A grid-connected and off-grid switching control method and system of a photovoltaic energy storage inverter

Abstract

The present application relates to and off-grid switching control technical field, especially in kind of photovoltaic energy storage inverter's parallel off-grid switching control method and system, including: based on the energy storage battery state of charge determines the power balance window;Power difference deviates from the window, voltage anomaly or detects the island when determining the power grid is abnormal;Energy storage converter switches to off-grid voltage source mode and executes the first power regulation, checks the unqualified and reduces the adjustable load power setting value or photovoltaic output power;During off-grid, reduce the battery output power when the discharge maintenance time is insufficient, and narrow the window and close the fixed load when the remaining dischargeable electric quantity decreases.The present application makes the grid-connected determination, off-grid switching and off-grid maintenance all take the same power balance window as the benchmark, which improves the continuity and stability of power balance control in the process of parallel off-grid switching.

Description

Technical Field

[0001] This invention relates to the field of grid-connected / off-grid switching control technology, and in particular to a grid-connected / off-grid switching control method and system for photovoltaic energy storage inverters. Background Technology

[0002] With the large-scale application of distributed photovoltaic and energy storage systems, grid-connected / off-grid switching control of photovoltaic energy storage inverters has become a key means to ensure uninterrupted power supply to critical loads. However, existing switching controls mostly perform one-time adjustments based on instantaneous power status at the switching moment, lacking continuous power balance verification after switching. When the state of charge of energy storage changes continuously during off-grid operation, the power difference is prone to imbalance again, leading to power outages or frequent load switching, making it difficult to guarantee the continuity and stability of off-grid power supply.

[0003] Chinese Patent Application Publication No. CN116418024A discloses an off-grid energy management method for a photovoltaic inverter-storage system, comprising: S1.1, the energy storage inverter is in off-grid mode, and when the energy consumed by the load is greater than the photovoltaic energy generated by the photovoltaic (PV) system, the battery enters a discharge state; S1.2, after the battery charge is discharged to a set minimum charge level, MPPT control is turned off, the energy storage inverter stops power output, and the PV system charges the battery; S1.3, when the battery is charged to a set minimum charge level + 3%, MPPT control is turned on, the energy storage inverter outputs the rated power of the load, if the energy consumed by the load is less than the photovoltaic energy generated by the PV system, the excess energy in the PV system charges the battery, and if the energy consumed by the load is still greater than the photovoltaic energy generated by the PV system, step S1.1 is repeated.

[0004] However, the above technical solution uses the minimum battery charge as a single fixed threshold. It only stops power output after discharging to this threshold and resumes operation after charging to above the threshold. It does not consider the impact of the dynamic change of the energy storage battery's discharge capacity with the state of charge on the power balance, making it difficult to ensure the stability of off-grid power supply during the process of dynamically narrowing the energy storage capacity. Summary of the Invention

[0005] To address this issue, the present invention provides a grid-connected / off-grid switching control method and system for photovoltaic energy storage inverters, which overcomes the problem in the prior art that only stops power output after discharging to a threshold and resumes operation after charging to above the threshold, without considering the impact of the dynamic changes in the discharge capacity of the energy storage battery with the state of charge on the power balance, making it difficult to ensure the stability of off-grid power supply during the process of dynamic narrowing of energy storage capacity.

[0006] To achieve the above objectives, the present invention provides a grid-connected / off-grid switching control method for a photovoltaic energy storage inverter, comprising: In grid-connected operation, the power balance window is determined based on the state of charge of the energy storage battery; The power difference is compared with the power balance window, and combined with the voltage status of the power grid and the islanding detection results, it is determined whether the power grid is in an abnormal state. The energy storage converter switches its current mode to off-grid voltage source mode in response to the abnormal state of the power grid, and performs the first power regulation based on the power difference. The difference between the adjusted power and the photovoltaic output power after the first adjustment is re-determined based on the measured load power and the difference between the adjusted power and the photovoltaic output power after the first adjustment. The difference between the adjusted power and the photovoltaic output power after the first adjustment is then checked to see if the difference between the adjusted power and the photovoltaic output power falls within the power balance window. If the verification fails, reduce the power setting of the adjustable load, turn off the adjustable load, or continue to reduce the photovoltaic output power until the verification passes. During off-grid power supply, when the discharge sustaining time is less than the discharge sustaining reference, the reduction in battery output power is determined based on the power of the closed load, the reduction in power of the adjustable load, and the current output power of the energy storage battery. The discharge duration is determined based on the difference between the remaining dischargeable capacity of the energy storage battery and the adjustable power. When the remaining dischargeable power shows a decreasing trend, the power balance window is narrowed, and when the adjusted power difference exceeds the narrowed power balance window again, several fixed loads are shut down.

[0007] Furthermore, the process of determining the power balance window includes: The discharge power and rechargeable power of the energy storage battery are obtained based on the state of charge. The upper limit of the power balance window is determined based on the dischargeable power; The lower limit of the power balance window is determined based on the rechargeable power.

[0008] Furthermore, the process of determining whether the power grid is in an abnormal state includes: Based on the power difference not falling into the power balance window, the voltage amplitude and voltage frequency in the voltage state not falling into the corresponding preset range, or the detection of an islanding state, the power grid is determined to be in an abnormal state. The power difference is determined based on the difference between the load power and the photovoltaic output power.

[0009] Furthermore, the process of switching the current mode to the off-grid voltage source mode includes: Disconnect the static transfer switch between the load and the power grid to enter the off-grid state; Based on the fact that the power difference is greater than the upper limit of the power balance window, it is determined to shut down the adjustable loads in order of load priority from low to high, and the power difference is re-determined when any adjustable load is shut down, until the power difference falls back to within the power balance window; Based on the fact that the power difference is less than the lower limit of the power balance window, it is determined to reduce the photovoltaic output power.

[0010] Furthermore, the process of failing the verification includes: Based on the fact that the power difference is greater than the upper limit of the power balance window, the power setting value for reducing the unclosed adjustable load is determined. Based on the fact that the adjustment power difference is less than the lower limit of the power balance window, it is determined to continue reducing the photovoltaic output power.

[0011] Furthermore, the process of determining the extent of the reduction in battery output power includes: The target discharge power is determined based on the remaining dischargeable capacity of the energy storage battery and the discharge sustaining benchmark. The amount of power to be reduced is determined based on the difference between the current output power of the energy storage battery and the target discharge power. The remaining adjustable capacity is determined based on the difference between the total adjustable capacity and the amount of power already reduced; Based on the remaining adjustable capacity and the output power limit and adjustment rate of the energy storage battery, the amount of power to be reduced is allocated as the power setpoint reduction of the adjustable load and the shutdown amount of the fixed load. The reduction in battery output power is determined based on the sum of the reduction in the power setting value and the shutdown amount.

[0012] Furthermore, the process of narrowing the power balancing window includes: Based on the decreasing trend of the remaining dischargeable capacity of the energy storage battery, the upper limit of reducing the power balance window is determined; When the power difference exceeds the narrowed power balance window again, determine to shut down a number of fixed loads in order of load priority from low to high, until the power difference falls back into the narrowed power balance window.

[0013] Furthermore, the process of restoring power to the load includes: When the adjusted power difference falls below the input boundary on which the load is restored and remains stable for a preset time, the power setting value for restoring the adjustable load with reduced power is determined, and the power supply to the load that has been turned off is restored in reverse order according to the load priority.

[0014] Furthermore, the process of restoring power to the load also includes: Hysteresis bands are provided between the upper limit of the power balance window and the upper limit side input boundary, and between the lower limit of the power balance window and the lower limit side input boundary, respectively. The upper limit side input boundary is smaller than the upper limit of the power balance window, and the lower limit side input boundary is larger than the lower limit of the power balance window.

[0015] The present invention also provides a grid-connected / off-grid switching control system for a photovoltaic energy storage inverter, comprising: Energy storage batteries are used to store electrical energy and discharge to the load in off-grid voltage source mode; A photovoltaic device for converting light energy into electrical energy and supplying power to a load as well as charging the energy storage battery; An energy storage converter is used to switch between grid-connected mode and off-grid voltage source mode, and to control the charging and discharging of the energy storage battery. A static transfer switch is used to disconnect or close the connection between the load and the power grid; The determination unit is used to determine the power balance window based on the state of charge of the energy storage battery, compare the power difference with the power balance window, and combine the voltage state of the power grid and the islanding detection results to determine whether the power grid is in an abnormal state. A power regulation unit is used to control the energy storage converter to switch to off-grid voltage source mode in response to the grid being in an abnormal state and to perform the first power regulation based on the power difference; The verification unit is used to determine the difference between the adjusted power and the photovoltaic output power after the first adjustment based on the measured load power and to verify whether the difference between the adjusted power and the photovoltaic output power falls within the power balance window. If the verification fails, the power setting value of the adjustable load is reduced, the adjustable load is turned off, or the photovoltaic output power is reduced until the verification passes. A battery power regulation unit is used to reduce the output power of the energy storage battery based on a discharge sustaining time being less than a discharge sustaining reference. A window narrowing unit is used to determine to narrow the power balance window based on the decreasing trend of the remaining dischargeable capacity of the energy storage battery, and to shut down several fixed loads based on the adjustment power difference exceeding the narrowed power balance window again.

[0016] Compared with existing technologies, the beneficial effects of this invention are as follows: In grid-connected operation, a power balance window is determined based on the state of charge of the energy storage battery. The upper limit of this window is the dischargeable power of the energy storage battery, and the lower limit is the negative value of the rechargeable power. The power difference is then compared with the power balance window, and the grid voltage state and islanding detection results are used to determine whether the grid is in an abnormal state. In response to an abnormal grid state, the energy storage converter switches its current mode to off-grid voltage source mode and performs the first power adjustment based on the power difference. Subsequently, the adjusted power difference is re-determined based on the measured load power and the photovoltaic output power after the first adjustment, and the adjustment is verified. Whether it falls within the power balance window, if it fails, the power setting value of the adjustable load is reduced or the photovoltaic output power is further reduced until the verification is qualified; during off-grid power supply, the battery output power is reduced based on the discharge maintenance time being less than the discharge maintenance benchmark, where the discharge maintenance time is determined based on the difference between the remaining dischargeable capacity and the adjustable power, and the reduction in battery output power is determined based on the power of the closed load, the reduction in the power of the adjustable load, and the current output power of the energy storage battery; then, as the remaining dischargeable capacity decreases, the power balance window is narrowed, and when the difference in adjustable power exceeds the narrowed power balance window again, it is determined to shut down several fixed loads. In this way, the present invention uses the same power balance window throughout the three stages of grid-connected status determination, off-grid switching, and off-grid maintenance, so that the independent voltage, frequency, or power thresholds of the grid-connected side and the off-grid side are merged into the same range, and the power difference can be brought back to the range that the energy storage battery can independently balance without the need to readjust the criteria before and after the switch, thereby improving the continuity of grid-connected and off-grid switching and reducing the degree of power change at the moment of switching.

[0017] Furthermore, this invention determines that the power grid is in an abnormal state based on the power difference not falling into the power balance window, either the voltage amplitude or the voltage frequency not falling into a preset voltage range, or the detection of an islanding state. Then, it shuts down adjustable loads one by one according to load priority from low to high. After each load is shut down, the load power is re-acquired and the power difference is re-determined until it falls back into the power balance window. This ensures that the number of adjustable loads shut down meets the minimum number required for power balance, improving the accuracy of grid connection status determination and reducing the scope of power outages.

[0018] Furthermore, this invention reduces the power setting value of the adjustable load when the power difference is greater than the upper limit of the power balance window and reduces the photovoltaic output power when it is less than the lower limit. After each adjustment, the power difference is redefined and re-verified. At the same time, the distribution of the power to be reduced is constrained by the remaining adjustable capacity, the output power limit and the adjustment rate. This allows the power difference to converge to the power balance window successively without having to readjust the criteria according to the off-grid conditions. It also ensures that each adjustment is within the range that the energy storage converter and each load can fulfill, thereby improving the reliability of off-grid switching and the feasibility of power regulation.

[0019] Furthermore, this invention determines the discharge maintenance benchmark based on the backup power supply duration and margin coefficient required by the important load, determines the discharge maintenance time based on the ratio of the difference between the remaining dischargeable capacity and the adjustable power, and then, when the discharge maintenance time is less than the discharge maintenance benchmark, determines the target discharge power based on the ratio of the remaining dischargeable capacity and the discharge maintenance benchmark, obtains the power to be reduced based on the difference between the current output power of the energy storage battery and the target discharge power, determines the remaining adjustable capacity based on the difference between the total adjustable capacity and the power already reduced, and allocates the smaller of the remaining adjustable capacity and the power to be reduced as the power setting reduction amount for the adjustable load, and the insufficient part is allocated as the shutdown amount for the fixed load. This allows the output power of the energy storage battery to be adjusted in advance according to its ability to maintain off-grid power supply, and the reduction amount on the load side is realized as the reduction range of the battery output power, improving the controllability of the off-grid power supply maintenance time, reducing the degree of over-discharge of the energy storage battery and the fluctuation of the power difference.

[0020] Furthermore, this invention reduces the upper limit of the power balance window when the remaining dischargeable capacity shows a decreasing trend, while keeping the lower limit at the value at the switching moment. Then, when the adjusted power difference exceeds the narrowed power balance window again, fixed loads are shut down one by one according to the load priority from low to high. This makes the power balance window narrow synchronously with the decay of the energy storage battery's discharge capacity, and the shutdown of loads is triggered sequentially and earlier as the discharge continues. This improves the margin of power balance in the later stage of off-grid operation and reduces the degree of concentrated load reduction when the energy storage battery is close to the discharge cutoff.

[0021] Furthermore, this invention sets hysteresis bands between the upper limit of the power balance window used to shut down the load and the upper limit side input boundary used to restore the load, and between the lower limit of the power balance window used to reduce photovoltaic output power and the lower limit side input boundary used to restore photovoltaic output power. Both input boundaries are located inside the corresponding window boundaries. This allows the power setting value of the adjustable load with reduced power to be restored first, and then the power supply of the shut-down load to be restored in reverse order according to the load priority. Each time a load is restored, the adjustment power difference is re-determined and re-verified. After the photovoltaic output power is reduced, it is restored when the adjustment power difference rises above the upper limit side input boundary and remains stable for a preset time. This makes the restoration conditions more lenient than the shutdown conditions. The power reduction and restoration are determined according to the same input boundary and the same stable preset time, which improves the stability of load switching and reduces the degree of repeated load switching caused by the fluctuation of the power difference near the upper limit of the power balance window and the degree to which the power difference exceeds the power balance window again during the restoration process. Attached Figure Description

[0022] Figure 1 This is a block diagram of the grid-connected / off-grid switching control system of the photovoltaic energy storage inverter according to an embodiment of the present invention; Figure 2 This is a flowchart of the grid-connected / off-grid switching control method for a photovoltaic energy storage inverter according to an embodiment of the present invention; Figure 3 This is a flowchart illustrating the logic of determining whether a power grid is in an abnormal state according to an embodiment of the present invention. Figure 4 This is a schematic diagram showing the relationship between the hysteresis bands on both sides of the upper and lower limits of the power balance window in an embodiment of the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described below with reference to embodiments; it should be understood that the specific embodiments described herein are merely for explaining the invention and are not intended to limit the invention.

[0024] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Those skilled in the art should understand that these embodiments are merely illustrative of the technical principles of the present invention and are not intended to limit the scope of protection of the present invention.

[0025] Please see Figure 1 As shown, this is a block diagram of the grid-connected / off-grid switching control system of the photovoltaic energy storage inverter according to an embodiment of the present invention. The system in this embodiment includes: Energy storage batteries are connected to the DC bus to store electrical energy and discharge to the load in off-grid mode.

[0026] A photovoltaic device, connected to a DC bus via a DC converter, is used to convert light energy into electrical energy and to supply power to the load and charge the energy storage battery via the DC bus.

[0027] An energy storage converter is connected between the DC bus and the load to switch between grid-connected and off-grid voltage source modes and to control the charging and discharging of the energy storage battery.

[0028] A static transfer switch is installed between the load and the power grid to disconnect or close the connection between the load and the power grid.

[0029] The control unit includes a determination unit, a power regulation unit, a verification unit, a battery power regulation unit, a window narrowing unit, and a recovery unit, and is respectively connected to the energy storage battery, the photovoltaic device, the energy storage converter, and the static transfer switch signal.

[0030] The determination unit is used to determine the power balance window based on the state of charge of the energy storage battery, compare the power difference with the power balance window, and combine the voltage state of the power grid and the islanding detection results to determine whether the power grid is in an abnormal state.

[0031] The power regulation unit is used to control the energy storage converter to switch to off-grid voltage source mode in response to an abnormal grid condition and to perform the first power regulation based on the power difference.

[0032] The verification unit is used to determine the difference between the adjusted power and the photovoltaic output power after the first adjustment based on the measured load power and verify whether the difference between the adjusted power and the photovoltaic output power falls within the power balance window. If the verification fails, the power setting value of the adjustable load is reduced or the photovoltaic output power is further reduced until the verification passes.

[0033] A battery power regulation unit is used to determine the reduction of the output power of the energy storage battery based on the discharge sustaining time being less than the discharge sustaining reference, wherein the discharge sustaining time is determined based on the difference between the remaining dischargeable capacity of the energy storage battery and the regulation power.

[0034] The window narrowing unit is used to determine the narrowing of the power balance window based on the decreasing trend of the remaining dischargeable power, and to shut down several fixed loads based on the adjustment power difference exceeding the narrowed power balance window again.

[0035] The recovery unit is used to restore the power setting value of the adjustable load that has reduced power based on the adjustment power difference falling back below the input boundary on which the load is restored and remaining stable for a preset time, and to restore the power supply of the load that has been turned off in reverse order according to the load priority.

[0036] Please see Figure 2 As shown, it is a flowchart of the grid-connected / off-grid switching control method for a photovoltaic energy storage inverter according to an embodiment of the present invention. The method in this embodiment includes at least the following steps: Step S1: In grid-connected mode, determine the power balance window based on the state of charge of the energy storage battery. Step S2: Compare the power difference with the power balance window, and combine the voltage status of the power grid and the islanding detection results to determine whether the power grid is in an abnormal state. Step S3: In response to the abnormal state of the power grid, the energy storage converter switches the current mode to off-grid voltage source mode and performs the first power regulation based on the power difference. Step S4: Based on the measured load power and the photovoltaic output power after the first adjustment, redetermine the adjustment power difference and verify whether the adjustment power difference falls within the power balance window. Step S5: If the verification fails, reduce the power setting of the adjustable load, turn off the adjustable load, or continue to reduce the photovoltaic output power until the verification passes. Step S6: During off-grid power supply, when the discharge sustaining time is less than the discharge sustaining reference, the reduction in battery output power is determined based on the power of the closed load, the reduction in power of the adjustable load, and the current output power of the energy storage battery. The discharge duration of the energy storage battery is determined based on the difference between the remaining dischargeable capacity of the energy storage battery and the adjustable power. Step S7: Based on the decreasing trend of remaining dischargeable power, determine to narrow the power balance window, and determine to shut down several fixed loads when the adjusted power difference exceeds the narrowed power balance window again. Step S8: Based on the power difference falling back below the input boundary and remaining stable for a preset time, restore the power setting value of the adjustable load that has reduced power, and restore the power supply of the shut-off loads in reverse order according to the load priority.

[0037] In step S1, the process of determining the power balance window based on the state of charge of the energy storage battery includes: Specifically, the determination unit acquires the state of charge (SOC) of the energy storage battery and determines the power balance window based on the SOC. The power balance window is the range of allowable power difference values, used to determine whether the power difference between the load and the photovoltaic device is within the range that the energy storage battery can independently balance.

[0038] The upper limit of the power balance window is taken as the discharge power of the energy storage battery, and the lower limit of the power balance window is taken as the negative value of the rechargeable power of the energy storage battery. The discharge power is the upper limit of the power that the energy storage battery can continuously release under the current state of charge, current temperature, and current health; the rechargeable power is the upper limit of the power that the energy storage battery can continuously absorb under the current state of charge, current temperature, and current health; both are obtained by the determination unit from the battery management system and determined by looking up a table based on the state of charge.

[0039] Understandably, when the photovoltaic output power exceeds the load power, the power difference is negative, and the excess energy needs to be absorbed by the energy storage battery. Therefore, the lower limit is represented by a negative value of the rechargeable power. When the load power equals the photovoltaic output power, the power difference is zero, and zero falls within the power balance window. In other words, the ideal power self-balancing state will not be judged as exceeding this window.

[0040] The power difference is the difference between the load power and the photovoltaic output power, with the dimension kW. The load power is the product of the load-side voltage and the load-side current, calculated by the voltage and current transformers installed on the load side. The photovoltaic output power is the product of the DC-side voltage and the DC-side current, calculated by the voltage and current sensors installed on the DC side.

[0041] The acquisition period is the time interval between two consecutive acquisitions of load power and photovoltaic output power. It is determined based on the control unit's operation cycle and the rate of change of load power, and is set to a value no greater than the control unit's operation cycle, so that the result obtained from each acquisition can participate in a complete comparison and adjustment calculation. For example, the control unit's operation cycle is 100ms, and the acquisition period is correspondingly set to 100ms.

[0042] Specifically, the window length of the moving average filter is the number of results involved in the averaging calculation, determined based on the acquisition period and the fluctuation period of the load power. It is chosen to ensure that the filtered power difference follows changes in load power without causing misjudgments due to instantaneous fluctuations. For example, a window length of 5 is used, meaning the arithmetic mean of the power differences obtained from the most recent 5 acquisition periods is taken, and the filtered power difference is used in subsequent comparison and verification calculations.

[0043] Specifically, the correspondence between dischargeable power and rechargeable power as a function of state of charge (SOC) was determined through preliminary experimental calibration. In the preliminary experiments, the sustainable upper limit of the energy storage battery was measured at multiple SOC points using constant power discharge and constant power charging methods. The criterion was that the battery terminal voltage had not reached the discharge cutoff voltage or the charging cutoff voltage. Each SOC point and its corresponding upper limit of power were fitted as a piecewise polygonal line representing the change in SOC, and this data was written into the battery management system. In actual operation, the corresponding dischargeable power and rechargeable power are retrieved based on the measured SOC.

[0044] In this embodiment, the discharge power and rechargeable power obtained from the above-mentioned segmented broken line are as follows: when the state of charge is 80%, the discharge power is 40kW and the rechargeable power is 20kW, with a corresponding power balance window of -20kW to +40kW; when the state of charge is 50%, the discharge power is 30kW and the rechargeable power is 25kW, with a corresponding power balance window of -25kW to +30kW.

[0045] In step S2, the process of determining whether the power grid is in an abnormal state includes: Specifically, the determination unit compares the power difference with the power balance window and obtains the voltage state of the power grid and the islanding detection results. The voltage state of the power grid is characterized by the grid connection point voltage, which includes voltage amplitude and voltage frequency, both of which are collected and calculated by voltage transformers installed at the grid connection point.

[0046] In this step, the power difference is compared in the same way as the power difference determined in step S1, that is, the power difference is the difference between the load power and the photovoltaic output power, with the dimension of kW; the acquisition location, acquisition period and filtering method of the load power and photovoltaic output power are the same as in step S1.

[0047] The preset voltage range refers to the allowable range of the grid connection point voltage, including the preset voltage amplitude range and the preset voltage frequency range. The voltage amplitude range is the allowable interval for the voltage amplitude at the grid connection point, and the voltage frequency range is the allowable interval for the voltage frequency at the grid connection point.

[0048] Specifically, the voltage not falling within the preset voltage range includes the voltage amplitude not falling within the preset voltage amplitude range or the voltage frequency not falling within the preset voltage frequency range. Either of these conditions constitutes the voltage not falling within the preset voltage range.

[0049] In this context, "power difference falling within the power balance window" means the power difference is not less than the lower limit and not greater than the upper limit of the power balance window; "power difference greater than the upper limit or less than the lower limit" means it does not fall within the power balance window. When the equality holds, the power difference is exactly equal to the power that the energy storage battery can continuously discharge or charge, and is still within the range that the energy storage battery can independently balance, thus it is considered to fall within the window. Similarly, "voltage amplitude falling within the preset voltage amplitude range" and "voltage frequency falling within the preset voltage frequency range" both mean that their values ​​are not less than the lower limit and not greater than the upper limit of the corresponding range.

[0050] Specifically, both the voltage amplitude range and voltage frequency range are based on the rated voltage and rated frequency of the grid connection point, and are determined according to the allowable values ​​for voltage and frequency deviations during grid-connected operation. That is, the allowable deviation of the amplitude is calculated by multiplying the rated voltage by the allowable value, and the allowable deviation of the frequency is directly taken from the allowable value. The allowable range is then obtained by adding or subtracting the corresponding allowable deviation from the rated value. Voltage frequency is measured in Hz, and its allowable deviation is an absolute quantity and is not converted based on the rated frequency. For example, if the rated voltage of the grid connection point is 380V and the rated frequency is 50Hz, the allowable voltage deviation is ±7%, and the voltage amplitude range is 353.4V to 406.6V; the allowable frequency deviation is ±0.5Hz, and the voltage frequency range is 49.5Hz to 50.5Hz. These allowable values ​​are consistent with the existing allowable ranges for voltage and frequency deviations during grid-connected operation.

[0051] Specifically, during grid-connected operation, the grid connection point voltage is clamped by the grid, and the power difference between the load and the photovoltaic device is balanced by the grid and the energy storage battery. Therefore, none of the three criteria are met. When the grid loses power or the power quality deteriorates, the grid connection point voltage is no longer clamped, and the power difference becomes unbalanced. Any one of the three criteria may then be met. Therefore, if any one of the following three conditions is met—the power difference not falling within the power balance window, the voltage not falling within the preset voltage range, or islanding is detected—the determination unit will determine that the grid is in an abnormal state. If none of the three conditions are met, the determination unit will determine that the grid is in a normal state, and the system will continue to operate in grid-connected mode.

[0052] Please see Figure 3The diagram shown is a logical flowchart of an embodiment of the present invention for determining whether the power grid is in an abnormal state. This diagram corresponds to the determination process in step S2 and the switching process in step S3. After determining that the power grid is in an abnormal state, the energy storage converter switches to off-grid voltage source mode, establishes off-grid voltage, and disconnects the static transfer switch, and the system enters the off-grid state.

[0053] Islanding mode refers to a state where the power grid fails but the photovoltaic energy storage inverter continues to supply power to the load.

[0054] Specifically, islanding detection is achieved using the frequency perturbation method. When using the frequency perturbation method, the control unit injects a frequency perturbation of a set amplitude into the current loop of the grid connection point and detects whether the grid connection point voltage frequency continuously deviates beyond the frequency deviation threshold within a set duration. If it exceeds the threshold, an islanding state is detected. The perturbation amplitude is determined based on the allowable range of current harmonics and power factor during grid-connected operation. It is a value that can be clamped by the grid when the grid is normal, and can cause the grid connection point frequency to deviate rapidly without affecting the grid power quality when the grid is de-energized; for example, it is 2% of the inverter's rated current. The frequency deviation threshold and the set duration are determined based on the islanding protection's requirements for detection time and false alarm rate. The frequency deviation threshold is a value greater than the normal fluctuation range of the grid connection point frequency, and the set duration is a value not greater than the required operating time of the islanding protection; for example, the frequency deviation threshold is 0.5Hz and the set duration is 0.5s.

[0055] As another implementation method, island detection can also be achieved using the reactive power disturbance method.

[0056] Understandably, when voltage drops, frequency shifts, or islanding occur on the grid side, the energy storage converter's ability to support the grid decreases, and the local power difference will shift from within the power balance window to outside. Therefore, the deviation of the power difference from the power balance window can be used as a criterion for determining abnormal grid conditions.

[0057] In step S3, the process of switching the current mode to off-grid voltage source mode and performing the first power regulation based on the power difference includes: Specifically, after the determination unit determines that the power grid is in an abnormal state, the power regulation unit responds to the determination result by controlling the energy storage converter to switch the current operating mode to the off-grid voltage source mode, and the energy storage converter establishes the off-grid voltage. Then, the static transfer switch is disconnected, and the system enters the off-grid state.

[0058] The amplitude and frequency of the off-grid voltage are set based on the rated voltage and frequency of the load, and are output by the energy storage converter in voltage source mode. Specifically, the amplitude of the off-grid voltage is taken as the rated voltage of the load, and the frequency is taken as the rated frequency of the load, so that the load does not need to change its operating state before and after switching. For example, the rated voltage of the load is 380V and the rated frequency is 50Hz, and the off-grid voltage established by the energy storage converter is 380V and 50Hz.

[0059] In this process, the energy storage converter first establishes an off-grid voltage that meets the load requirements in off-grid voltage source mode, and then disconnects the static transfer switch. The time interval between these two steps is determined based on the voltage build-up time when the energy storage converter switches from current control mode to voltage source control mode, and is set to a value not less than this voltage build-up time, so that the load side already has a stable off-grid voltage when the static transfer switch is disconnected. For example, the voltage build-up time is 20ms, and the static transfer switch is disconnected after the voltage build-up is completed.

[0060] It is understandable that if the static transfer switch is disconnected first and then the off-grid voltage is established by the energy storage converter, the load will lose power during the period between disconnection and voltage establishment. In this embodiment, the off-grid voltage is established first and then the static transfer switch is disconnected, so the load does not lose voltage during the switching process.

[0061] Specifically, the power regulation unit performs the first power regulation based on the power difference during switching to correct the power difference at the moment of switching in the direction of deviation. When the power difference is greater than the upper limit of the power balance window, it indicates that the portion of the load power exceeding the photovoltaic output power has exceeded the discharge capacity of the energy storage battery. The power regulation unit shuts down several adjustable loads in order of load priority, from low to high. When the power difference is less than the lower limit of the power balance window, it indicates that the portion of the photovoltaic output power exceeding the load power has exceeded the charge capacity of the energy storage battery. The power regulation unit reduces the photovoltaic output power.

[0062] The number of adjustable loads to be shut down is determined as follows: Adjustable loads are shut down one by one in order of load priority from low to high. At the same time as each adjustable load is shut down, the load power is re-acquired and the power difference is re-determined until the power difference falls back to within the power balance window. The number of adjustable loads shut down at this time is the number of loads shut down.

[0063] Understandably, when shutting down loads one by one, it is necessary to verify that the number of loads shut down is the minimum required to meet power balance, so as to avoid shutting down too many loads at once and expanding the scope of power outage.

[0064] Specifically, the reduction in photovoltaic output power is determined based on the amount by which the power difference exceeds the lower limit of the power balance window, i.e., the difference between the reduced photovoltaic output power and the load power does not exceed the rechargeable power of the energy storage battery. The reduction in photovoltaic output power is achieved by causing the photovoltaic device's maximum power point tracking operating point to deviate from the maximum power point, i.e., the photovoltaic device's operating point shifts from the maximum power point in the direction of reduced output power. For example, if the lower limit of the power balance window is -25kW and the power difference is -30kW, the photovoltaic output power will decrease by 5kW.

[0065] Among them, load priority is the ranking of loads according to their importance in power supply.

[0066] Specifically, load priority is determined based on the importance and interruptibility of the load. The importance of a load is categorized according to the consequences of power interruption; the more severe the consequences, the higher the priority. The interruptibility of a load is categorized according to whether its power setting can be continuously reduced; loads that can be continuously reduced have higher interruptibility and lower priority. Combining these two categories, loads with minor interruption consequences and those that can be continuously reduced are assigned the lowest priority, while loads with severe interruption consequences and those that cannot be reduced are assigned the highest priority. Therefore, this embodiment divides loads into three priority levels, from lowest to highest: interruptible loads, secondary loads, and important loads. Adjustable loads are classified as interruptible loads and secondary loads, while fixed loads are classified as important loads. Fixed loads have the highest priority because they cannot be operated at reduced power.

[0067] Loads are categorized into adjustable loads and fixed loads based on whether their power settings can be adjusted. Adjustable loads have adjustable power settings, such as air conditioners, water heaters, and charging stations, which allow for continuous power reduction. Fixed loads, on the other hand, have non-adjustable power settings and can only be kept running or turned off, such as lighting, monitoring, and communication equipment, which do not allow for power reduction.

[0068] Understandably, the first power adjustment causes the power difference to fall back to a range that the energy storage battery can independently balance at the moment of switching, which avoids overloading of the energy storage battery in the early stage of off-grid operation and avoids repeated actions to correct the power deviation in the future.

[0069] In step S4, the process of verifying whether the adjusted power difference falls within the power balance window includes: Specifically, after the system enters off-grid mode, the power regulation unit collects the measured load power and the photovoltaic output power after the first regulation, and obtains the regulated power difference by subtracting the load power from the photovoltaic output power. The power difference and the regulated power difference are the values ​​of the same physical quantity at different points in time. The former is the measured value during the judgment and switching phase, and the latter is the measured value after the first power regulation. Different names are used for easy distinction.

[0070] The determination unit compares the adjusted power difference with the power balance window to verify whether the adjusted power difference falls within the power balance window. The power balance window used in steps S4 to S6 is the same power balance window used in steps S1 to S3, and its upper and lower limits remain the values ​​at the corresponding switching time. To avoid affecting the convergence determination due to simultaneous changes in the window and the criterion, steps S4 to S6 all use the power balance window at the switching time for verification. That is, the upper and lower limits of the power balance window are not updated during steps S4 to S6, and the narrowing of the window is uniformly performed in step S7.

[0071] It is understood that in this embodiment, the same power balance window is used as the comparison benchmark in each stage of grid connection determination, off-grid switching, off-grid maintenance and load recovery. The input boundary used in the load recovery stage is determined by the upper limit of the power balance window according to a preset ratio. It is not necessary to readjust the various criteria according to the off-grid conditions before and after the switching.

[0072] In step S5, the process of continuing adjustment when the verification fails includes: Specifically, when the verification result is unqualified, the power regulation unit continues to adjust based on the deviation direction of the power difference. When the power difference is greater than the upper limit of the power balance window, the power regulation unit reduces the power setpoint of the adjustable load that has not yet been turned off; when the power difference is less than the lower limit of the power balance window, the power regulation unit continues to reduce the photovoltaic output power.

[0073] The reduction range of the power setting value for adjustable loads is determined based on the amount by which the power difference exceeds the upper limit of the power balance window. Specifically, the sum of the power reduction ranges must not be less than this excess amount, and the power is allocated to the remaining adjustable loads in ascending order of load priority. For a single adjustable load, the reduction range is the difference between its current power setting value and its adjustable lower limit. If the reduction range for that adjustable load is insufficient, the next adjustable load is then reduced in ascending order of load priority. The adjustable lower limit is the lowest permissible power setting value under basic operating conditions for the adjustable load, determined based on its operating requirements. For example, the adjustable lower limit for an air conditioner is 50% of its rated power, and for a water heater, it is 30% of its rated power.

[0074] Each adjustment involves re-collecting the load power and photovoltaic output power, re-determining the adjustment power difference, and re-verifying until the adjustment power difference falls within the power balance window.

[0075] It is understandable that after the first power adjustment is completed, the power setting value of the adjustable load that has not yet been turned off is further reduced in priority, rather than turning off the load, because the adjustment of the power setting value is reversible and continuous, which can minimize the impact on the power supply of the load while satisfying power balance.

[0076] In step S6, the process of determining the reduction of battery output power based on the discharge sustaining time includes: Specifically, during off-grid power supply, the power regulation unit determines the discharge sustaining time of the energy storage battery and compares it with a discharge sustaining reference to determine whether to reduce the output power of the energy storage battery. The discharge sustaining time is the duration for which the energy storage battery can maintain off-grid power supply when continuously discharging at the current regulation power difference; the discharge sustaining reference is the expected duration of continuous off-grid power supply.

[0077] Specifically, the discharge sustaining reference is calculated by multiplying the required backup power supply duration for the critical load by a margin factor, with dimensions h; the backup power supply duration is determined based on the operational requirements of the critical load. The margin factor is a dimensionless quantity, representing the amplification factor of the discharge sustaining reference relative to the backup power supply duration.

[0078] The margin coefficient is set as follows: Before the system is put into operation for the first time, the margin coefficient is set to a preset constant value. This preset constant value is obtained through pre-experiment calibration. The calibration process is as follows: the energy storage battery is subjected to off-grid discharge test with multiple margin coefficients of sequential values. The maximum ratio of the adjustment power difference to the power difference at the switching time in each test and the maximum relative deviation of the remaining dischargeable capacity obtained from the table to the measured value are recorded. The sum of the two is taken as the lower limit of the margin coefficient required for this test. The lower limit of the margin coefficient that makes the discharge maintenance benchmark not less than the backup power supply duration required by the important load and has the smallest value is taken as the preset constant value. For example, the preset constant value is 1.1.

[0079] After the system completes at least one full off-grid discharge cycle, the value obtained from the pre-experiment is replaced with the maximum change ratio obtained from historical data statistics. That is, the maximum ratio of the adjustment power difference to the power difference at the switching moment in the historical off-grid operation data is taken, and the sum of this ratio and the power estimation deviation of the remaining dischargeable power is taken as the lower limit of the margin coefficient. A value not less than this lower limit is taken as the margin coefficient. The power estimation deviation is obtained from the table lookup error of the piecewise linear curve, and the maximum relative deviation of the remaining dischargeable power obtained from the table lookup is taken as the maximum relative deviation of the remaining dischargeable power to the measured value.

[0080] Among them, the power estimation deviation and the maximum change ratio are both dimensionless quantities; a complete off-grid discharge cycle refers to the process from when the system enters the off-grid state until the remaining dischargeable power drops to the value corresponding to the discharge cutoff state of charge.

[0081] Specifically, the state of charge (SOC) is determined by the battery management system based on a table of SOC calibration curves. The remaining dischargeable capacity is then calculated by converting the rated capacity to the SOC. Therefore, the deviation between the remaining dischargeable capacity obtained from the table lookup and the measured value originates from the table lookup error of the SOC calibration curves. The SOC calibration curves are obtained by measuring the open-circuit voltage at multiple SOC points after allowing the battery to rest for a preset time, and fitting each SOC point with its corresponding open-circuit voltage. Adjacent curves are approximated by straight line segments.

[0082] Specifically, the remaining dischargeable capacity obtained from the table and the measured value are recorded and compared point by point within an off-grid discharge cycle. The measured value is obtained by the battery charge and discharge test equipment in the off-grid discharge test, that is, the cumulative electrical energy released from any discharge point to the discharge termination point when discharging at a constant power from the start of the off-grid discharge to the discharge cutoff voltage. The difference between the table value and the measured value in each collection cycle is divided by the measured value, and the maximum value is taken as the capacity estimation deviation for that cycle. When multiple off-grid discharge cycles are completed, the maximum value of the deviations obtained in each cycle is taken.

[0083] Understandably, the margin factor ensures that the discharge sustainment reference is not less than the backup power supply duration required by the critical load, so that the adjustment timing calculated based on the discharge sustainment reference is not later than the end of the backup power supply duration.

[0084] The discharge sustaining time is determined based on the difference between the remaining dischargeable capacity of the energy storage battery and the adjustable power. The remaining dischargeable capacity is the electrical energy that the energy storage battery can still release under the current state of charge and discharge cutoff conditions.

[0085] Specifically, the remaining dischargeable capacity is calculated by the battery management system based on the product of the rated capacity and the current state of charge minus the discharge cutoff state of charge, with the dimension being kWh. The discharge cutoff state of charge is the state of charge corresponding to the energy storage battery when it discharges to the discharge cutoff voltage. The discharge cutoff state of charge is obtained through pre-experiment calibration: in the pre-experiment, the energy storage battery is discharged at a set power at a constant power, and the corresponding data of the battery terminal voltage change with discharge are recorded. The state of charge corresponding to the battery terminal voltage reaching the discharge cutoff voltage is determined as the discharge cutoff state of charge and written into the battery management system. For example, with a rated capacity of 200 kWh, a discharge cutoff state of charge of 5%, and a current state of charge of 80%, the remaining dischargeable capacity is 150 kWh.

[0086] Specifically, the discharge sustaining time is calculated based on the ratio of the remaining dischargeable charge to the adjustable power, with the dimension being h.

[0087] Among them, the power difference is adjusted with the direction of the energy storage battery discharging towards the load as positive; when the photovoltaic output power is not less than the load power, the energy storage battery is in a charging state, and the discharge maintenance time calculation in step S6 is not performed.

[0088] Specifically, when the discharge sustain time is less than the discharge sustain reference, it indicates that the current discharge level cannot sustain the expected power supply duration, and the power regulation unit reduces the output power of the energy storage battery in advance. In off-grid mode, the difference between the load power and the photovoltaic output power is borne by the energy storage battery, which reduces the output power of the energy storage battery by lowering the power setpoint of the adjustable load and shutting down the fixed load.

[0089] The reduction in battery output power is determined as follows: the target discharge power is determined based on the remaining dischargeable capacity of the energy storage battery and the discharge maintenance benchmark. Specifically, the target discharge power is calculated based on the ratio of the remaining dischargeable capacity to the discharge maintenance benchmark, with the dimension being kW. The target discharge power is the allowable discharge power when the remaining dischargeable capacity can be maintained for a duration equal to the discharge maintenance benchmark.

[0090] Then, the power to be reduced is obtained by subtracting the current output power of the energy storage battery from the target discharge power, with the dimension of kW; the power to be reduced is used to represent the part of the current output power of the energy storage battery that exceeds the target discharge power.

[0091] Then, the remaining adjustable capacity is obtained by subtracting the predetermined total adjustable capacity from the amount of power already reduced.

[0092] The total adjustable capacity is the sum of the power reduction capabilities of all adjustable loads under the condition of ensuring their basic operation. Specifically, the power reduction capability of each adjustable load is the difference between the rated power of the adjustable load and the adjustable lower limit of the adjustable load. The total adjustable capacity is determined by summing the power reduction capabilities of each adjustable load, and its dimension is kW. The total adjustable capacity is determined once before the system is put into use and written into the control unit, and does not change during operation. In this embodiment, for example, the power reduction capabilities of each adjustable load are 20kW, 12kW, and 8kW, and the corresponding total adjustable capacity is 40kW.

[0093] The power of the closed load is the sum of the power of the closed adjustable load and the fixed load, and both are calculated at their rated power. The power of the closed fixed load is not included in the total adjustable capacity. After deducting it, the remaining adjustable capacity is less than the total power that can be reduced. Therefore, its value is conservative.

[0094] The power reduction is the sum of the reductions in the power settings of each adjustable load. The power of the off load and the power reduction are both accumulated successively based on the power settings at the time of switching.

[0095] The power reduction is the sum of the power of the load that has been turned off and the power reduction magnitude; the remaining adjustable capacity is the total power that can still be reduced, which is the difference between the total adjustable capacity and the power reduction, and the unit is kW.

[0096] Finally, based on the remaining adjustable capacity and the output power limit and adjustment rate of the energy storage battery, the amount of power to be reduced is allocated as the power setpoint reduction of the adjustable load and the shutdown amount of the fixed load, and the sum of the power setpoint reduction and the shutdown amount is used as the reduction range of the battery output power.

[0097] Specifically, the allocation is determined as follows: First, the smaller of the remaining adjustable capacity and the power to be reduced is taken as the power setting reduction amount for the adjustable load; if the remaining adjustable capacity is not less than the power to be reduced, the entire power to be reduced is allocated as the power setting reduction amount for the adjustable load, and the shutdown amount for the fixed load is zero; if the remaining adjustable capacity is less than the power to be reduced, the power setting reduction amount for the adjustable load is taken as the remaining adjustable capacity, and the difference between the power to be reduced and the remaining adjustable capacity is allocated as the shutdown amount for the fixed load. The power setting reduction amount for the adjustable load is allocated to each adjustable load in ascending order of load priority until all allocations are completed; the shutdown amount for the fixed load is allocated to each fixed load in ascending order of load priority, with the last fixed load to be shut down receiving its required power.

[0098] The output power limit is the permissible output power range of the energy storage converter, determined based on its rated power and permissible output capacity. Its upper limit is the rated power of the energy storage converter, and its lower limit is the minimum permissible output power. Specifically, the difference between the current output power of the energy storage battery and the reduction in its output power must not be lower than the lower limit of the output power limit; that is, the reduced battery output power should remain within the permissible output power range of the energy storage converter. Any portion not allocated due to this lower limit will continue to be allocated in the next adjustment cycle.

[0099] The regulation rate is the allowable power adjustment per unit time, determined based on the switching frequency of the power devices in the energy storage converter and the allowable voltage fluctuation range of the DC bus. Specifically, the regulation rate is a value that ensures the DC bus voltage fluctuation caused by power adjustment does not exceed the allowable range and does not trigger the overcurrent protection of the energy storage converter. This value is directly given by the selection parameters of the energy storage converter, and its value depends on the model of the energy storage converter and the parameters of its power devices; for example, it is taken as 10 kW / s. The reduction in battery output power is performed in stages according to the regulation rate, meaning that the actual reduction in each regulation cycle does not exceed the product of the regulation rate and the duration of that regulation cycle.

[0100] Understandably, the remaining adjustable capacity limits the amount of power reduction that can be borne by the adjustable load in a single adjustment, the output power limit limits the lower limit of the output power that the energy storage battery can achieve in this adjustment, and the adjustment rate limits the time feasibility of this adjustment; the three together constrain the allocation of the amount of power to be reduced, so that each allocated adjustment amount is the amount that the energy storage converter and each load can actually realize.

[0101] In this allocation process, the amount of power to be reduced is prioritized as the reduction amount of the power setpoint for adjustable loads. If the remaining adjustable capacity is less than the amount of power to be reduced, the insufficient portion is allocated as the shutdown amount for fixed loads. Fixed loads are shut down according to their load priority from low to high. This allocation ensures that the adjustment methods are first exhausted within a reversible and continuously adjustable range before irreversible shutdown methods are used.

[0102] In this embodiment, after a period of off-grid operation, the state of charge decreases, and the remaining dischargeable capacity is 33kWh; the adjustable power difference is 25kW, and the discharge sustaining time is 1.32h; the backup power supply duration required by the important load is 2h. The change factor of the power difference and the deviation of the remaining dischargeable capacity estimation are negligible, so the margin coefficient is taken as 1.1, and the corresponding discharge sustaining benchmark is 2.2h. The discharge sustaining time is less than the discharge sustaining benchmark. The target discharge power is 15kW, and the power to be reduced is 10kW. The total adjustable capacity is 40kW. The adjustable load with a rated power of 15kW is turned off, and its power setting is reduced by 5kW. The power of the turned-off load is 15kW, the power reduction is 5kW, the power reduction is 20kW, and the remaining adjustable capacity is 20kW. The remaining adjustable capacity is greater than the power to be reduced, so the power to be reduced is entirely allocated to the reduction of the adjustable load's power setting, and the reduction in battery output power is 10kW. The adjusted energy storage battery has an output power of 15kW and a remaining discharge capacity that can sustain operation for 2.2 hours, consistent with the discharge sustainment benchmark.

[0103] In step S7, the process of narrowing the power balancing window and shutting off the fixed load includes: Specifically, during off-grid power supply, the remaining dischargeable charge decreases as discharge continues. The power regulation unit determines the upper limit of the power balance window based on the decreasing trend of the remaining dischargeable charge. The dischargeable power and the remaining dischargeable charge change in the same direction, both determined by the state of charge. As the remaining dischargeable charge decreases, the dischargeable power also decreases, and the upper limit of the power balance window decreases accordingly, thus narrowing the power balance window on the discharge side.

[0104] The decreasing trend of remaining dischargeable energy refers to a continuous decrease in remaining dischargeable energy over multiple consecutive sampling periods, with the cumulative decrease not less than a preset decrease threshold. The determination process involves continuously acquiring the remaining dischargeable energy according to the sampling period, comparing the value of the current sampling period with the value of the previous sampling period, and determining that a decreasing trend exists if the remaining dischargeable energy decreases for a preset number of consecutive sampling periods and the cumulative decrease is not less than the decrease threshold.

[0105] The acquisition period is determined based on the control unit's operation period and the rate of change of remaining dischargeable energy, and is set to a value no greater than the control unit's operation period. The preset number and reduction threshold are determined based on the sampling accuracy of the state of charge and the adjustment frequency of the power balance window. The preset number is set to a value that excludes single fluctuations caused by sampling noise, and the reduction threshold is set to a value greater than the energy deviation corresponding to the state of charge sampling error. For example, the acquisition period is 100ms, the preset number is 10, and the reduction threshold is 0.5kWh.

[0106] It is understandable that step S6 converges the battery output power to the level corresponding to the discharge maintenance benchmark in terms of discharge duration, while step S7 converges the adjustment power difference to the range of sustainable discharge of the energy storage battery in terms of discharge power. The two respectively constrain the discharge duration and discharge power.

[0107] Among them, the charging acceptance capability of the energy storage battery does not decrease when the state of charge decreases, the change in rechargeable power is relatively small, and the lower limit of the power balance window remains at the value at the switching moment.

[0108] Specifically, the upper limit of the narrowed power balance window is determined by looking up the piecewise linear curve showing the change in dischargeable power with state of charge, obtained through pre-experimental calibration in step S1, based on the measured state of charge. Its lower limit remains the value at the switching moment without being updated, thus gradually narrowing the power balance window on the discharge side. It can be understood that after the power balance window narrows, the allowable power difference range shrinks towards the discharge side, allowing the load to be shut down earlier, thereby preventing deep discharge of the energy storage battery.

[0109] The number of fixed loads to be shut down is determined as follows: fixed loads are shut down one by one in order of load priority from low to high. At the same time as each fixed load is shut down, the load power is re-acquired and the adjustment power difference is re-determined until the adjustment power difference falls back to within the narrowed power balance window. The cumulative number of fixed loads shut down is the total number of fixed loads shut down.

[0110] Specifically, during the process of shutting down fixed loads one by one, if the power difference exceeds the narrowed power balance window again, the power adjustment unit continues to shut down the next fixed load according to the load priority from low to high, until the power difference falls back to within the narrowed power balance window. The shutdown order is from low to high load priority until the power difference falls back to within the narrowed power balance window.

[0111] In this embodiment, as the discharge continues, the dischargeable power decreases from 40kW to 12kW, and the upper limit of the power balance window decreases accordingly from 40kW to 12kW. The adjusted power difference is 15kW, which exceeds the upper limit. The power adjustment unit shuts down the fixed load with the lowest load priority, which has a power of 5kW. The adjusted power difference falls back to 10kW, falling within the narrowed power balance window.

[0112] In step S8, the process of restoring power supply to the load includes: Specifically, hysteresis bands are provided between the upper limit of the power balance window used to shut down the load and the input boundary used to restore the load, as well as between the lower limit of the power balance window used to reduce photovoltaic output power and the input boundary on the lower limit side of restoring photovoltaic output power. Both input boundaries are located inside the corresponding window boundaries. The input boundary is the power difference boundary used to determine whether load power can be restored, and is a preset proportion of the corresponding boundary of the power balance window. The upper limit input boundary is a preset proportion of the upper limit of the power balance window and is less than the upper limit; the lower limit input boundary is the same preset proportion of the lower limit of the power balance window. Since the lower limit of the power balance window is negative, the value obtained after multiplying by this preset proportion is greater than the lower limit and located inside the power balance window. For example, if the upper limit of the power balance window is 40kW and the lower limit is -25kW, and the preset proportion is 0.9, then the upper limit input boundary is 36kW and the lower limit input boundary is -22.5kW.

[0113] Please see Figure 4 As shown, this is a schematic diagram of the relationship between the hysteresis bands on both sides of the upper and lower limits of the power balance window in an embodiment of the present invention. This diagram corresponds to the load recovery process described above. Figure 4In the diagram, the horizontal axis represents the discharge duration in hours (h), and the vertical axis represents the power in kilowatts (kW). In the positive power region, the solid line represents the upper limit of the power balance window, which decreases stepwise as the discharge continues. The dashed line represents the upper limit side input boundary, which is the same shape as the upper limit of the power balance window and is generally lower than the upper limit of the power balance window. The interval between the two lines is the upper limit side hysteresis band, and the upper limit side hysteresis band narrows proportionally as the upper limit of the power balance window decreases stepwise. In the negative power region, the solid line represents the lower limit of the power balance window, which remains at the value taken at the switching moment. The dashed line represents the lower limit side input boundary, which is the same shape as the lower limit of the power balance window and is generally higher than the lower limit of the power balance window. The interval between the two lines is the lower limit side hysteresis band.

[0114] It is understandable that if the boundary conditions for restoring the load and shutting down the load are the same, the load will be repeatedly switched on and off when the power difference fluctuates around that value. The hysteresis band makes the restoration conditions more lenient than the shutdown conditions, thereby avoiding repeated switching of the load.

[0115] Specifically, the preset ratio for the input boundary is obtained through pre-experiment calibration. The calibration process is as follows: first, the allowable number of switching operations is determined based on the load switching characteristics and the allowable number of operations of the load switching device; then, switching tests are conducted at several progressively decreasing ratios, and the number of load switching operations per unit time is measured at each ratio. The ratio with the largest value that ensures the number of switching operations does not exceed the allowable number is taken as the preset ratio. The allowable number of operations is determined based on the load switching characteristics and the service life of the load switching device, i.e., the number of operations per unit time calculated from the allowable number of operations of the load switching device. Since the input boundary is a preset ratio of the upper limit of the power balance window, the input boundary is smaller than the upper limit of the power balance window.

[0116] Specifically, when the adjusted power difference falls below the input boundary and remains stable for a preset duration, the recovery unit restores power supply to the load. The preset duration of continuous stability is the time during which the adjusted power difference needs to remain stable before the load is restored.

[0117] Specifically, the preset duration for continuous stability is obtained through pre-experiment calibration. The calibration process is as follows: A recovery operation is performed on the closed load, and the number of times the adjusted power difference exceeds the power balance window again after recovery is recorded. This experiment is repeated for several progressively larger durations, and the minimum value where the number of times the adjusted power difference exceeds the power balance window again does not exceed the allowable number is taken as the preset duration for continuous stability. The allowable number of times is determined based on the switching characteristics of the load and the service life of the load switching device, i.e., the number of times calculated per unit time based on the allowable number of operations of the load switching device.

[0118] The recovery unit first restores the power setting of adjustable loads that have had their power reduced, and then restores the power supply to the loads that have been turned off in reverse order according to their load priority, from high to low. This means that restoring the power setting of adjustable loads first buffers the power balance, and then restoring the loads that have been turned off in order of their importance, preventing the power difference from exceeding the power balance window again during the recovery process.

[0119] Specifically, each time the power setting of an adjustable load is restored or the power supply of a previously shut-off load is restored, the load power and photovoltaic output power are re-acquired to redetermine the adjustment power difference and re-verify it. When the adjustment power difference rises back above the input boundary, the restoration is paused. After the adjustment power difference falls back below the input boundary and remains stable for a preset time, the restoration continues in the original order.

[0120] Furthermore, when the adjusted power difference rises above the lower limit input boundary and remains stable for a preset duration, the previously reduced photovoltaic output power due to the photovoltaic output power exceeding the load power is restored. It can be understood that the reduction in photovoltaic output power and the shutdown of the load correspond to opposite sides of the power difference. The input boundaries on both sides are taken as the same preset proportion of the corresponding boundaries of the power balance window; the upper limit input boundary is smaller than the upper limit of the power balance window, and the lower limit input boundary is larger than the lower limit of the power balance window. Both sides are determined to recover based on their respective input boundaries and the same preset stable duration, ensuring symmetrical recovery conditions for adjustments on both sides of the power difference.

[0121] The technical solution of the present invention has been described above with reference to the preferred embodiments shown in the accompanying drawings. However, it will be readily understood by those skilled in the art that the scope of protection of the present invention is obviously not limited to these specific embodiments. Without departing from the principles of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will all fall within the scope of protection of the present invention.

Claims

1. A grid-connected / off-grid switching control method for a photovoltaic energy storage inverter, characterized in that, include: In grid-connected operation, the power balance window is determined based on the state of charge of the energy storage battery; The power difference is compared with the power balance window, and combined with the voltage status of the power grid and the islanding detection results, it is determined whether the power grid is in an abnormal state. The energy storage converter switches its current mode to off-grid voltage source mode in response to the abnormal state of the power grid, and performs the first power regulation based on the power difference. The difference between the adjusted power and the photovoltaic output power after the first adjustment is re-determined based on the measured load power and the difference between the adjusted power and the photovoltaic output power after the first adjustment. The difference between the adjusted power and the photovoltaic output power after the first adjustment is then checked to see if the difference between the adjusted power and the photovoltaic output power falls within the power balance window. If the verification fails, reduce the power setting of the adjustable load, turn off the adjustable load, or continue to reduce the photovoltaic output power until the verification passes. During off-grid power supply, when the discharge sustaining time is less than the discharge sustaining reference, the reduction in battery output power is determined based on the power of the closed load, the reduction in power of the adjustable load, and the current output power of the energy storage battery. The discharge duration is determined based on the difference between the remaining dischargeable capacity of the energy storage battery and the adjustable power. When the remaining dischargeable power shows a decreasing trend, the power balance window is narrowed, and when the adjusted power difference exceeds the narrowed power balance window again, several fixed loads are shut down.

2. The grid-connected / off-grid switching control method for a photovoltaic energy storage inverter according to claim 1, characterized in that, The process of determining the power balance window includes: The discharge power and rechargeable power of the energy storage battery are obtained based on the state of charge. The upper limit of the power balance window is determined based on the dischargeable power; The lower limit of the power balance window is determined based on the rechargeable power.

3. The grid-connected / off-grid switching control method for photovoltaic energy storage inverters according to claim 2, characterized in that, The process of determining whether the power grid is in an abnormal state includes: Based on the power difference not falling into the power balance window, the voltage amplitude and voltage frequency in the voltage state not falling into the corresponding preset range, or the detection of an islanding state, the power grid is determined to be in an abnormal state. The power difference is determined based on the difference between the load power and the photovoltaic output power.

4. The grid-connected / off-grid switching control method for a photovoltaic energy storage inverter according to claim 3, characterized in that, The process of switching the current mode to the off-grid voltage source mode includes: Disconnect the static transfer switch between the load and the power grid to enter the off-grid state; Based on the fact that the power difference is greater than the upper limit of the power balance window, it is determined to shut down the adjustable loads in order of load priority from low to high, and the power difference is re-determined when any adjustable load is shut down, until the power difference falls back to within the power balance window; Based on the fact that the power difference is less than the lower limit of the power balance window, it is determined to reduce the photovoltaic output power.

5. The grid-connected / off-grid switching control method for a photovoltaic energy storage inverter according to claim 4, characterized in that, The process of failing the verification includes: Based on the fact that the power difference is greater than the upper limit of the power balance window, the power setting value of the unclosed adjustable load is determined to be reduced. Based on the fact that the power difference is less than the lower limit of the power balance window, it is determined to continue reducing the photovoltaic output power.

6. The grid-connected / off-grid switching control method for a photovoltaic energy storage inverter according to claim 5, characterized in that, The process of determining the extent of the reduction in battery output power includes: The target discharge power is determined based on the remaining dischargeable capacity of the energy storage battery and the discharge sustaining benchmark. The amount of power to be reduced is determined based on the difference between the current output power of the energy storage battery and the target discharge power. The remaining adjustable capacity is determined based on the difference between the total adjustable capacity and the amount of power already reduced; Based on the remaining adjustable capacity and the output power limit and adjustment rate of the energy storage battery, the amount of power to be reduced is allocated as the power setpoint reduction of the adjustable load and the shutdown amount of the fixed load. The reduction in battery output power is determined based on the sum of the reduction in the power setting value and the shutdown amount.

7. The grid-connected / off-grid switching control method for a photovoltaic energy storage inverter according to claim 6, characterized in that, The process of narrowing the power balance window includes: Based on the decreasing trend of the remaining dischargeable capacity of the energy storage battery, an upper limit for reducing the power balance window is determined; When the power difference exceeds the narrowed power balance window again, determine to shut down a number of fixed loads in order of load priority from low to high, until the power difference falls back into the narrowed power balance window.

8. The grid-connected / off-grid switching control method for a photovoltaic energy storage inverter according to claim 7, characterized in that, The process of restoring power to the load includes: When the adjusted power difference falls below the input boundary on which the load is restored and remains stable for a preset time, the power setting value for restoring the adjustable load with reduced power is determined, and the power supply to the load that has been turned off is restored in reverse order according to the load priority.

9. The grid-connected / off-grid switching control method for a photovoltaic energy storage inverter according to claim 8, characterized in that, The process of restoring power to the load also includes: Hysteresis bands are provided between the upper limit of the power balance window and the upper limit side input boundary, and between the lower limit of the power balance window and the lower limit side input boundary, respectively. The upper limit side input boundary is smaller than the upper limit of the power balance window, and the lower limit side input boundary is larger than the lower limit of the power balance window.

10. A grid-connected / off-grid switching control system for a photovoltaic energy storage inverter, used to implement the grid-connected / off-grid switching control method for a photovoltaic energy storage inverter as described in any one of claims 1-9, characterized in that, include: Energy storage batteries are used to store electrical energy and discharge to the load in off-grid voltage source mode; A photovoltaic device for converting light energy into electrical energy and supplying power to a load as well as charging the energy storage battery; An energy storage converter is used to switch between grid-connected mode and off-grid voltage source mode, and to control the charging and discharging of the energy storage battery. A static transfer switch is used to disconnect or close the connection between the load and the power grid. The determination unit is used to determine the power balance window based on the state of charge of the energy storage battery, compare the power difference with the power balance window, and combine the voltage state of the power grid and the islanding detection results to determine whether the power grid is in an abnormal state. A power regulation unit is used to control the energy storage converter to switch to off-grid voltage source mode in response to the grid being in an abnormal state and to perform the first power regulation based on the power difference; The verification unit is used to determine the difference between the adjusted power and the photovoltaic output power after the first adjustment based on the measured load power and to verify whether the difference between the adjusted power and the photovoltaic output power falls within the power balance window. If the verification fails, the power setting value of the adjustable load is reduced, the adjustable load is turned off, or the photovoltaic output power is reduced until the verification passes. A battery power regulation unit is used to reduce the output power of the energy storage battery based on a discharge sustaining time being less than a discharge sustaining reference. A window narrowing unit is used to determine to narrow the power balance window based on the decreasing trend of the remaining dischargeable capacity of the energy storage battery, and to shut down several fixed loads based on the adjustment power difference exceeding the narrowed power balance window again.

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