string decreases, the SOC stops declining continuously, and gradually stabilizes.

[0098] Step Six: Dynamic Reactive Power Distribution and Grid Connection Point Voltage Support:

[0099] The station-level collaborative grid controller monitors the voltage at the grid connection point in real time. and the preset grid connection point reference voltage (In this embodiment, the voltage is set to 10kV, the effective value of the line voltage) The voltage deviation is calculated by comparing the values. The unit is kilovolts.

[0100] when That is, when the voltage deviation exceeds 3%, the station-level collaborative grid controller calculates the total reactive power required by the entire station based on the voltage deviation. The calculation uses a proportional-integral control algorithm: ;

[0101] in This is the proportionality coefficient. The integral coefficient is determined through engineering debugging to ensure the speed and stability of reactive power regulation.

[0102] The station-level collaborative network controller determines the current apparent power capacity margin of each network unit. and response priority coefficient The total reactive power demand is allocated proportionally, and the specific allocation formula is as follows: ;

[0103] The parameters are explained below:

[0104] Apparent power capacity margin The unit is kilovolt-amperes; among which Let i be the rated apparent power of the i-th grid cell. This represents the current active power output value. This represents the current reactive power output value.

[0105] Response Priority Coefficient Setting rules: Network elements in an idle state, Network elements under light load conditions Mesh elements under heavy load 3.

[0106] The station-level collaborative grid controller sends reactive power adjustment commands to each grid unit through the communication network. Each cascade self-balancing grid unit and the bidirectional converter of the station-level central grid energy storage system, according to the commands, coordinates the generation or absorption of reactive power while completing their respective active power adjustment tasks, so as to support the voltage stability of the grid connection point.

[0107] Example: Assume the grid connection point reference voltage Actual monitoring ,but Calculate the total reactive power requirement: (The integral term is taken as 2s in steady state, i.e.) ),but .

[0108] Assume there are 10 network units in the system, of which 3 are idle ( , ), 5 light-load ( , ), 2 heavy-duty ( , Then the total weight sum is:

[0109] ;

[0110] The reactive power allocation of a certain idle grid unit is:

[0111] ;

[0112] The grid unit sends out 18.5kVar of reactive power according to the instruction, and other grid units allocate reactive power adjustment tasks proportionally to support the grid connection point voltage to recover to the range of 10kV±0.3kV.

[0113] Step 7, Virtual Synchronizer Cluster Control Execution:

[0114] The station-level collaborative grid controller is a bidirectional converter for all string-level self-balancing grid units and the station-level central grid energy storage system, with unified configuration of virtual rotational inertia parameters. and virtual damping parameters (In this embodiment) This parameter configuration is sent to the controller of each network unit through the communication network and stored in the local register.

[0115] When the grid frequency changes, the bidirectional converters of each grid unit simulate the rotor motion equations of a synchronous generator according to the virtual synchronous machine control algorithm, providing inertial support and damping for the grid. The frequency-active power droop characteristic equation implemented by the virtual synchronous machine control algorithm is as follows:

[0116] ;

[0117] Detailed explanations of each parameter are as follows:

[0118] The active power output value that the network unit needs to adjust according to the frequency deviation, in kilowatts. A positive value indicates an increase in active power output, and a negative value indicates a decrease in active power output.

[0119] Frequency droop factor, in kilowatts per hertz, is set to 50 kW / Hz in this embodiment and is used to adjust the active power according to the static frequency deviation.

[0120] : Rated frequency reference value, taken as 50Hz, unit is Hertz.

[0121] The actual frequency of the power grid, measured in Hertz, is collected in real time and obtained through the frequency monitoring module at the grid connection point.

[0122] The virtual moment of inertia is set in kilograms per square meter, simulating the moment of inertia of the synchronous generator rotor. Increasing this parameter can improve the frequency stability of the system, but will reduce the frequency response speed; decreasing it will have the opposite effect. The value in this embodiment was determined through engineering experiments.

[0123] The rate of change of frequency deviation, measured in Hertz per second, is obtained by differentiating the frequency deviation signal using a first-order difference algorithm.

[0124] Virtual damping parameter, in kilowatts per hertz, is used to suppress frequency oscillations and improve the dynamic stability of the system. The value in this embodiment was determined through engineering debugging.

[0125] Under the unified coordination of the station-level collaborative grid controller, all grid-connecting units adopt consistent virtual rotational inertia parameters and virtual damping parameters, so that the entire photovoltaic power station appears to the outside world as an equivalent virtual synchronous generator with uniform inertia and damping characteristics, participating in the primary frequency regulation of the power grid.

[0126] Example: Assume the actual frequency of the power grid Frequency deviation Frequency deviation change rate ,but:

[0127]

[0128] Each grid unit increases its active power output according to its own power allocation ratio, thereby increasing the total active power of the entire photovoltaic power station, which suppresses further decline in grid frequency and supports grid frequency stability.

[0129] Step 8: Implementation of protective off-grid and seamless reconnection:

[0130] The station-level collaborative network controller monitors the operating status of the string AC bus and the main power grid of the power station in real time. It determines whether a serious fault has occurred by using the characteristic quantities of voltage and current signals. The criteria for determining a serious fault are as follows:

[0131] The string AC bus voltage drops to below 50% of the rated value for more than 10ms.

[0132] The voltage of the power plant's main grid drops to below 60% of its rated value and lasts for more than 10ms.

[0133] The power grid frequency deviation exceeds ±2Hz and lasts for more than 5ms.

[0134] Upon detecting the aforementioned severe fault, the station-level collaborative network controller immediately issues an islanding operation command via the communication network, instructing all network units within the affected area to switch to islanding operation mode. In islanding operation mode, each network unit continues to employ a voltage source control strategy to collaboratively maintain the voltage and frequency stability of the local power grid. The voltage is stabilized within the range of 380V±10V, and the frequency is stabilized within the range of 50Hz±0.5Hz, ensuring continuous power supply to critical loads within the islanded area.

[0135] After the fault is cleared, the station-level collaborative grid controller monitors and confirms that the grid voltage and frequency have returned to normal, and then initiates the seamless reconnection process:

[0136] The station-level collaborative grid controller adjusts the output voltage amplitude of each grid unit to ensure that the voltage amplitude difference between the station and the main grid is ≤5%.

[0137] Adjust the output frequency of each grid unit to make the frequency difference between it and the main grid ≤ 0.2Hz, and use a phase tracking algorithm to make the phase difference between the output voltage of each grid unit and the main grid voltage ≤ 10°.

[0138] Once the voltage amplitude, frequency, and phase all meet the synchronization conditions, the station-level collaborative grid controller issues a grid connection command, and the grid connection switches of each grid unit are closed, achieving smooth grid connection without impact. During the grid connection process, the peak value of the inrush current does not exceed 1.2 times the rated current.

[0139] In some alternative embodiments, the energy storage battery is a lithium iron phosphate battery, the bidirectional converter adopts a three-level topology, and the switching device is an insulated gate bipolar transistor; the energy storage battery is replaced with a lithium titanate battery, the bidirectional converter is replaced with a two-level three-phase voltage source converter with a full-bridge topology, and the switching device is replaced with a silicon carbide metal oxide semiconductor field-effect transistor.

[0140] The lithium titanate battery has a rated capacity of 10kWh, a rated voltage of 50V, a charge / discharge rate of 2C, and a state of charge range of 5% to 95%, offering better cycle life and fast charge / discharge performance. The two-level bidirectional converter with a full-bridge topology has a rated power of 12kW and a switching frequency of 20kHz. The silicon carbide metal oxide semiconductor field-effect transistor has lower switching losses and better high-temperature resistance.

[0141] The implementation logic of each step is the same as in the above embodiment; only the relevant parameters need to be adjusted.

[0142] In the calculation of theoretical output power, no parameter adjustment is required because the battery type does not affect the theoretical output power of the photovoltaic string.

[0143] The discharge power command for the energy storage battery can be adjusted to a maximum discharge power of 20kW based on the 2C charge / discharge rate of the lithium titanate battery.

[0144] In the voltage source control strategy of the bidirectional converter, the switching frequency is adjusted to 20kHz, and the PI regulator parameters are optimized accordingly to ensure voltage and frequency control accuracy.

[0145] The system can still achieve photovoltaic power fluctuation smoothing, string AC bus stabilization and grid support functions, and has certain advantages in terms of fast response and long life.

[0146] In some optional embodiments, based on the above embodiments, the power coordinated scheduling model in step five is optimized by introducing environmental prediction information, adjusting the objective function and constraints of the scheduling model, and improving the foresight and rationality of the scheduling.

[0147] The solar irradiance prediction system installed in the photovoltaic power station obtains the predicted value for the next hour. And input it into the station-level collaborative network controller.

[0148] The objective function of the scheduling model is:

[0149]

[0150]

[0151] in To add weighting coefficients, This is the theoretical output power prediction calculated based on the predicted irradiance. This represents the predicted total output power.

[0152] Add the following to the constraints: This means limiting the future discharge power of the energy storage battery based on the current SOC and predicted irradiance to avoid over-discharge.

[0153] When performing power coordination scheduling, the station-level collaborative grid controller not only considers the current power balance and SOC balance, but also predicts future power changes in advance, rationally allocates the power support tasks of each controllable unit, reduces the frequent adjustment of the station-level central energy storage system, and improves the system's operating efficiency and stability.

[0154] This embodiment utilizes a hierarchical control method combining string-level local compensation and station-level collaborative scheduling to effectively mitigate power fluctuations in photovoltaic strings caused by local shading, bringing the output power of the string-level self-balancing grid unit closer to the theoretical output power and improving the overall power generation efficiency of the photovoltaic array. Through the grid configuration mode of the string-level self-balancing grid unit, it autonomously maintains the voltage and frequency stability of the string AC bus, reducing dependence on the external power grid. The station-level collaborative grid controller's power collaborative scheduling and dynamic reactive power allocation reduce the operational burden on the station-level central energy storage system, enhance the photovoltaic power station's proactive support capability for the grid, and improve the stability of voltage and frequency at the grid connection point. Virtual synchronous machine cluster control and protective off-grid and seamless re-grid functions further enhance the grid friendliness and operational reliability of the photovoltaic power station.

[0155] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. A method for graded charge and discharge regulation of energy storage to smooth photovoltaic power fluctuations, characterized in that, The method, applicable to a photovoltaic power station system comprising multiple photovoltaic strings, string-level self-balancing grid-connecting units corresponding to each photovoltaic string, a station-level central grid-connecting energy storage system, and a station-level collaborative grid-connecting controller, includes the following steps: Step 1: Each string-level self-balancing grid unit monitors the actual output power of its corresponding photovoltaic string in real time, and calculates the theoretical output power of the photovoltaic string under the current environment based on the operating environment parameters of the photovoltaic string. The string-level self-balancing grid unit includes an energy storage battery and a bidirectional converter with independent grid control capability. Step 2: Each string-level self-balancing grid unit calculates the power difference between the actual output power and the theoretical output power, and compares the power difference with a preset first power threshold. When the power difference is greater than the first power threshold, the photovoltaic string is determined to be a weak string affected by local shading, and the local compensation mode is activated. Step 3: The string-level self-balancing grid unit in the local compensation mode controls the energy storage battery to discharge through the bidirectional converter, so that the total active power output by the string-level self-balancing grid unit approaches the theoretical output power. At the same time, the bidirectional converter of the string-level self-balancing grid unit operates in grid mode to maintain the voltage and frequency stability of the string AC bus connected to the string-level self-balancing grid unit. Step four: The station-level collaborative grid controller collects the operating status, power difference, and state of charge information of all string-level self-balancing grid units through the communication network, and monitors the voltage and frequency of the photovoltaic power station grid connection point. Step 5: When the state of charge (SOC) of the energy storage battery of any string-level self-balancing grid unit in local compensation mode is lower than the preset SOC threshold, the station-level collaborative grid controller initiates power collaborative scheduling. The power collaborative scheduling includes instructing the station-level central grid energy storage system to inject power into the string AC bus, or instructing other string-level self-balancing grid units with an SOC higher than the SOC threshold to adjust the output power of the other string-level self-balancing grid units, so as to provide power support to the string-level self-balancing grid unit with the low SOC of the energy storage battery through the string AC bus. Step six: When the voltage at the grid connection point deviates from the reference voltage value set for that grid connection point, the station-level collaborative grid controller calculates the total reactive power required for the entire station based on the voltage deviation, and dynamically distributes reactive power adjustment commands to all string-level self-balancing grid units and the station-level central grid energy storage system. This enables each string-level self-balancing grid unit to coordinate the generation or absorption of reactive power while completing its own active power adjustment tasks, thereby supporting the stability of the grid connection point voltage.

2. The method for graded charge and discharge regulation of energy storage to smooth photovoltaic power fluctuations according to claim 1, characterized in that, In step two, the algorithms for calculating the power difference and determining the weak string groups are executed by the local controller embedded in each string-level self-balancing network unit. The theoretical output power... The calculation formula is: ;in, This represents the theoretical output power of the photovoltaic string under the current operating environment. This indicates the photoelectric conversion efficiency of a photovoltaic module under standard test conditions. This represents the total area of ​​all photovoltaic modules in the photovoltaic string. This represents the solar irradiance collected in real time by the irradiance sensor installed on the string. This indicates the real-time temperature of the photovoltaic module's backsheet, as collected by a temperature sensor. This indicates the reference temperature under standard test conditions. This represents the power temperature coefficient of a photovoltaic module.

3. The method for graded charge and discharge regulation of energy storage to smooth photovoltaic power fluctuations according to claim 1, characterized in that, In step three, the string-level self-balancing grid-building unit operates in grid-building mode as follows: the bidirectional converter adopts a voltage source control strategy. The bidirectional converter adjusts the modulation amplitude and phase through its own controller to autonomously establish and maintain the voltage amplitude and frequency reference of the string AC bus, providing voltage and frequency support for other power electronic devices connected to the same bus, without relying on the voltage and frequency signals of the external power grid.

4. The method for graded charge and discharge regulation of energy storage to smooth photovoltaic power fluctuations according to claim 1, characterized in that, The power coordinated scheduling in step five specifically includes: the station-level coordinated network controller establishing a scheduling model with the optimization objectives of overall station power balance and balanced state of charge of each energy storage unit, and solving for the optimal power command of each controllable unit. The objective function of the scheduling model is based on minimizing the output power change rate of the station-level central energy storage and maximizing the consistency of the state of charge of each energy storage unit.

5. The method for graded charge and discharge regulation of energy storage to smooth photovoltaic power fluctuations according to claim 1, characterized in that, In step six, the specific method for the station-level collaborative grid controller to dynamically allocate reactive power adjustment commands is as follows: based on the current apparent power capacity margin and response priority coefficient of each grid unit, namely each group of cascade self-balancing grid units and the station-level central grid energy storage system, the total reactive power demand is allocated proportionally. Among them, units with larger capacity margins are allocated larger reactive power adjustment amounts, and units in idle or light-load states have higher response priorities.

6. The method for graded charge and discharge regulation of energy storage to smooth photovoltaic power fluctuations according to claim 1, characterized in that, The method further includes: Step 7, Virtual Synchronous Machine Cluster Control: The station-level collaborative grid controller uniformly configures virtual rotational inertia parameters and virtual damping parameters for all string-level self-balancing grid units and the bidirectional converters of the station-level central grid energy storage system. When these bidirectional converters respond to changes in grid frequency, they simulate the rotor motion equations of synchronous generators, providing inertial support and damping for the grid. The frequency-active power droop characteristic equation implemented by the virtual synchronous machine control algorithm is as follows: ;in, This indicates the active power output value that the network unit needs to adjust based on the frequency deviation. Indicates the frequency droop factor. Indicates the reference value of the rated frequency. This represents the actual frequency of the power grid as collected in real time. This represents the set virtual moment of inertia.

7. The method for graded charge and discharge regulation of energy storage to smooth photovoltaic power fluctuations according to claim 6, characterized in that, In step seven, all grid-building units, under the unified coordination of the station-level collaborative grid-building controller, adopt consistent virtual rotational inertia parameters and virtual damping parameters, so that the entire photovoltaic power station appears to the outside world as an equivalent virtual synchronous generator with uniform inertia and damping characteristics, participating in the primary frequency regulation of the power grid.

8. The method for graded charge and discharge regulation of energy storage to smooth photovoltaic power fluctuations according to claim 1, characterized in that, The string-level self-balancing network building unit communicates with the station-level collaborative network building controller, as well as with each string-level self-balancing network building unit, via high-speed industrial Ethernet based on time-sensitive networking technology.

9. The method for graded charge and discharge regulation of energy storage to smooth photovoltaic power fluctuations according to claim 1, characterized in that, The energy storage battery is a lithium iron phosphate battery or a lithium titanate battery, the bidirectional converter is a three-phase voltage source converter with a full-bridge or three-level topology, and the switching device used in the bidirectional converter is an insulated gate bipolar transistor or a silicon carbide metal oxide semiconductor field-effect transistor.

10. The method for graded charge and discharge regulation of energy storage to smooth photovoltaic power fluctuations according to claim 1, characterized in that, The method further includes: Step 8, Protective Off-Grid and Seamless Reconnection: When a serious fault is detected in the string AC bus or the main power grid of the power station, the station-level collaborative grid controller instructs all grid units in the affected area to switch to islanded operation mode. The grid units in this area continue to work together to maintain the voltage and frequency stability of the local power grid. After the fault is cleared and the power grid voltage and frequency return to normal, the station-level collaborative grid controller adjusts the output voltage phase and amplitude of each grid unit.

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