Power conversion system control method and power conversion system

Abstract

This application discloses a control method and a power conversion system for a power conversion system, belonging to the field of electronic power technology. The method, applied to a power conversion system, includes: acquiring the photovoltaic voltage of a photovoltaic module and the inverter voltage output from the off-grid inverter to the load, and acquiring the voltage output frequency corresponding to the inverter voltage; in response to a load start-up command, if the photovoltaic voltage is less than a voltage collapse warning threshold, setting the target output voltage of the photovoltaic module to the output voltage corresponding to the maximum output power of the photovoltaic module, reducing the inverter voltage to a first target output voltage, and reducing the voltage output frequency to a first target output frequency; and if the load has completed start-up, increasing the inverter voltage to the rated inverter voltage and increasing the voltage output frequency to the rated output frequency. This application improves the load-carrying capacity of the power conversion system and ensures the power supply reliability and practicality of the power conversion system.

Description

Technical Field

[0001] This application relates to the field of power electronics technology, specifically to a power conversion system control method and a power conversion system. Background Technology

[0002] In underdeveloped countries and remote areas, power shortages and unstable grid power are common problems, creating an urgent need for renewable energy equipment that provides stable power supply. Due to economic constraints, users often prioritize low-priced, simplified photovoltaic off-grid inverters that only have an off-grid operating mode, and to further control costs, they typically do not include energy storage battery units.

[0003] In the application scenarios of this type of off-grid photovoltaic (PV) inverter, fluctuations in sunlight intensity directly affect the output power of the PV modules, thus impacting the inverter's load-carrying capacity. Related technologies often optimize the load-carrying capacity of off-grid PV inverters by adding energy storage modules and optimizing the inverter's hardware structure. However, optimizing the hardware structure increases manufacturing costs and cannot fundamentally solve the voltage drop problem during load startup, leading to insufficient load-carrying capacity and consequently affecting the reliability and practicality of the off-grid PV inverter's power supply. Summary of the Invention

[0004] This application provides a power conversion system control method and a power conversion system, aiming to improve the load capacity of the power conversion system and ensure the power supply reliability and practicality of the power conversion system without increasing hardware.

[0005] In a first aspect, embodiments of this application provide a power conversion system control method, applied to a power conversion system comprising a photovoltaic module and an off-grid inverter connected in sequence, the off-grid inverter also being connected to a load; the method comprising: acquiring the photovoltaic voltage of the photovoltaic module and the inverter voltage output by the off-grid inverter to the load, and acquiring the voltage output frequency corresponding to the inverter voltage; in response to a load start-up command, when the photovoltaic voltage is less than a voltage collapse warning threshold, setting the target output voltage of the photovoltaic module to the output voltage corresponding to the maximum output power of the photovoltaic module, reducing the inverter voltage to a first target output voltage, and reducing the voltage output frequency to a first target output frequency; when the load has completed start-up, increasing the inverter voltage to a rated inverter voltage, and increasing the voltage output frequency to a rated output frequency.

[0006] Secondly, embodiments of this application also provide a power conversion system, the power conversion system including a photovoltaic module and an off-grid inverter connected in sequence, the off-grid inverter being further connected to a load; the power conversion system also includes a controller, the controller being configured to execute the power conversion system control method as described in any of the above embodiments.

[0007] The power conversion system control method in this application embodiment acquires the photovoltaic voltage and the inverter voltage and frequency output from the inverter to the load in real time. When the photovoltaic voltage drops below the voltage collapse warning threshold due to load startup, the target output voltage of the photovoltaic module is set to the output voltage corresponding to the maximum output power of the photovoltaic module. The inverter voltage and frequency are actively reduced to a lower first target value, so that the power required by the load during startup is reduced and matched with the current output capacity of the photovoltaic. This avoids the photovoltaic side from continuously collapsing due to excessive power gap and triggering the inverter undervoltage shutdown, ensuring that the load can smoothly pass through the startup impact stage. After the load has completed startup and the power demand has dropped to a steady state, the inverter voltage and frequency are gradually restored to the rated value, so that the load can operate stably under normal operating conditions. Therefore, this application embodiment solves the technical problem of off-grid photovoltaic inverters without energy storage being prone to shutdown due to power mismatch and insufficient load capacity at the moment of load startup by simply dynamically adjusting the output parameters of the off-grid inverter without the need to add energy storage batteries or optimize hardware. This significantly improves the power supply reliability and practicality of the system under conditions of light fluctuations and load impacts. Attached Figure Description

[0008] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0009] Figure 1 This is a schematic diagram of the structure of a power conversion system provided in an embodiment of this application; Figure 2 A schematic diagram illustrating the voltage (V)-power (P) output characteristics of a photovoltaic module provided in an embodiment of this application; Figure 3 A flowchart illustrating a power conversion system control method provided in an embodiment of this application; Figure 4 This is a control block diagram for the photovoltaic side provided in an embodiment of this application; Figure 5 This is a flowchart illustrating another power conversion system control method provided in an embodiment of this application. Detailed Implementation

[0010] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0011] In the description of this application, it should be understood that the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of technical features indicated. Therefore, features defined as "first" or "second" may explicitly or implicitly include one or more of the stated features. In the description of this application, "multiple" means two or more, unless otherwise explicitly specified.

[0012] "A and / or B" includes the following three combinations: A only, B only, and a combination of A and B.

[0013] The use of "applies to" or "configured to" in this application implies open and inclusive language, which does not exclude the applicability to or configuration to devices performing additional tasks or steps. Additionally, the use of "based on" implies openness and inclusivity, because processes, steps, calculations, or other actions "based on" one or more of the stated conditions or values ​​may in practice be based on additional conditions or values ​​beyond those stated.

[0014] In this application, the term "exemplary" is used to mean "used as an example, illustration, or description." Any embodiment described as "exemplary" in this application is not necessarily to be construed as being more preferred or advantageous than other embodiments. The following description is provided to enable any person skilled in the art to make and use this application. Details are set forth in the following description for purposes of explanation. It should be understood that those skilled in the art will recognize that this application can be made without using these specific details. In other instances, well-known structures and processes are not described in detail to avoid obscuring the description of this application with unnecessary detail. Therefore, this application is not intended to be limited to the embodiments shown, but is consistent with the broadest scope of the principles and features disclosed in this application.

[0015] Please see Figure 1 , Figure 1 This is a schematic diagram of a power conversion system provided in an embodiment of this application. Figure 1 As shown, the power conversion system includes a photovoltaic module 100 and an off-grid inverter 200 connected in sequence, and the off-grid inverter is also connected to the load.

[0016] The power conversion system in this embodiment is a photovoltaic off-grid power supply system without energy storage batteries. This type of power conversion system is inexpensive, has simplified functions, and only has an off-grid operating mode, making it suitable for underdeveloped countries and remote areas.

[0017] A photovoltaic (PV) module is a photovoltaic power generation unit composed of PV cells connected in series or parallel, used to convert solar energy into direct current (DC) electricity. The output power of a PV module varies with environmental conditions such as light intensity and temperature, exhibiting non-linear voltage-power output characteristics. Please refer to [link / reference]. Figure 2 , Figure 2 This is a schematic diagram of the voltage (V)-power (P) output characteristics of a photovoltaic module provided in an embodiment of this application.

[0018] like Figure 2 As shown, when the maximum output power of the photovoltaic module is greater than the load power, the photovoltaic module operates at point (V1, P1). When the maximum output power of the photovoltaic module is equal to the load power, the photovoltaic module operates at point (V2, P2). When the maximum output power of the photovoltaic module is less than the load surge power, the power conversion system control method in this embodiment will limit the operating point to (V3, P3). After the load start-up impact phase ends, the photovoltaic module will adjust the operating point towards (V2, P2). Here, V0 represents the open-circuit voltage of the photovoltaic module; V1, V2, V3, and P1, P2, P3 represent the schematic operating voltage and output power of the photovoltaic module under different load conditions, respectively; V2 and P2 correspond to the operating state near the maximum power point.

[0019] An off-grid inverter refers to a power electronic conversion device operating in off-grid mode. Its input is connected to photovoltaic modules, and its output is connected to the load. It is used to convert the direct current (DC) output from the photovoltaic modules into alternating current (AC) required by the load. Off-grid inverters are used to convert DC power into AC power and supply it to the load.

[0020] In some embodiments, the power conversion system further includes a controller (not shown in the figures). The photovoltaic module and inverter provided in this application embodiment are both controlled by the controller and execute the control method of the power conversion system of this application embodiment. The specific process and principle of the control method are described in the following method embodiments, and will not be detailed here.

[0021] In such off-grid photovoltaic power supply systems, fluctuations in sunlight intensity directly affect the output power of photovoltaic modules, thereby impacting the load-carrying capacity of the power conversion system. Furthermore, everyday electronic devices (such as small motors, water pumps, and household appliances) typically exhibit high starting current and low steady-state power, meaning they require significant instantaneous power upon startup, while the power required decreases considerably once the system is running stably.

[0022] When a load generates a large instantaneous power demand upon startup, the output power of the photovoltaic (PV) modules cannot match this demand in time, leading to voltage sag (voltage drop) on the PV side. In related technologies, due to the lack of an effective dynamic adjustment mechanism, after a voltage sag on the PV side, the inverter will trigger overcurrent or undervoltage protection and shut down, or its load-carrying capacity will decrease significantly, making it unable to drive the load to start up and operate stably. This severely affects the reliability and practicality of off-grid PV power supply systems and fails to meet the actual electricity needs of underdeveloped countries and remote areas.

[0023] Most solutions for optimizing the load capacity of off-grid photovoltaic inverters focus on adding energy storage modules, optimizing the inverter hardware structure, or improving the maximum power point (MPP) tracking algorithm to improve photovoltaic power harvesting efficiency. However, adding hardware modules will significantly increase equipment costs, which contradicts the cost requirements of underdeveloped countries and remote areas. Optimizing the hardware structure will also increase manufacturing costs and cannot fundamentally solve the voltage drop problem at the moment of load start-up. Simply improving the MPP tracking algorithm can only improve the photovoltaic power harvesting efficiency and cannot cope with the power mismatch problem caused by sudden load increases.

[0024] Based on this, embodiments of this application provide a power conversion system control method. This method requires no additional hardware costs, can effectively cope with load startup shocks, fundamentally solves the voltage drop problem during load startup, and thus enhances the load-carrying capacity of the power conversion system.

[0025] Please see Figure 3 , Figure 3 This is a flowchart illustrating a power conversion system control method provided in an embodiment of this application. The method is applied to the aforementioned power conversion system. Figure 3 As shown, the method includes the following steps S310 to S330.

[0026] S310. Obtain the photovoltaic voltage of the photovoltaic module and the inverter voltage output from the off-grid inverter to the load, and obtain the voltage output frequency corresponding to the inverter voltage.

[0027] Wherein, photovoltaic voltage is the DC voltage value at the output terminal of the photovoltaic module, denoted as V. pv The inverter voltage is the effective value U of the AC voltage at the output terminal of the off-grid inverter. ac This refers to the AC voltage supplied to the load. The voltage output frequency is the frequency f of the AC voltage output by the off-grid inverter. ac This refers to the frequency of the AC power supplied to the load. In the embodiments of this application, the voltage output frequency can also be understood as the frequency of the AC voltage output by the off-grid inverter.

[0028] In this embodiment of the application, the photovoltaic voltage V of the photovoltaic module is collected in real time by the detection unit in the controller.pv Output current I pv Collect the inverter voltage U of the off-grid inverter. ac Output current I ac Output frequency f ac Inverter bus voltage U bus and the real-time power P of the load load .

[0029] Meanwhile, the output power P of the photovoltaic module is detected in real time through the MPP tracking module. pv And record the maximum power P in real time. mpp The relative position of the current photovoltaic operating point and the MPP is determined, and the photovoltaic side voltage is monitored simultaneously for any signs of voltage collapse. In some embodiments, the photovoltaic module output power P... pv and maximum power P mpp The calculation frequency can be matched with the sampling frequency of the photovoltaic module's output voltage and output current (e.g., 20kHz) to meet the need for rapid detection of photovoltaic power changes when the load suddenly increases.

[0030] S320, in response to the load start command, when the photovoltaic voltage is less than the voltage collapse warning threshold, the target output voltage of the photovoltaic module is set to the output voltage corresponding to the maximum output power of the photovoltaic module, the inverter voltage is reduced to the first target output voltage, and the voltage output frequency is reduced to the first target output frequency.

[0031] The load start-up command is a power surge signal generated when the load begins to start. In this application, it can be represented by a jump in load power from zero or low power to start-up power. This command can be identified by detecting the load power change rate or the photovoltaic voltage drop trend. The first target output voltage is the target value of the AC output voltage actively reduced by the off-grid inverter when the load starts and the photovoltaic side voltage shows a collapse trend. This value is less than the inverter's rated inverter voltage, but must meet the minimum voltage requirement for the load to start normally. For example, the first target output voltage can be 85% to 95% of the preset normal output voltage value.

[0032] The first target output frequency is the target value of the AC output frequency after the off-grid inverter actively reduces its frequency during the load startup phase. This value is lower than the inverter's rated output frequency, but must be within the frequency range where the load can operate normally. For example, the adjustment range of the first target output frequency can be 95% to 99% of the preset normal output frequency.

[0033] The voltage collapse warning threshold is a pre-set lower limit value V of the photovoltaic module output voltage. lowThis threshold is used to determine if the photovoltaic (PV) voltage is showing a downward trend. When the PV voltage falls below this threshold, it indicates that the current output power of the PV modules cannot meet the load demand, and there is a risk that the inverter will shut down due to undervoltage. This threshold can be flexibly set according to the rated output voltage of the PV modules and the actual application scenario.

[0034] In this embodiment of the application, when the photovoltaic voltage V is detected pv The voltage drops below the voltage collapse warning threshold V. low If the instantaneous power required for the current load to start exceeds the maximum power that the photovoltaic module can currently output, and there is a risk of further voltage collapse in the photovoltaic voltage, then the target output voltage of the photovoltaic module will be reduced from the current set value (i.e., the voltage collapse warning threshold V). low Switch to the output voltage V corresponding to the maximum output power of the photovoltaic module. mpp Then, the inverter voltage is reduced from the preset normal output voltage value (rated inverter voltage) to the first target output voltage, and the inverter's voltage output frequency is reduced from the preset normal output frequency (rated output frequency) to the first target output frequency. It should be noted that the output voltage V corresponding to the maximum output power... mpp This refers to the photovoltaic module reaching its maximum output power P under the current light and temperature conditions. mpp The voltage value corresponding to the time.

[0035] To avoid sudden drops in inverter voltage and voltage output frequency from impacting the load or causing drastic fluctuations in inverter parameters, the implementation method of this application adopts a stepped adjustment method.

[0036] Specifically, the inverter voltage is gradually reduced from its current value to the first target output voltage, adjusted once every first adjustment cycle, with each adjustment not exceeding the preset first adjustment range. Simultaneously, the voltage output frequency is gradually reduced from its current value to the first target output frequency, adjusted once every first adjustment cycle, with each adjustment not exceeding the preset first adjustment range.

[0037] During the adjustment process, the photovoltaic voltage and load power are collected in real time to ensure that the photovoltaic voltage is stable near the voltage collapse warning threshold, and the load power is gradually reduced to match the power that the photovoltaic modules can stably output, so as to avoid further photovoltaic voltage collapse, ensure that the inverter does not trigger protection shutdown, and maintain continuous power supply during the load startup process.

[0038] This application embodiment improves the upper limit of the load-carrying capacity of the off-grid inverter by switching the target output voltage of the photovoltaic module from the voltage collapse warning threshold to the output voltage corresponding to the maximum power point when the photovoltaic voltage drops below the warning threshold, thus ensuring that the voltage collapse is suppressed while the photovoltaic energy is maximized.

[0039] S330: When the load has started, the inverter voltage is increased to the rated inverter voltage and the voltage output frequency is increased to the rated output frequency.

[0040] Rated inverter voltage is the output voltage of the off-grid inverter under normal operating conditions, i.e., the voltage value required for the load to operate under rated conditions. Rated output frequency is the output frequency value of the off-grid inverter under normal operating conditions.

[0041] During the execution of the above dynamic adjustment strategy, load power changes are continuously monitored. When the load completes startup and enters a stable operating state, the system executes the recovery strategy.

[0042] The determination that the load has completed startup can be achieved by: detecting that the real-time load power is stabilizing, i.e., the rate of change of the load power is less than a preset threshold; and that the real-time load power is lower than the current maximum power P of the photovoltaic module. mpp This indicates that the photovoltaic energy is sufficient to support the stable operation of the load.

[0043] After determining that the load has completed startup, the controller initiates a recovery strategy, increasing the inverter voltage from the current value to the rated inverter voltage, and increasing the voltage output frequency from the current value to the rated output frequency.

[0044] Similarly, to avoid sudden shocks during the parameter recovery process, the inverter voltage and voltage output frequency recovery process also adopts a step-by-step recovery method.

[0045] Specifically, the inverter voltage is gradually restored from its current value to the rated inverter voltage, adjusted once every second adjustment cycle, with each restoration amplitude not exceeding the preset second adjustment amplitude; the voltage output frequency is gradually restored from its current value to the rated output frequency, adjusted once every second adjustment cycle, with each restoration amplitude not exceeding the preset second adjustment amplitude.

[0046] During the recovery process, photovoltaic voltage and load power are continuously monitored to ensure a smooth recovery and avoid power surges that could impact the load and inverter.

[0047] The power conversion system control method in this application embodiment acquires the photovoltaic voltage and the inverter voltage and frequency output from the inverter to the load in real time. When the photovoltaic voltage drops below the voltage collapse warning threshold due to load startup, the target output voltage of the photovoltaic module is set to the output voltage corresponding to the maximum output power of the photovoltaic module. The inverter voltage and frequency are actively reduced to a lower first target value, so that the power required by the load during startup is reduced and matched with the current output capacity of the photovoltaic. This avoids the photovoltaic side from continuously collapsing due to excessive power gap and triggering the inverter undervoltage shutdown, ensuring that the load can smoothly pass through the startup impact stage. After the load has completed startup and the power demand has dropped to a steady state, the inverter voltage and frequency are gradually restored to the rated value, so that the load can operate stably under normal operating conditions. Therefore, this application embodiment solves the technical problem of off-grid photovoltaic inverters without energy storage being prone to shutdown due to power mismatch and insufficient load capacity at the moment of load startup by simply dynamically adjusting the output parameters of the off-grid inverter without the need to add energy storage batteries or optimize hardware. This significantly improves the power supply reliability and practicality of the system under conditions of light fluctuations and load impacts.

[0048] In some embodiments, the power conversion system control method further includes: setting the target output voltage of the photovoltaic module to the voltage collapse warning threshold when the photovoltaic voltage is greater than or equal to the voltage collapse warning threshold; wherein the voltage collapse warning threshold is less than the rated output voltage of the photovoltaic module.

[0049] The above steps constitute a preventative setting strategy for the target output voltage of the photovoltaic module. After the system starts up and enters normal operation, the detection unit acquires the photovoltaic voltage V of the photovoltaic module in real time. pv and V pv Compared with the preset voltage collapse warning threshold V low Compare them.

[0050] When the photovoltaic voltage V is detected pv Greater than or equal to the voltage collapse warning threshold V low This indicates that the current output voltage of the photovoltaic module is within a safe range and there is no risk of voltage collapse. Under this condition, the target output voltage V of the photovoltaic module is... ref Pre-set voltage collapse warning threshold V low .

[0051] As an example, the voltage collapse warning threshold V low It can be set to between 80% and 90% of the rated output voltage of the photovoltaic module.

[0052] For example, when the rated output voltage of a photovoltaic module is 36V, the voltage collapse warning threshold can be set to 30.6V (i.e., 85% of the rated value). In practical applications, this threshold can be flexibly adjusted according to the specific model of the photovoltaic module, the lighting conditions, and the load characteristics to balance the response sensitivity and the prevention of false triggering of the dynamic adjustment strategy.

[0053] Please see Figure 4 , Figure 4 This is a control block diagram for the photovoltaic side provided in an embodiment of this application. Figure 4 As shown, control loop 1 is the photovoltaic voltage V. pv The control loop is used to maintain the photovoltaic voltage and prevent voltage collapse on the photovoltaic side. The input to control loop 1 is the target output voltage value V of the photovoltaic module. ref And the actual photovoltaic output voltage V pv The output is a current control command. Control loop 2 is the bus voltage control loop, used to maintain the bus voltage at the target bus voltage value Bu. sref The target voltage value of the bus voltage, Bu. sref It can be determined based on the target effective value of the inverter output AC voltage. For example, the target effective value of the inverter output AC voltage can be multiplied by 1.414 and then a preset margin can be added to ensure that the bus voltage is higher than the peak value of the inverter output AC voltage and meets the inverter conditions.

[0054] When the power conversion system is operating under normal conditions, the load power is less than the maximum power of the photovoltaic module, and the target output voltage V of the photovoltaic module is... ref It can be set as a voltage collapse warning threshold V low The current command output by control loop 1 is at a large value or saturation value. At this time, the output current of control loop 1 is greater than the output current of control loop 2, and the target current value I of the current loop is... ref Take the output of control loop 2. The proportional-integral (PI) controller takes the target value I of the input current loop. ref The duty cycle of the modulation signal is determined, and the off-grid inverter is controlled by the duty cycle to make the photovoltaic modules work near the MPP and achieve maximum power point tracking.

[0055] When the load suddenly increases and the instantaneous power exceeds the maximum power of the photovoltaic modules, the bus voltage will drop due to insufficient input energy. This occurs in control loop 2, where the Bus... ref Greater than U bus The current control command output by control loop 2 increases, I refAccording to the current (I)-voltage (V) curve characteristics of photovoltaic (PV) modules, an increase in the output current will cause a decrease in the output voltage. When the output voltage of the PV module exceeds the voltage corresponding to the maximum power point but still cannot meet the load power demand, the output voltage of the PV module will continue to decrease. At this point, the target output voltage V of the PV module... ref The value is set as the voltage collapse warning threshold V. low When the output voltage of the photovoltaic module is pulled down to V low When this happens, the current control command output by control loop 1 will decrease. At this time, the output current of control loop 1 is less than the output current of control loop 2, I... ref The output current of control loop 1 is obtained. This method limits the output current of the photovoltaic module, ensuring that the output voltage of the photovoltaic module does not continue to decrease and avoiding photovoltaic collapse.

[0056] With the above settings, when the load suddenly increases and causes the photovoltaic voltage V to drop... pv Descending to V low When the photovoltaic module is nearby, control loop 1 can intervene and limit the output current of the photovoltaic module, so that the output voltage V of the photovoltaic module is reduced. pv The voltage dropped further, preventing the photovoltaic system from collapsing and causing the off-grid inverter to shut down.

[0057] This application embodiment sets the target output voltage of the photovoltaic module to a warning threshold higher than the minimum operating voltage in advance, enabling the control loop to intervene early and respond quickly when a load impact occurs. This effectively shortens the response delay between voltage drop and protection action, achieving preventive protection against photovoltaic voltage collapse.

[0058] In some embodiments, the output voltage corresponding to the maximum output power of the photovoltaic module is the voltage value corresponding to the maximum output power of the photovoltaic module under the current illumination and temperature conditions.

[0059] The maximum output power of a photovoltaic module is P, which is the maximum output power of the photovoltaic module under the current light intensity and ambient temperature conditions. mpp At that time, the output voltage value V corresponding to the photovoltaic module mpp V mpp The value can be dynamically obtained through real-time detection and tracking, rather than using a fixed value, to ensure that the dynamic adjustment strategy is always based on the actual maximum power output capability of the photovoltaic modules.

[0060] In the embodiment of this application, at the most critical moment of load surge, the operating point of the photovoltaic module is guided to the true maximum power point under the current conditions, ensuring that the photovoltaic module supports the system with all available power, delaying the bus voltage drop to the greatest extent, and buying valuable time for load startup.

[0061] In some embodiments, reducing the inverter voltage to a first target output voltage and reducing the voltage output frequency to a first target output frequency includes: reducing the inverter voltage to a first target output voltage based on a preset first adjustment period and a preset first adjustment amplitude, and reducing the voltage output frequency to a first target output frequency.

[0062] The first adjustment period is the time interval between two consecutive adjustment actions of the inverter voltage, and the first adjustment period can be 50 to 100 ms.

[0063] The first adjustment range is the amount by which the inverter voltage or voltage output frequency changes in a single adjustment operation. For example, the adjustment range of the inverter voltage each time does not exceed 5% of the current inverter voltage value, and the adjustment range of the voltage output frequency each time does not exceed 5% of the current voltage output frequency.

[0064] In this embodiment, the inverter voltage and voltage output frequency are adjusted in a stepwise manner and can be performed simultaneously. That is, the two parameters are adjusted at the same time within the same adjustment cycle to ensure that the load experiences coordinated changes in voltage and frequency during startup, thus avoiding load anomalies caused by sudden changes in a single parameter.

[0065] In a specific example, the current inverter voltage is 220V, the first target output voltage is 200V, and the total difference is 20V. The first adjustment period is set to 80ms, and the first adjustment increment is set to 5% of the current value (i.e., a maximum adjustment of 11V each time).

[0066] The inverter voltage adjustment process is as follows: at 80ms, the inverter voltage is reduced from 220V to 209V (an adjustment of 11V, or 5%); at 160ms, it is reduced from 209V to 200V, at which point the first target output voltage of 200V is reached, and the adjustment stops. The voltage output frequency is reduced from 50Hz to 49Hz, adjusted every 80ms, with each adjustment not exceeding 5% of the current value (i.e., a maximum adjustment of 2.5Hz each time). Usually, the adjustment from 50Hz to 49Hz can be completed in one step (an adjustment of 1Hz, or 2%).

[0067] During the aforementioned inverter voltage and voltage output frequency adjustment process, the photovoltaic voltage and load power are collected in real time to ensure that the photovoltaic voltage is stable between 30 and 30.6V, and the load power is gradually reduced to the inverter's rated load power so that the load power matches the power that the photovoltaic modules can stably output, thereby preventing further collapse of the photovoltaic voltage, ensuring that the off-grid inverter does not trigger protection shutdown, and maintaining continuous power supply during the load startup process.

[0068] Through the above-mentioned step-by-step adjustment, the inverter voltage and voltage output frequency are gradually adjusted to their respective first target values ​​in a slow manner, avoiding the electrical shock to the load and off-grid inverter caused by sudden parameter changes.

[0069] In some embodiments, the first target output voltage is determined based on the bus voltage of the off-grid inverter, and the minimum value of the first target output voltage is determined based on the minimum operating voltage of the load; the first target output frequency is the minimum output frequency of the off-grid inverter when the load can operate normally.

[0070] After detecting that the photovoltaic voltage is lower than the voltage collapse warning threshold and triggering the dynamic adjustment strategy, V ref The value is set to V mpp .

[0071] At this time, because the load power is greater than the maximum output power of the photovoltaic module, the bus voltage U will be affected. bus The voltage U decreases. In this embodiment of the application, the bus voltage U is detected in real time. bus And according to U bus Real-time calculation of the first target output voltage U of the off-grid inverter Rms_ref Specifically, U Rms_ref Calculate according to the following formula: U Rms_ref = U bus / 1.414. Where, U Rms_ref This indicates that based on the current bus voltage U bus The calculated target effective value of the inverter output AC voltage; 1.414 is the square root of 2, used to represent the conversion relationship between the peak value and the effective value of the AC voltage.

[0072] Off-grid inverters convert bus voltage into AC voltage using pulse width modulation (PWM). Under a fixed modulation ratio, the effective value of the inverter voltage is directly proportional to the bus voltage. Therefore, when the bus voltage drops due to insufficient energy, the target value of the inverter voltage is dynamically adjusted accordingly, ensuring a matching relationship between the inverter voltage and the bus voltage and preventing modulation saturation or waveform distortion caused by setting the target value too high.

[0073] And set U Rms_ref The minimum value is U Rms_min Among them, U Rms_min U represents the lower limit of the target effective value of the inverter output AC voltage. Rms_min The value is greater than or equal to the minimum operating voltage of the load. By setting this minimum constraint, it is ensured that the inverter voltage will not drop below the minimum voltage that the load can withstand during dynamic adjustment. This setting method reduces the load power to match the photovoltaic output capacity, without causing the load to fail to start or stop midway due to excessively low voltage.

[0074] The first target output frequency is set to the minimum output frequency of the off-grid inverter when the load can operate normally. This minimum output frequency is determined based on the lowest supply frequency when the load can operate normally, and can typically be set to 49Hz.

[0075] In some embodiments, the power conversion system control method further includes: determining a target voltage value for the bus voltage of the off-grid inverter based on a first target output voltage, wherein the target voltage value for the bus voltage is greater than the peak AC voltage corresponding to the first target output voltage.

[0076] During dynamic adjustments due to sudden load surges, the bus voltage U... bus A downward trend will occur. If U bus If the voltage drops to an excessively low level, the AC voltage waveform output by the inverter will be distorted or even unable to maintain a normal sine wave output, which will lead to the load failing to start or the inverter shutting down for protection.

[0077] Based on this, in the dynamic adjustment phase, the embodiments of this application adjust the bus voltage U based on the real-time detected value. bus Dynamically calculate the current allowable target value U of inverter output AC voltage. Rms_ref That is, the first target output electricity.

[0078] The target voltage value of the bus voltage is the target setpoint that the control loop wants the bus voltage to stabilize at. It is the reference input of the outer loop controller of the bus voltage and is denoted as Bus. ref .

[0079] The peak AC voltage corresponding to the first target output voltage is the target effective value of the AC voltage that the off-grid inverter currently wants to output. During the dynamic adjustment phase when the load suddenly increases, this value will be adjusted downward from the rated value (e.g., 220V) (e.g., adjusted to 200V, 187V, etc.) to reduce the instantaneous power demand of the load.

[0080] In this embodiment of the application, the target voltage value of the bus voltage is Bus. ref The calculation formula is: Bus ref =U Rms_ref 1.414 + 30. To satisfy the inverting condition, Bus... ref The peak AC voltage is greater than the output voltage of the first target.

[0081] In the process of a sudden load increase causing a dynamic drop in the actual value of the bus voltage, this embodiment of the application links and synchronously adjusts the target value of the bus voltage with the target value of the output voltage. On the one hand, this ensures that the target value of the bus voltage always follows the changing trend of the actual bus voltage and retains a positive margin, ensuring that the modulation of the off-grid inverter always operates in the linear modulation region, and avoiding overmodulation, waveform distortion, harmonic surge and control failure caused by the bus voltage being lower than the peak value of the sine wave.

[0082] In some embodiments, increasing the inverter voltage to the rated inverter voltage and increasing the voltage output frequency to the rated output frequency includes: increasing the inverter voltage to the rated inverter voltage and increasing the voltage output frequency to the rated output frequency based on a preset second adjustment period and a preset second adjustment range.

[0083] Based on the above embodiments, this application provides a step-by-step recovery method for inverter voltage and voltage output frequency during the recovery phase.

[0084] After determining that the load has completed startup and entered a stable operating state, the controller does not abruptly increase the inverter voltage and voltage output frequency to their respective rated values ​​all at once. Instead, it adopts a stepped recovery method, gradually increasing the inverter voltage to the rated inverter voltage and simultaneously gradually increasing the voltage output frequency to the rated output frequency with a preset second adjustment period as the time interval and a preset second adjustment amplitude as the single change amount. The second adjustment period ranges from 100 to 200 ms, and the second adjustment amplitude does not exceed 5% of the inverter voltage value or the current voltage output frequency.

[0085] As an optional implementation, the step-wise recovery of inverter voltage and voltage output frequency can be carried out simultaneously, that is, the two parameters are adjusted at the same time within the same recovery cycle to ensure that the load feels the coordinated changes in voltage and frequency at the same time during the recovery process, and to avoid abnormal load operation caused by sudden changes in a single parameter.

[0086] When the load real-time power is detected to be stabilizing (i.e., the load power change rate is less than the preset threshold), and the load real-time power is lower than the current maximum power of the photovoltaic module, while the bus voltage reaches the target value of the bus voltage, it indicates that the load has completed the start-up and is ready to be restored to the rated working state, and then the step-by-step recovery strategy is activated.

[0087] In a specific example, after the water pump starts up and enters a stable operating state, the current inverter voltage is 200V, the rated inverter voltage is 220V, and the total difference is 20V. The second adjustment cycle is set to 150ms, and the second adjustment increment is set to 5% of the current value (for example, when the current inverter voltage is 200V, the maximum recovery is 10V each time). The controller performs the recovery as follows: at 150ms, the inverter voltage is increased from 200V to 210V; at 300ms, it is increased from 210V to 220V, and the recovery stops after reaching the rated inverter voltage of 220V. The voltage output frequency is similar, increasing from 49Hz to 50Hz, adjusted every 150ms, with each adjustment increment not exceeding 5% of the current value (i.e., a maximum recovery of 2.45Hz each time). Usually, the recovery from 49Hz to 50Hz can be completed in one step (recovering 1Hz, approximately 2%).

[0088] Through the aforementioned step-by-step recovery, the inverter voltage and voltage output frequency reach their respective rated values ​​in a slow and gradual manner, avoiding the impact of parameter mutations on the stable operation of the load and the inverter itself.

[0089] In some embodiments, the power conversion system control method further includes: if the photovoltaic voltage is less than the voltage collapse warning threshold, and the power conversion system is detected to be in an abnormal state, controlling the off-grid inverter to reduce the output power until it shuts down.

[0090] In this embodiment of the application, when the system is detected to be in an abnormal state, it is determined that the current dynamic adjustment strategy can no longer effectively restore the system to normal operating conditions, and then a protective shutdown operation is performed: the off-grid inverter is controlled to gradually reduce the inverter output power until it is completely shut down.

[0091] Specifically, the inverter's output power can be gradually reduced according to a preset power reduction rate until the output power drops to zero, at which point the inverter completely shuts down. This process is a controllable soft shutdown process, aiming to smoothly exit operation while protecting equipment safety.

[0092] An abnormal state refers to a dangerous operating state in which, after the above dynamic adjustment strategy is triggered and executed, the system still cannot return to normal operating conditions within a preset time, and continued operation may lead to damage to the inverter or load.

[0093] The abnormal state includes at least one of the following: the duration for which the photovoltaic voltage is less than the voltage collapse warning threshold is greater than or equal to a first preset duration; or the duration for which the load power is greater than the maximum output power of the photovoltaic module is greater than a second preset duration.

[0094] For example, the first preset duration and the second preset duration can be set to 1 to 3 seconds.

[0095] As a specific example, during load startup, the inverter voltage is reduced to 200V and the frequency to 49Hz. However, continuous monitoring under this state revealed that the photovoltaic voltage remained below 30.6V for 2.5 seconds, exceeding the first preset duration (2 seconds). This indicated that the photovoltaic output power was severely insufficient under the current sunlight conditions, triggering a protective shutdown. The inverter output power was gradually reduced at a rate of 20% per second, completing a soft shutdown within 5 seconds. The inverter then entered standby protection mode, preventing damage to the power conversion system due to overcurrent and undervoltage, thus improving system operational safety.

[0096] Please see Figure 5 , Figure 5 This is a flowchart illustrating another power conversion system control method provided in an embodiment of this application.

[0097] The load in this embodiment is illustrated using a small water pump as an example. The load starting power is 1.5kW, the steady-state power is 0.8kW, the rated load capacity of the off-grid inverter is 1.2kW, the rated output voltage of the photovoltaic module is 36V, and the maximum power corresponding to the MPP is 1.3kW.

[0098] like Figure 5 As shown, the power conversion system control method includes the following steps S510 to S550.

[0099] Step S510: Real-time detection of key parameters.

[0100] The output voltage and current of the photovoltaic modules, the output voltage, bus voltage, output current, and output power of the inverter, as well as the real-time power of the load, are collected in real time using voltage and current sensors. The sampling frequency for the output voltage and current of the photovoltaic modules and the inverter bus voltage is 20kHz. The sampling frequency for the output voltage, output current, and output power of the inverter is 100Hz.

[0101] The detection unit can also calculate the output power P of the photovoltaic module based on its output voltage and output current. pv And record the maximum output power P mpp and the corresponding photovoltaic module output voltage V mpp .

[0102] Step S520: Determine the sudden increase in load and obtain the operating status of the photovoltaic module.

[0103] In this embodiment of the application, the voltage collapse warning threshold is set to 85% of the rated output voltage of the photovoltaic module, i.e., 30.6V.

[0104] When the small water pump starts, the real-time load power is detected to surge from 0kW to 1.5kW. Since the maximum output power of the photovoltaic module is less than the power at the moment the water pump starts, the output voltage of the photovoltaic module is pulled down.

[0105] Figure 4 Control loop 1 intervenes when the output voltage of the photovoltaic module drops to around 30.6V to limit the output current of the photovoltaic module and suppress further voltage drop. Combined with a sudden increase in load power from 0kW to 1.5kW and a drop in the photovoltaic module output voltage to near the voltage collapse warning threshold, this can be identified as a load surge, triggering the dynamic adjustment strategy.

[0106] The detection unit records the maximum output power P of the photovoltaic module in real time. mpp and the corresponding photovoltaic module output voltage V mpp Among them, P mpp It is 1.3kW, V mpp It is 36V.

[0107] Step S530: Execute the dynamic adjustment strategy during the load surge phase.

[0108] After the dynamic adjustment strategy is triggered, the inverter controller initiates a step-by-step adjustment. The inverter's output voltage is gradually adjusted from the preset normal value of 220V to 200V (an adjustment increment of approximately 9.1%, within the preset range of 85% to 95%); the inverter's voltage output frequency is gradually adjusted from the preset normal value of 50Hz to 49Hz (an adjustment increment of 2%, within the preset range of 95% to 99%). Adjustments are made every 80ms, with each adjustment increment not exceeding 5%.

[0109] During this dynamic adjustment process, the controller can also adjust the target output voltage V of the photovoltaic module. ref Voltage collapse warning threshold V low Switch to the output voltage V corresponding to the maximum output power mpp And based on the real-time detected bus voltage U bus Calculate the target effective value UR of the inverter output AC voltage. ms_ref This is to ensure that the inverter output voltage matches the current bus voltage.

[0110] During the adjustment process, the output voltage and load power of the photovoltaic modules are collected in real time to ensure that the photovoltaic side voltage is stable between 30V and 30.6V, and the load power is gradually reduced to 1.2kW to match the power that the photovoltaic modules can stably output. This prevents the photovoltaic side voltage from collapsing further and ensures that the inverter does not trigger protection shutdown, so as to maintain continuous power supply during the water pump startup process.

[0111] Step S540: Restore the power conversion system to normal operating condition.

[0112] The system continuously monitors load power changes. Once the water pump has started and entered a stable operating state, the real-time load power is detected to be stable at 0.8kW, lower than the 1.3kW corresponding to the MPP photovoltaic module. At this point, the controller initiates a stepped recovery process, gradually restoring the inverter's output voltage from 200V to 220V and the voltage output frequency from 49Hz to 50Hz, adjusting every 150ms with each adjustment not exceeding 5%, until it returns to the preset normal operating value. The inverter then operates at its normal load capacity, ensuring a stable power supply to the water pump.

[0113] Step S550: Perform abnormal protection on the power conversion system.

[0114] During the dynamic adjustment process, if the output voltage of the photovoltaic module is detected to be continuously less than or equal to 30.6V for more than 2 seconds, or the load power is continuously higher than 1.3kW for more than 2 seconds, the off-grid inverter will be controlled to gradually reduce the output power until it shuts down, so as to avoid damage to the off-grid inverter due to overcurrent or undervoltage and improve the safety of system operation.

[0115] This application embodiment effectively avoids the problem of photovoltaic side voltage collapse during water pump startup using the above method. When the small water pump starts, the inverter does not shut down and can successfully drive the water pump to start and operate stably. This method effectively enhances the load-carrying capacity of off-grid photovoltaic inverters, improves the reliability of off-grid photovoltaic power supply systems, and requires no additional hardware costs, making it suitable for the needs of underdeveloped countries and remote areas.

[0116] It should be noted that the voltage collapse warning threshold, output voltage and frequency adjustment range and adjustment interval in the embodiments of this application can be flexibly adjusted according to the actual use scenario, inverter model, photovoltaic module parameters and load type, and are not limited to the specific values ​​in the embodiments of this application.

[0117] In the above embodiments, the descriptions of each embodiment have different focuses. For parts not described in detail in a certain embodiment, please refer to the relevant descriptions in other embodiments.

[0118] The above provides a detailed description of a power conversion system control method and a power conversion system provided in the embodiments of this application. Specific examples have been used to illustrate the principles and implementation methods of this application. The description of the above embodiments is only for the purpose of helping to understand the method and core ideas of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A control method for a power conversion system, characterized in that, The method is applied to a power conversion system, the power conversion system comprising photovoltaic modules and an off-grid inverter connected in sequence, the off-grid inverter also being connected to a load; the method includes: Obtain the photovoltaic voltage of the photovoltaic module and the inverter voltage output from the off-grid inverter to the load, and obtain the voltage output frequency corresponding to the inverter voltage; In response to a load start command, if the photovoltaic voltage is less than the voltage collapse warning threshold, the target output voltage of the photovoltaic module is set to the output voltage corresponding to the maximum output power of the photovoltaic module, the inverter voltage is reduced to the first target output voltage, and the voltage output frequency is reduced to the first target output frequency. When the load has started, the inverter voltage is increased to the rated inverter voltage, and the voltage output frequency is increased to the rated output frequency.

2. The method according to claim 1, characterized in that, The method further includes: If the photovoltaic voltage is greater than or equal to the voltage collapse warning threshold, the target output voltage of the photovoltaic module is set to the voltage collapse warning threshold; wherein the voltage collapse warning threshold is less than the rated output voltage of the photovoltaic module.

3. The method according to claim 1 or 2, characterized in that, The output voltage corresponding to the maximum output power of the photovoltaic module is the voltage value corresponding to the maximum output power of the photovoltaic module under the current light and temperature conditions.

4. The method according to claim 1, characterized in that, The steps of reducing the inverter voltage to a first target output voltage and reducing the voltage output frequency to a first target output frequency include: Based on a preset first adjustment period and a preset first adjustment range, the inverter voltage is reduced to the first target output voltage, and the voltage output frequency is reduced to the first target output frequency.

5. The method according to claim 1, characterized in that, The first target output voltage is determined based on the bus voltage of the off-grid inverter, and the minimum value of the first target output voltage is determined based on the minimum operating voltage of the load. The first target output frequency is the minimum output frequency of the off-grid inverter when the load is operating normally.

6. The method according to claim 5, characterized in that, The method further includes: The target voltage value of the bus voltage of the off-grid inverter is determined based on the first target output voltage, and the target voltage value of the bus voltage is greater than the peak AC voltage corresponding to the first target output voltage.

7. The method according to claim 1, characterized in that, The steps of increasing the inverter voltage to the rated inverter voltage and increasing the voltage output frequency to the rated output frequency include: Based on a preset second adjustment period and a preset second adjustment range, the inverter voltage is increased to the rated inverter voltage, and the voltage output frequency is increased to the rated output frequency.

8. The method according to claim 1, characterized in that, The method further includes: If the photovoltaic voltage is less than the voltage collapse warning threshold, and the power conversion system is detected to be in an abnormal state, the off-grid inverter is controlled to reduce its output power until it shuts down.

9. The method according to claim 8, characterized in that, The abnormal state includes at least one of the following: The duration during which the photovoltaic voltage is less than the voltage collapse warning threshold is greater than or equal to a first preset duration; The duration during which the load power is greater than the maximum output power of the photovoltaic module is greater than a second preset duration.

10. The method according to claim 1, characterized in that, The load start command is determined by detecting the load power change rate and / or detecting the downward trend of the photovoltaic voltage.

11. The method according to claim 1, characterized in that, The start-up of the load is determined by the following method: the real-time power change rate of the load is less than a preset threshold, and the real-time power of the load is lower than the maximum output power of the photovoltaic module.

12. The method according to claim 3, characterized in that, The method further includes: determining the output power of the photovoltaic module based on the photovoltaic voltage and output current of the photovoltaic module, and recording the maximum output power of the photovoltaic module and the output voltage corresponding to the maximum output power.

13. A power conversion system, characterized in that, The power conversion system includes a photovoltaic module and an off-grid inverter connected in sequence, the off-grid inverter being further connected to a load; the power conversion system also includes a controller configured to execute the power conversion system control method as described in any one of claims 1 to 12.

Images