A frequency control method for a photovoltaic power storage system based on photovoltaic power backup

Abstract

The application discloses a kind of photovoltaic power backup based on light storage system coordination frequency control method, light storage system is flexibly converted between power backup control and frequency control two modes: under power backup control, photovoltaic system can reserve part of output, increase the available power capacity of system frequency modulation under the premise of avoiding a large number of energy storage demand, and energy storage battery effectively absorbs the periodic peak power generated by photovoltaic power backup control, smooths photovoltaic output;When frequency is disturbed, light storage system coordination control effectively utilizes the backup power of photovoltaic array and the storage capacity of energy storage battery, enhances the support ability of power grid frequency, can reduce the degree of frequency disturbance under large disturbance, can maintain system frequency within the allowable range fluctuation when facing small disturbance;In addition, energy storage can also suppress the photovoltaic output fluctuation caused by irradiance fluctuation, improve light storage output power quality and power grid frequency support performance.

Description

Technical Field

[0001] This invention relates to the field of new energy grid-connected coordinated frequency control technology, specifically a method for coordinated frequency control of photovoltaic power backup photovoltaic energy storage systems. Background Technology

[0002] With the rapid development of the new energy industry, the proportion of new energy installed capacity has increased significantly. Photovoltaics is an ideal sustainable energy source with promising development prospects. However, the continuous increase in photovoltaic power generation units reduces the inertia of modern power systems, weakens the system's frequency regulation capability, and may lead to critical frequency stability issues. Frequency is one of the key variables in AC power grids, determining the synchronization state of various devices in the grid and greatly affecting the safe and stable operation of the grid.

[0003] To alleviate this problem and maintain system frequency stability, current approaches primarily involve two methods for actively participating in frequency regulation: configuring additional energy storage systems for photovoltaic (PV) systems to participate in frequency regulation, or using power reserve control for PV systems to reserve some output in advance to respond to grid power demands. Research on the former focuses on the types and composition of energy storage configurations and studies the cooperative use of PV-energy storage systems, but does not particularly emphasize its supporting role in grid frequency regulation. Research on the latter focuses on PV systems utilizing their own power reserve control to reserve output for subsequent frequency regulation, and scholars have designed numerous power reserve control schemes. However, configuring only energy storage or using only PV power reserve control presents the system with high energy storage configuration costs or severe curtailment issues, which is not cost-effective. Therefore, it is necessary to explore simpler, more effective, and lower-cost control methods and frequency regulation strategies for PV systems.

[0004] The existing technical solutions and their disadvantages are as follows:

[0005] Measure 1: Equip photovoltaic systems with hybrid energy storage devices consisting of batteries and supercapacitors. The following reference can be consulted: [Reference 1] Z. Cabrane, M. Ouassaid and M. Maaroufi, "Management and control of the integration of supercapacitor in photovoltaic energy storage", 2017 International Conference on Green Energy Conversion Systems (GECS), 2017, pp.1-6.

[0006] This measure uses a hybrid energy storage system of batteries and supercapacitors, which can extend battery life by eliminating peak current and reducing battery stress, but it increases the complexity of the system structure and the difficulty of control, making it relatively difficult to implement.

[0007] Measure 2: Utilize batteries to quickly absorb or release power to smooth and stabilize photovoltaic output power. See the following references for further information:

[0008] [Reference 2] H.Yong, S.Jun, Z.Qiang, Z.Liang and W.Chuanxiu, "ControlStrategy of Grid Connected Photovoltaic Power with Energy Storage System," 2020IEEE 4th Information Technology, Networking, Electronic and AutomationControl Conference (ITNEC), 2020, pp.2409-2413.

[0009] [Reference 3] PCSekhar and S.Mishra, "Storage free smart energy management for frequency control in a diesel-PV-fuel cell-based hybrid ACmicrogrid," IEEE Trans.Neural Netw.Learn.Syst., vol.27, no.8, pp.1657–1671, Aug.2016.

[0010] However, this measure only considers smoothing and stabilizing the output power of grid-connected photovoltaics when configuring energy storage battery systems, and does not focus on the process of photovoltaic energy storage systems participating in frequency regulation.

[0011] Measure 3: Implement power reserve control for photovoltaic systems to participate in system frequency regulation. Refer to the following references:

[0012] [Reference 4] Q.Peng, Z.Tang, Y.Yang and F.Blaabjerg, "Event-TriggeringPower Reserve Control for Grid-Connected PV Systems," 2020IEEE Applied PowerElectronics Conference and Exposition(APEC), 2020, pp.417-423.

[0013] This measure uses traditional photovoltaic power backup control methods, which are difficult to avoid due to computational complexity or high sensor costs. Furthermore, due to the volatility and randomness of photovoltaic output, it cannot provide stable frequency regulation power.

[0014] Measure 4: A direct and efficient photovoltaic power reserve control method based on maximum power point tracking (MPPT) can be found in the following references:

[0015] [Reference 5] Q.Peng, Z.Tang, Y.Yang, T.Liu and F.Blaabjerg, "Event-Triggering Virtual Inertia Control of PV Systems With Power Reserve," in IEEETransactions on Industry Applications, vol.57, no.4, pp.4059-4070, July-Aug.2021.

[0016] While this measure optimizes and solves the problem of the difficulty in implementing traditional photovoltaic power reserve control methods, the curtailment problem caused by its power reserve control is quite prominent, which essentially limits photovoltaic production and is less economical.

[0017] Measure 5: A relatively stable photovoltaic power backup control algorithm was designed by utilizing the IV characteristics of photovoltaic panels. The following references can be consulted:

[0018] [Reference 6] F.Soltanian, "A Stable Power Reserve Control Method inPhotovoltaic Systems Using IV Curve Characteristics," 2019 10th International Power Electronics, Drive Systems and Technologies Conference (PEDSTC), Shiraz, Iran, 2019, pp.487-491.

[0019] This measure employs a simple photovoltaic power reserve control based on current limitation, which increases the stability of the photovoltaic reserve output mode to some extent. However, it does not discuss the supporting role of photovoltaic power on the system frequency under this control and ignores the instability caused by changes in photovoltaic output to the system frequency. Summary of the Invention

[0020] To address the aforementioned problems, the present invention aims to provide a frequency control method for a photovoltaic-storage system based on photovoltaic power reserve. This method can fully and flexibly utilize the regulation capability of the energy storage battery in different control modes, achieving smooth output power of the photovoltaic-storage system in photovoltaic power reserve control; when the grid frequency is disturbed, the energy storage battery and photovoltaic array work together to provide frequency support to the grid; and it can suppress photovoltaic output fluctuations caused by irradiance fluctuations, improving the output power quality of the photovoltaic-storage system and the grid frequency support performance. The technical solution is as follows:

[0021] A method for coordinated frequency control of a photovoltaic power backup system based on photovoltaic power storage includes the following steps:

[0022] Step 1: Determine the photovoltaic power backup control method

[0023] The power-voltage variation of the photovoltaic system during two consecutive power reserve control cycles is as follows:

[0024] During the nth cycle, the photovoltaic system quickly finds the maximum power point A through maximum power point tracking control (e.g., incremental conductance method), and then operates at the power reserve point B through power reserve control, so that the photovoltaic system generates constant power until the end of the cycle;

[0025] At the beginning of the (n+1)th cycle, the photovoltaic system switches from power reserve mode to maximum power mode. This means the actual photovoltaic output no longer remains at the reference value in power reserve mode, but instead operates at the actual power point C based on the actual irradiance and temperature on the photovoltaic panels. The voltage values ​​at point C are the same as at point B. Then, under maximum power point tracking control, the operating point moves from point C until it reaches a new peak value at point D. Afterward, it switches back to power reserve mode: based on the given power reserve reference value, the operating point moves from the new peak value at point D. Proportional-integral (PI) control is used to quickly move the photovoltaic system's operating point to the left-hand power reserve point E, and it reaches a stable state within this cycle. Then, it operates with a constant power output, waiting for the maximum power mode trigger signal to enter the next power reserve cycle.

[0026] Step 2: Establish a photovoltaic-energy storage battery coordinated control model

[0027] A photovoltaic system is configured with energy storage batteries to form a photovoltaic-storage system. In the DC-DC section of the photovoltaic system, during each power reserve cycle, the photovoltaic system continuously switches between maximum power mode and power reserve mode to realize photovoltaic power reserve control, thereby enabling the photovoltaic system to reserve some output to prepare for participation in frequency regulation.

[0028] The energy storage battery uses a bidirectional DC-DC circuit in the DC circuit section, enabling power to flow bidirectionally between the energy storage battery and the grid, and then be connected to the grid through an inverter; in the DC-AC section, a dual closed-loop PI control is used, with the outer loop being the power loop and the inner loop being the current loop. The reference value of the outer loop power loop consists of the sum of two parts: the power change caused by the change in the photovoltaic system environment and the power corresponding to the frequency support provided by the photovoltaic and energy storage system to the grid.

[0029] Step 3: Achieve coordinated control of photovoltaic backup power and energy storage battery system

[0030] The system monitors the frequency change of the power grid system in real time. When the frequency change exceeds the action threshold, the photovoltaic system's virtual inertial control is prioritized. This means the photovoltaic system responds to the power change rate based on the system frequency and stops operating in power reserve mode until the power grid system frequency returns to within the action threshold. During this process, the energy storage battery participates in auxiliary frequency regulation: when the photovoltaic system can restore the power grid system frequency, the energy storage battery does not operate; when the photovoltaic system is operating at its maximum power point and still cannot restore the power grid frequency, the energy storage battery will take charging and discharging measures to reduce the degree of system frequency change, thereby reducing the number of times it actively operates.

[0031] When the frequency change of the power grid system is less than the action threshold, the photovoltaic system operates normally under power reserve control. The energy storage battery uses its fast charging and discharging characteristics to pull back the small fluctuations in the power grid frequency, so that the power grid frequency returns to the allowable fluctuation range.

[0032] Furthermore, the state of charge (SOC) of the energy storage battery is used as a constraint condition for the coordinated control of the photovoltaic-energy storage system:

[0033] When the state of charge (SOC) of the energy storage battery is within the safe upper and lower limits... up / low During this period, the energy storage battery participates in grid frequency regulation at its maximum charge and discharge rate;

[0034] When the state of charge (SOC) of the energy storage battery is within the alarm upper and lower limits... max / min During this period, the energy storage battery assists frequency regulation with a safe charge and discharge rate;

[0035] When the state of charge (SOC) of the energy storage battery is within the alarm upper and lower limits... max / min In the event of an abnormal condition, the energy storage battery stops charging and discharging and issues an alarm signal.

[0036] The beneficial effects of this invention are as follows: In the photovoltaic power reserve-based photovoltaic-storage system coordinated frequency control method proposed in this invention, the photovoltaic-storage system flexibly switches between power reserve control and frequency control modes. Under power reserve control, the photovoltaic system can reserve some output, increasing the system's available power capacity for frequency regulation while avoiding large energy storage demands. Meanwhile, the energy storage battery effectively absorbs the periodic peak power generated by the photovoltaic power reserve control, smoothing the photovoltaic output. When the frequency is disturbed, the coordinated control of the photovoltaic-storage system effectively utilizes the reserve power of the photovoltaic array and the storage capacity of the energy storage battery, enhancing the support capability for the grid frequency. Under large disturbances, it can reduce the degree of frequency disturbance, and under small disturbances, it can maintain the system frequency within the allowable fluctuation range. In addition, energy storage can also smooth out the photovoltaic output fluctuations caused by irradiance intensity fluctuations, improving the output power quality of photovoltaic-storage and the grid frequency support performance. In summary, the control method proposed in this invention is simple, effective, and low-cost. Attached Figure Description

[0037] Figure 1(a) illustrates the photovoltaic power reserve control principle: the power-voltage characteristics of the photovoltaic system in two consecutive power reserve cycles. Figure 1(b) illustrates the photovoltaic power reserve control principle: the output power curve of the photovoltaic system under power reserve control with varying light intensity. Figure 2 To account for the fluctuations in photovoltaic power output, the left side shows the photovoltaic output power curve under backup control. The right side shows the photovoltaic output power fluctuation without energy storage batteries, while the left side shows the photovoltaic output power fluctuation with energy storage batteries.

[0038] Figure 3 A coordinated control structure for photovoltaic-energy storage battery systems.

[0039] Figure 4 This is a flowchart of the coordinated control process for a photovoltaic and energy storage system used for frequency regulation under a large disturbance to the power grid.

[0040] Figure 5(a) shows the simulation results of the operation status of the photovoltaic-storage system under fluctuating light intensity.

[0041] Figure 5(b) shows the dynamic simulation results of the power grid frequency with and without frequency support from the photovoltaic and energy storage system under the condition of huge interference.

[0042] Figure 6(a) shows the simulation results of the operation status of the photovoltaic-storage system under different operating conditions.

[0043] Figure 6(b) shows the dynamic simulation results of the power grid frequency with and without frequency support from the photovoltaic and energy storage system under minor disturbances. Detailed Implementation

[0044] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0045] 1. Design a photovoltaic power backup control method

[0046] The working principle of power reserve control considering photovoltaic fluctuations is shown in Figure 1, where Figure 1(a) illustrates the power-voltage relationship of the photovoltaic system during two consecutive power reserve control cycles. In the nth cycle, the photovoltaic system quickly finds the maximum power point (point A) through maximum power point tracking control (e.g., incremental conductance method), and then operates at the power reserve point B through power reserve control. At this point, the photovoltaic system generates constant power until the end of the cycle. At the beginning of the (n+1)th cycle, the photovoltaic system switches from power reserve mode to maximum power mode. That is, the actual output of the photovoltaic system no longer remains at the reference value in power reserve mode, but rather, based on the actual irradiance and temperature on the photovoltaic panel, the operating point is lowered to the actual power point C. Due to the limitation that the photovoltaic voltage cannot change abruptly, the voltage values ​​at point C and point B are the same. Then, under maximum power point tracking control, the operating point increases from point C until it reaches a new peak value at point D. After that, the system switches back to power reserve mode. Based on the given power reserve reference value, the operating point moves from the new peak value D. From the photovoltaic system's power-voltage curve, it can be seen that for the same power reserve level, there are two operating points, E and E', at different voltage levels. Considering the operational stability of the photovoltaic system and to avoid the photovoltaic voltage jumping to open-circuit voltage when the irradiance decreases rapidly, proportional-integral (PI) control is used to quickly move the photovoltaic system's operating point to the operating point E on the left side of the power-voltage curve, and the photovoltaic system reaches a stable state within this cycle. Then, it operates with a constant power output, waiting for the maximum power mode trigger signal to enter the next power reserve cycle.

[0047] The period length of the photovoltaic power reserve control adopted in this invention can be flexibly modified according to objectives such as minimizing curtailment, maximizing the lifespan of energy storage batteries, and adjusting power reserve capacity, and is not limited to a fixed period length. The photovoltaic power reserve control of this invention requires periodic maximum power detection; in addition to the incremental conductance method, any maximum power point tracking algorithm can also be applied during this process.

[0048] The change in active power of the photovoltaic system under power reserve control is shown in Figure 1(b). There are two main reasons for the fluctuation in its output:

[0049] (1) The mode switching between maximum power mode and power reserve mode will result in peak power pulses in adjacent cycles.

[0050] (2) The actual working environment of the photovoltaic array is always changing. Changes in light intensity and ambient temperature will cause instability in the output of the photovoltaic system during each power reserve cycle.

[0051] Without effective peak shaving and valley filling mechanisms, the power output of photovoltaic (PV) systems to the grid will not only fluctuate severely but also generate periodic spikes, potentially affecting system stability. By configuring energy storage batteries for PV systems, these batteries can effectively smooth out power output fluctuations during each power reserve cycle: on the one hand, when sunlight intensity decreases or increases, the energy storage batteries can supplement or store additional PV active power; on the other hand, the energy storage batteries can directly employ instantaneous power pulse absorption control to absorb and store peak power between adjacent power reserve cycles, thus smoothing the overall PV power output curve. The effect is as follows: Figure 2 As shown.

[0052] 2. Establish a photovoltaic-energy storage battery coordinated control model

[0053] Figure 3 The diagram illustrates the grid-connected coordinated control structure of the photovoltaic-storage system designed in this invention. The system consists of two parts: a photovoltaic system and a storage battery. In the DC-DC section of the photovoltaic system, the system switches between maximum power mode and power reserve mode: the photovoltaic system first operates to the maximum power point using maximum power point tracking control (e.g., incremental conductance method), and then uses proportional-integral (PI) control to operate to the power reserve point, achieving a stable operating state. These actions are repeatedly executed within each power reserve cycle, realizing photovoltaic power reserve control, thus allowing the photovoltaic system to reserve a portion of its output in preparation for participating in frequency regulation.

[0054] Figure 3 The symbol in the middle is defined as: V PV_array / I PV_array : Photovoltaic panel output voltage / current; P PV_max : Maximum photovoltaic power; P PV_prc Photovoltaic backup power; VPV_dc / V PV_dcref : Actual / reference voltage of the photovoltaic DC side bus; V PV_ac / I PV_ac Photovoltaic AC side voltage / current; P PV_average : Average grid-connected photovoltaic power; SOC (State of Charge): Battery state of charge; V BESS_dc / V BESS_dcref : Actual / reference voltage of the DC side bus of the energy storage battery; P BESS_response : The power response of the energy storage battery to the system frequency; V BESS_ac / I BESS_ac : AC side voltage / current of energy storage battery; H B : Inertia constant. Common parts: PWM (Pulse Width Modulation); PI: Proportional-Integral Controller; df grid / dt: Rate of change of grid frequency; df grid : Change in power grid frequency; f grid : Actual frequency of the power grid; DC-DC: DC-DC converter; DC-AC: Three-phase inverter.

[0055] Before grid connection of the photovoltaic system, a constant DC voltage is assumed. The DC-AC section employs a DC voltage-current dual closed-loop PI control strategy to achieve DC bus voltage tracking with zero steady-state error. The energy storage battery uses a bidirectional DC-DC circuit in the DC circuit section, enabling bidirectional power flow between the energy storage battery and the grid, before being connected to the grid via an inverter. In the DC-AC section, dual closed-loop PI control is used, with an outer power loop and an inner current loop. The reference value of the outer power loop consists of the sum of two parts: the power change caused by environmental variations in the photovoltaic system and the power corresponding to the frequency support provided by the photovoltaic-energy storage system to the grid.

[0056] 3. Design a coordinated control method for photovoltaic backup power and energy storage battery system.

[0057] When the power grid experiences a large disturbance (system frequency change > action threshold of 0.2Hz), both the photovoltaic system and the energy storage battery can actively support the grid frequency, reducing the degree of grid frequency fluctuation. To minimize the number of charging and discharging actions of the energy storage battery in frequency control mode, reduce its aging rate, and maximize its lifespan, this invention proposes prioritizing the use of virtual inertial control of the photovoltaic system when the grid experiences a large disturbance in frequency control mode. That is, the photovoltaic system responds to power changes based on the system frequency change rate and no longer operates in power reserve mode until the grid system frequency returns to within the action threshold. During this process, the energy storage battery participates in auxiliary frequency regulation: when the photovoltaic system can restore the grid system frequency, the energy storage battery does not operate; when the photovoltaic system is operating at its maximum power point and still cannot restore the grid frequency, the energy storage battery will take charging and discharging measures to reduce the degree of system frequency change, thereby reducing the number of its active actions.

[0058] When the power grid experiences minor disturbances (grid frequency changes less than the operating threshold, such as between 0.01Hz and 0.2Hz), the photovoltaic system operates normally under power reserve control. The energy storage battery utilizes its rapid charge and discharge characteristics to pull back the small fluctuations in the grid frequency, restoring it to the allowable fluctuation range. Furthermore, the state of charge (SOC) of the energy storage battery is a limiting condition for the coordinated control of the photovoltaic-energy storage system: when the SOC is within the safe upper and lower limits... up / low Within this range, the energy storage battery participates in grid frequency regulation at its maximum charge and discharge rate; when the State of Charge (SOC) is within the alarm upper and lower limits... max / min Within this range, the energy storage battery assists frequency regulation at a safe charge and discharge rate; when the SOC is within the alarm upper and lower limits... max / min In the event of an abnormal condition, the energy storage battery stops charging and discharging and issues an alarm signal.

[0059] The proposed photovoltaic-storage coordinated control method, such as Figure 4 As shown. The symbol in the figure is defined as: f grid : Actual frequency of the power grid; df grid / dt: Rate of change of grid frequency; Δf grid : Frequency change action threshold under large disturbance; H B Inertia constant; SOC (State of Charge): Battery state of charge; I L / I Lmax : Actual / maximum value of energy storage battery current; VIC (Virtual Inertial Control): Virtual inertial control; SOC max / min : Upper / lower limits for energy storage battery state of charge alarm; SOC up / low : Upper / lower limits of safe state of charge for energy storage batteries.

[0060] Furthermore, the method proposed in this invention aims to achieve coordinated frequency support for photovoltaic-storage systems based on photovoltaic power backup. This method can fully utilize the photovoltaic backup power and the storage capacity of the energy storage battery, smoothing the photovoltaic curve and enhancing the stability of photovoltaic output on the one hand, and effectively suppressing the fluctuation of the photovoltaic-storage system under different disturbances on the other, thus enhancing frequency stability. However, in other scenarios, such as when certain functions are not required or other additional functions are needed, the control strategy proposed in this invention can be used separately or in conjunction with other control strategies.

[0061] Example verification:

[0062] This invention constructs an integrated power grid for a photovoltaic-storage system in Matlab / Simulation software. The system topology is as follows: Figure 3 As shown, the system parameters are displayed in Table 1. To verify whether the photovoltaic-storage system has a better operational response to support the grid frequency, this invention designed three different operating conditions: coordinated operation of the system under varying light intensity; coordinated operation of the system under large and small grid disturbances.

[0063] In operating condition 1, the solar irradiance varied by 1000 W / m² (corresponding to a maximum power point of 10.07 kW for the photovoltaic system), the photovoltaic power reserve control cycle was 20 s, and the energy storage battery capacity was 1.97 kWh, approximately 20% of the photovoltaic system capacity. Figure 5(a) shows the simulation results of the photovoltaic-energy storage system under several consecutive control cycles. It can be seen that the photovoltaic system operates normally under different light intensities using the proposed method. Furthermore, the energy storage battery can effectively smooth the photovoltaic output power in different cycles, effectively improving the quality of the photovoltaic-energy storage system's output power.

[0064] Table 1. Some parameters of the simulated photovoltaic-energy storage system

[0065]

[0066] In operating condition 2, the grid frequency support performance of the photovoltaic-storage system under large disturbances was analyzed. The simulation results are shown in Figure 5(b). At 34 seconds, the grid was disturbed, with a step load of 9kW. Without energy storage, the virtual inertial control did not activate due to the frequency change being <0.2Hz, and the lowest frequency point only changed by 0.0033Hz. After configuring energy storage, the problem of photovoltaic power backup control not operating at non-operating frequencies was solved, and the lowest frequency point changed from 49.8086Hz to 49.8309Hz, improving the stability of the grid frequency. At 50 seconds, the grid was disturbed, with a load step of 11kW. Without energy storage batteries, due to frequency changes >0.2Hz, virtual inertial control is activated, and the photovoltaic system switches from power reserve to maximum power mode, continuously outputting maximum power to cope with frequency issues. Therefore, the lowest frequency point changes from 49.7963Hz to 49.8060Hz. After configuring energy storage batteries, the lowest grid frequency point increases from 49.7963Hz to 49.8133Hz, effectively enhancing frequency stability.

[0067] In operating condition 3, when the power grid experiences minor disturbances, the energy storage battery absorbs or releases power to alleviate over- or under-frequency problems and maintain frequency fluctuations within acceptable limits. For the power grid, these minor disturbances might be caused by fluctuations in photovoltaic output power due to changes in sunlight intensity or ambient temperature, or by source-load imbalances due to changes in flexible loads. Simulation results are as follows... Figure 6(a) and 6(b) As shown, when the charging and discharging power of the energy storage battery does not reach its maximum value (2kW), it can help maintain the grid frequency within the allowable fluctuation range by rapidly absorbing and releasing power. However, between 30 and 35 seconds, with the grid load remaining constant, the illuminance drops below the reference value, causing the frequency to drop to 49.9082Hz; after configuring the energy storage battery, the grid frequency changes from 49.9082Hz to exactly 49.9404Hz. Additionally, between 50 and 60 seconds, the grid load changes from 9kW to 7.5kW, causing the frequency to increase from 50.0100Hz to 50.0491Hz by 0.0391Hz; after configuring the energy storage battery, the highest point of the grid frequency drops to 50.0173Hz. In summary, energy storage batteries can effectively address small disturbances, enabling the grid to operate within the allowable safe frequency range, or mitigating frequency changes within the allowable battery power range.

[0068] In summary, this invention utilizes an energy storage system to support the power reserve control of photovoltaic (PV) power in a photovoltaic-storage system. On the one hand, it can absorb the peak power generated by periodically detecting the maximum available power during the PV power reserve control process; on the other hand, it can smooth out PV output fluctuations caused by environmental changes. Based on the support provided by the energy storage system for PV power reserve control, the method proposed in this invention efficiently coordinates the reserve power of the PV array and the storage power of the energy storage system, which can increase the upper limit of the PV-storage system's active support for grid frequency output and avoid the need for large-capacity energy storage configuration.

Claims

1. A method for coordinated frequency control of a photovoltaic power backup photovoltaic-storage system, characterized in that, Includes the following steps: Step 1: Determine the photovoltaic power backup control method The power-voltage variation of the photovoltaic system during two consecutive power reserve control cycles is as follows: During the nth cycle, the photovoltaic system quickly finds the maximum power point A through maximum power point tracking control, and then operates at the power reserve point B through power reserve control, so that the photovoltaic system generates constant power until the end of the cycle; At the beginning of the (n+1)th cycle, the photovoltaic system switches from power reserve mode to maximum power mode. This means the actual photovoltaic output no longer remains at the reference value in power reserve mode, but instead operates at the actual power point C based on the actual irradiance and temperature on the photovoltaic panels. The voltage values ​​at point C are the same as at point B. Then, under maximum power point tracking control, the operating point moves from point C until it reaches a new peak value at point D. Afterward, it switches back to power reserve mode: based on the given power reserve reference value, the operating point moves from the new peak value at point D. Proportional-integral (PI) control is used to quickly move the photovoltaic system's operating point to the left-hand power reserve point E, and it reaches a stable state within this cycle. Then, it operates with a constant power output, waiting for the maximum power mode trigger signal to enter the next power reserve cycle. Step 2: Establish a photovoltaic-energy storage battery coordinated control model A photovoltaic system is configured with energy storage batteries to form a photovoltaic-storage system. In the DC-DC section of the photovoltaic system, during each power reserve cycle, the photovoltaic system continuously switches between maximum power mode and power reserve mode to realize photovoltaic power reserve control, thereby enabling the photovoltaic system to reserve some output to prepare for participation in frequency regulation. The energy storage battery uses a bidirectional DC-DC circuit in the DC circuit section, allowing power to flow bidirectionally between the energy storage battery and the grid, and then be connected to the grid via an inverter. In the DC-AC section, a dual closed-loop PI control is used, with an outer loop as the power loop and an inner loop as the current loop. The reference value of the outer loop power loop is the sum of the two parts. Composition: Power changes caused by environmental changes in the photovoltaic system and the power corresponding to the frequency support provided by the photovoltaic and energy storage system to the power grid; Step 3: Achieve coordinated control of photovoltaic backup power and energy storage battery system The system monitors the frequency change of the power grid system in real time. When the frequency change exceeds the action threshold, the photovoltaic system's virtual inertial control is prioritized. This means the photovoltaic system responds to the power change rate based on the system frequency and stops operating in power reserve mode until the power grid system frequency returns to within the action threshold. During this process, the energy storage battery participates in auxiliary frequency regulation: when the photovoltaic system can restore the power grid system frequency, the energy storage battery does not operate; when the photovoltaic system is operating at its maximum power point and still cannot restore the power grid frequency, the energy storage battery will take charging and discharging measures to reduce the degree of system frequency change, thereby reducing the number of times it actively operates. When the frequency change of the power grid system is less than the action threshold, the photovoltaic system operates normally under power reserve control. The energy storage battery uses its fast charging and discharging characteristics to pull back the small fluctuations in the power grid frequency, so that the power grid frequency returns to the allowable fluctuation range.

2. The photovoltaic power backup-based photovoltaic-storage system coordinated frequency control method according to claim 1, characterized in that, Using the state of charge (SOC) of the energy storage battery as a constraint condition for the coordinated control of the photovoltaic-energy storage system: When the state of charge (SOC) of the energy storage battery is within the safe upper and lower limits... up / low During this period, the energy storage battery participates in grid frequency regulation at its maximum charge and discharge rate; When the state of charge (SOC) of the energy storage battery is within the alarm upper and lower limits... max / min During this period, the energy storage battery assists frequency regulation with a safe charge and discharge rate; When the state of charge (SOC) of the energy storage battery is within the alarm upper and lower limits... max / min In the event of an abnormal condition, the energy storage battery stops charging and discharging and issues an alarm signal.

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