A virtual inertia grid-connected control method and system for a photovoltaic power generation system

Abstract

This invention relates to the field of photovoltaic power generation grid-connected control technology, specifically disclosing a virtual inertia grid-connected control method and system for photovoltaic power generation systems. The method includes: S1: Real-time detection of the grid connection frequency; S2: Calculation of the frequency deviation based on the grid connection frequency and the rated frequency; when the frequency deviation is less than the frequency drop threshold, a grid frequency drop is determined to have occurred, and a virtual inertia response is initiated; S3: Virtual inertia response is performed using closed-loop control based on short-time super-maximum power point active power enhancement; S4: After the virtual inertia response ends, the photovoltaic power generation system is controlled to seamlessly return to maximum power point tracking steady-state operation. This invention can quickly provide active inertia support without derated operation when the grid frequency drops, thereby improving the energy utilization rate of the photovoltaic system.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic power generation grid-connected control technology, and in particular to a virtual inertia grid-connected control method and system for photovoltaic power generation systems. Background Technology

[0002] With the continuous increase in the penetration rate of new energy sources, the impact of large-scale photovoltaic (PV) power generation systems on the frequency stability of power systems is becoming increasingly prominent. Traditional synchronous generators rely on the rotational kinetic energy of their rotors to provide inertial support, while PV power generation systems, connected to the grid via power electronic inverters, lack inherent rotational inertia. This leads to a faster frequency drop rate and a lower frequency minimum point in the grid during frequency disturbance events, resulting in a significant decrease in the system's frequency stability margin. Therefore, enabling PV power generation systems to possess virtual inertial response capabilities has become a key technical issue in the field of PV grid-connected control.

[0003] In existing technologies, there are two main technical approaches for virtual inertia control of photovoltaic (PV) power generation systems. The first is a power reserve-type virtual inertia control scheme. This scheme degrades the PV system during normal operation, reserving a certain power reserve margin. When a grid frequency drop is detected, the reserved power is released to simulate an inertia response. However, because the PV system needs to operate in a derated state for an extended period, the PV energy utilization rate decreases, resulting in a loss of power generation efficiency under normal operating conditions. The second is a collaborative virtual inertia control scheme that relies on additional energy storage devices. This scheme provides energy for the inertia response by configuring battery energy storage or utilizing DC-side capacitor energy storage. However, this type of scheme requires additional hardware investment, increasing system complexity and equipment size.

[0004] Neither of the aforementioned existing technical solutions fully exploits the transient physical characteristics of photovoltaic cells, and they cannot achieve virtual inertia support for photovoltaic power generation systems without derating or relying on additional energy storage devices. Furthermore, existing technologies lack a systematic analysis of the time-scale differences between the junction temperature dynamics of photovoltaic modules and their electrical transient inertia response, and fail to establish a decoupling mechanism between junction temperature thermal inertia and the inertia response time window. Therefore, they cannot provide a theoretical basis and control strategy for utilizing the transient current margin of photovoltaic cells to achieve super-maximum power point active power generation. Summary of the Invention

[0005] This invention aims to solve the aforementioned problems. To this end, this invention provides a virtual inertia grid-connected control method and system for photovoltaic power generation systems. When the grid frequency drops, it can quickly provide active inertia support without derating operation, thereby improving the energy utilization rate of the photovoltaic system.

[0006] This invention provides a virtual inertia grid-connected control method for a photovoltaic power generation system, the technical solution of which includes: S1: Real-time detection of grid connection frequency; S2: Calculate the frequency deviation based on the grid connection frequency and the rated frequency. When the frequency deviation is less than the frequency drop judgment threshold, it is determined that a grid frequency drop has occurred, and the virtual inertia response is initiated. S3: Virtual inertial response is achieved using closed-loop control based on short-time super-maximum power point active power generation, including: Calculate the grid frequency change rate based on the grid connection frequency, and then calculate the target value of active power generation required for the virtual inertial response. Calculate the current increment command based on the active power generation target value; The actual operating current of the photovoltaic module is adjusted according to the current increment command through a current closed-loop controller. S4: After the virtual inertia response ends, control the photovoltaic power generation system to seamlessly return to maximum power point tracking steady-state operation.

[0007] Furthermore, the target value of active power generation is limited by using the maximum limit value of active power generation.

[0008] Furthermore, the formula for calculating the maximum limit of active power generation is as follows: in, This is the maximum limit value for active power generation. This represents the short-circuit current of the photovoltaic module under the current irradiance and junction temperature conditions. The maximum power point current, The voltage at the maximum power point. This is the safety margin coefficient.

[0009] Furthermore, the formula for the limiting process is expressed as follows: in, This represents the target value for the increased active power generation after the limit is applied. To obtain the minimum value, This is the maximum limit value for active power generation. This is the target value for active power generation.

[0010] Furthermore, after determining that a grid frequency drop has occurred, the time scale ratio is calculated based on the thermal time constant of the photovoltaic module junction temperature and the current virtual inertia response time window; when the time scale ratio is greater than the safety ratio threshold, the virtual inertia response is initiated.

[0011] Furthermore, the time scale ratio is the ratio of the thermal time constant of the photovoltaic module junction temperature to the virtual inertia response time window.

[0012] Furthermore, in step S4, the active power generation target value is gradually reduced with a preset slope until the operating point of the photovoltaic module returns to the maximum power point.

[0013] Furthermore, during the virtual inertia response, the grid connection frequency is measured in real time. When the grid connection frequency recovers to the preset frequency recovery range, the virtual inertia response is considered to have ended.

[0014] Furthermore, in step S3, the current closed-loop controller adopts a proportional-integral controller, whose input is the difference between the current increment command and the actual current increment, and whose output is the duty cycle adjustment of the inverter.

[0015] This invention also provides a virtual inertia grid-connected control system for a photovoltaic power generation system, the technical solution of which includes: The frequency detection module is used to detect the grid connection frequency in real time. The frequency drop determination module is used to calculate the frequency deviation based on the grid connection frequency and the rated frequency. When the frequency deviation is less than the frequency drop determination threshold, it is determined that a grid frequency drop has occurred and the virtual inertia response is initiated. The virtual inertia response module is used to perform virtual inertia response using closed-loop control based on short-time super-maximum power point active power enhancement, including: Calculate the grid frequency change rate based on the grid connection frequency, and then calculate the target value of active power generation required for the virtual inertial response. Calculate the current increment command based on the active power generation target value; The actual operating current of the photovoltaic module is adjusted according to the current increment command through a current closed-loop controller. After the virtual inertia response ends, the photovoltaic power generation system is controlled to seamlessly return to maximum power point tracking steady-state operation.

[0016] The above-described one or more technical solutions in the embodiments of the present invention have at least one of the following technical effects: 1. This invention utilizes the transient current margin of photovoltaic cells to achieve short-term active power generation above the maximum power point. It can provide virtual inertia support for the power grid without reserving power margin, avoiding the photovoltaic power generation efficiency loss caused by traditional derating operation schemes and improving the energy utilization rate of photovoltaic power generation systems.

[0017] 2. Based on the physical characteristic that there is a difference of more than two orders of magnitude between the thermal time constant of the junction temperature of a photovoltaic module and the virtual inertia response time window, this invention treats the junction temperature as approximately constant during the inertia response window, thereby achieving decoupling of transient virtual inertia control and steady-state maximum power point tracking control on the time scale. This effectively eliminates the coupling conflict between the two control modes, simplifies the design complexity of the control system, and improves the stability and reliability of the control system.

[0018] 3. This invention makes a safety determination by calculating the time scale ratio and uses the maximum limit value of active power generation for limit control, ensuring that the active power generation exceeding the maximum power point for a short time does not exceed the safe operating range of the photovoltaic module, thus avoiding irreversible damage to the photovoltaic cell caused by overcurrent.

[0019] 4. After the virtual inertia response ends, the present invention gradually reduces the active power increase command by a preset slope, realizing a seamless and smooth switch from transient over-generation state to maximum power point tracking steady-state operation, thus avoiding the impact of power surges on the power grid and photovoltaic power generation system.

[0020] Additional aspects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0022] Figure 1 This is a flowchart of the method provided by the present invention. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention. The following embodiments are used to illustrate this invention but should not be used to limit the scope of this invention.

[0024] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of the present invention. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0025] The following is combined Figure 1 The present invention will be further described in detail below, including a virtual inertia grid-connected control method and system for a photovoltaic power generation system: In this embodiment, as Figure 1 As shown, a virtual inertia grid-connected control method for a photovoltaic power generation system is provided, comprising the following steps: S1: Real-time detection of grid connection frequency.

[0026] Specifically, a frequency detection device is deployed at the grid connection point of the photovoltaic power generation system to collect the grid connection frequency in real time at a preset sampling period. In this embodiment, the sampling period is set to 5 milliseconds, that is, the instantaneous value of the grid connection frequency is acquired every 5 milliseconds.

[0027] Frequency detection employs phase-locked loop (PLL) technology. By performing Park transformation on the three-phase voltage signals at the grid connection point, the phase angle information of the voltage vector is extracted, and the phase angle information is differentiated to obtain the real-time measurement value of the grid connection frequency.

[0028] S2: Calculate the frequency deviation based on the grid connection frequency and the rated frequency. When the frequency deviation is less than the preset frequency drop judgment threshold, determine that a grid frequency drop has occurred, start the virtual inertia response, and execute step S3.

[0029] Specifically, the real-time detected grid connection frequency With rated frequency Compare and calculate frequency deviation The calculation formula is: .

[0030] When frequency deviation The frequency drop threshold is less than the preset threshold. When a frequency drop is detected in the power grid, a virtual inertia response is initiated. In this embodiment, the rated frequency... Set the frequency drop threshold to 50 Hz. The setting is -0.05 Hz, meaning that the virtual inertia response is triggered when the grid connection frequency is below 49.95 Hz.

[0031] S3: Virtual inertial response is achieved using closed-loop control based on short-time super-maximum power point active power generation.

[0032] During steady-state operation, the inverter unit uses maximum power point tracking (MPPT) to ensure the photovoltaic (PV) modules operate at their maximum power point. The output characteristics of the PV cell are determined by its current-voltage characteristic curve. Under given irradiance and junction temperature conditions, a PV cell has a unique maximum power point, corresponding to a maximum power point voltage. and maximum power point current At this point, the output power is at its maximum. , In steady-state grid-connected conditions, this embodiment uses closed-loop control to briefly and slightly increase the operating current of the photovoltaic modules, utilizing the transient current margin of the photovoltaic cells to achieve short-term active power generation exceeding the maximum power point, thus providing inertia support for the grid.

[0033] When a drop in grid frequency triggers a virtual inertial response, closed-loop control is used to briefly and slightly increase the operating current of the photovoltaic module (from...). (Referring to the current-voltage characteristic curve, an increase in operating current will lead to a decrease in operating voltage (from...) (Point). The increase in power due to the increase in current is greater than the decrease in power due to the decrease in voltage, so that the instantaneous output power of the photovoltaic module temporarily exceeds the maximum power point power, realizing short-term active power increase above the maximum power point.

[0034] The realization of short-term super-maximum power point (SMP) active power generation relies on the transient current margin of photovoltaic (PV) cells. Transient current margin refers to the ability of a PV cell to safely output additional current above its maximum power point current within a short time window where the junction temperature is approximately constant. The physical basis of this transient current margin lies in the short-circuit current of the PV cell. Current greater than the maximum power point The difference between the two constitutes the available current margin space.

[0035] Based on the characteristic that the junction temperature of a photovoltaic (PV) module is approximately constant within the virtual inertia response window, time-scale decoupling between transient virtual inertia control and steady-state maximum power point tracking (MPPT) control is achieved. The variation in the PV module junction temperature is constrained by both its heat capacity and heat dissipation conditions, resulting in a large thermal time constant. Generally, the thermal time constant of the PV module junction temperature... The time range is 30 to 300 seconds, representing the characteristic time required for the junction temperature to change from one steady-state value to another. The virtual inertia response time window... The duration, ranging from 200 milliseconds to 2 seconds, characterizes the duration during which inertial support is required in a power grid frequency drop event. Because... and There is a difference of about two orders of magnitude between them. Therefore, the junction temperature of the photovoltaic module can be regarded as approximately constant during the virtual inertia response window.

[0036] Based on the condition of approximately constant junction temperature, the current-voltage characteristic curve of the photovoltaic cell remains unchanged, and the position of the maximum power point also remains unchanged within the virtual inertia response window. This means that the short-term super-maximum power point active power generation performed by transient virtual inertia control will not change the operating target point of steady-state maximum power point tracking control, and the two control modes are decoupled on the time scale. Transient virtual inertia control only operates within a short time window of milliseconds to seconds, while steady-state maximum power point tracking control runs continuously on a time scale of seconds or more, and the two do not interfere with each other.

[0037] The specific process of virtual inertial response based on closed-loop control of short-time super-maximum power point active power generation is as follows: S31: Calculate the grid frequency change rate based on the grid connection frequency, and calculate the target value of active power generation required for the virtual inertial response based on the grid frequency change rate. The target value for active power generation is obtained through calculation using the virtual inertia equation: in, This represents the target value for active power generation. This is the virtual inertia coefficient; The rate of change of the power grid frequency deviation; t is time. Virtual inertia coefficient. The value is determined based on the rated capacity of the photovoltaic power generation system and the grid's requirements for inertia support. In this embodiment, for a string photovoltaic power generation system with a rated capacity of 60 kW, The value is set at 50 kW·s / Hz, which is in accordance with the recommended range for the inertial response capability of distributed power sources in IEEE Standard 1547-2018.

[0038] S32: According to Current increment command for calculating the operating current of photovoltaic modules The calculation formula is: in, The maximum power point voltage determined for the current maximum power point tracking control.

[0039] S33: The actual operating current of the photovoltaic module is adjusted according to the current increment command through the current closed-loop controller.

[0040] In this embodiment, the current closed-loop controller uses a proportional-integral controller, which adjusts the inverter's switching duty cycle to make the actual operating current track the target current. The input to the current closed-loop controller is a current increment command. Compared with actual current increment The difference is the output as the duty cycle adjustment of the inverter.

[0041] During the virtual inertia response, the grid connection frequency is measured in real time. When the grid connection frequency recovers to the preset frequency recovery range, the virtual inertia response is considered to have ended, and step S4 is executed. In this embodiment, the frequency recovery range is within -0.03 Hz of the rated frequency, that is, when the grid connection frequency recovers to above 49.97 Hz, the regression process is initiated.

[0042] S4: After the virtual inertia response ends, control the photovoltaic power generation system to seamlessly return to maximum power point tracking steady-state operation.

[0043] The specific process is as follows: the active power generation target value is gradually reduced at a preset slope until the operating point of the photovoltaic module returns to the maximum power point. In this embodiment, the preset slope is set to reduce the active power generation target value by 20% per second, that is, the active power generation target value is linearly reduced from the current value to zero within 5 seconds. The preset slope is set based on the following: an excessively large slope will cause power surges that impact the power grid, while an excessively small slope will prolong the operation time above the maximum power point and increase the risk of heat load on the photovoltaic module. A slope of 20% per second strikes a balance between the two, and this value was obtained through simulation verification. When the active power generation target value is reduced to zero, the operating current of the photovoltaic module returns to the maximum power point current. The inverter unit resumes normal maximum power point tracking control.

[0044] Example 2 In this embodiment, a virtual inertia grid-connected control method for a photovoltaic power generation system is provided. The difference between this embodiment and Embodiment 1 is that the active power increase at the short-term maximum power point is limited.

[0045] Specifically, in step S31, after calculating the target value of active power generation, the maximum limit value of active power generation of the photovoltaic module is calculated. Then, the target value of active power generation is limited using the maximum limit value of active power generation to obtain the limited target value of active power generation. The limited target value of active power generation is used for the current increment command calculation in step S32.

[0046] The formula for calculating the maximum limit of active power generation of photovoltaic modules is as follows: in, This is the maximum limit value for active power generation. This represents the short-circuit current of the photovoltaic module under the current irradiance and junction temperature conditions. This is the maximum power point current; This is the voltage at the maximum power point; The safety margin factor ranges from 0.5 to 0.9. The safety margin factor is determined based on the photovoltaic module's specifications and operating conditions, and its selection is based on the ratio of the maximum allowable operating current to the short-circuit current provided by the photovoltaic module manufacturer. In this embodiment, The value is 0.6.

[0047] use right The formula for applying amplitude limiting is as follows: in, This represents the target value for the increased active power generation after the limit is applied. To minimize the value, this embodiment uses limiting control to ensure that the active power increase exceeding the maximum power point for a short period does not exceed the safe operating range of the photovoltaic module, thus avoiding irreversible damage to the photovoltaic cells caused by overcurrent.

[0048] At this point, the calculation formula for the current increment command in step S32 is modified as follows: .

[0049] Step S4 then gradually reduces the target value of the active power increase after the limit is set by a preset slope until the operating point of the photovoltaic module returns to the maximum power point.

[0050] This embodiment illustrates a specific operating scenario. A string photovoltaic (PV) power generation system has a rated capacity of 60 kW. Under standard test conditions, the PV module's maximum power point voltage (MPPT) is 600 volts, its maximum power point current (MPPT) is 100 amperes, and its short-circuit current is 108 amperes. The thermal time constant of the PV module junction temperature is 120 seconds. During steady-state operation, the inverter unit performs maximum power point tracking (MPPT) using the perturbation-observation method, and the PV module output power is 60 kW.

[0051] At a certain moment, the grid connection frequency begins to drop from 50 Hz, with a frequency change rate of -2 Hz per second. When the frequency detection device detects a frequency deviation exceeding -0.05 Hz, it initiates a virtual inertia response. Based on the virtual inertia equation, the target value for increased active power generation is calculated as follows: = -50 × (-2) = 100 kW. That is, the target value for active power generation is 100 kW. After limiting control, the target value for active power generation is limited to 4.8 kW, and the corresponding current increment command is: .

[0052] The current closed-loop controller increases the operating current of the photovoltaic module from 100 amps to 108 amps. At this point, the operating voltage of the photovoltaic module decreases slightly due to the increased current, dropping to approximately 592 volts. Therefore, the instantaneous output power is approximately... The power output is 63.9 kilowatts, an increase of about 3.9 kilowatts from the maximum power point of 60 kilowatts. This additional power is injected into the grid through the inverter to provide inertia support for the grid.

[0053] After approximately 1.5 seconds of virtual inertia response, the grid connection frequency recovered to above 49.97 Hz, entering the frequency recovery range. At this point, the target value of the limited active power generation (4.8 kW) was gradually reduced at a preset slope of 20% per second. After approximately 5 seconds, the target value of the limited active power generation dropped to zero, and the operating current of the photovoltaic modules smoothly returned to the maximum power point current of 100 amperes, and the system resumed steady-state operation of maximum power point tracking.

[0054] The change in the junction temperature of the photovoltaic module during the entire virtual inertia response process can be estimated as follows. Assuming that all the excess power is converted into additional heat dissipation of the photovoltaic module (the most conservative estimate), the additional heat energy within a 1.5-second response time is... J. Based on a photovoltaic module heat capacity of approximately 3000 joules per degree Celsius, the junction temperature change is approximately... ℃. The thermal time constant of the junction temperature of a photovoltaic module is 120 seconds. Within a 1.5-second response window, the actual change in junction temperature is much smaller than the most conservative estimate mentioned above, and the assumption that the junction temperature is approximately constant holds true.

[0055] Example 3 This embodiment provides a virtual inertia grid-connected control method for a photovoltaic power generation system. The difference between this embodiment and Embodiments 1 and 2 is that the safety boundary is determined based on the magnitude difference between the thermal time constant and the response time window. After determining that a grid frequency drop has occurred, before starting the virtual inertia response, it is determined whether the condition of approximately constant junction temperature is met based on the safety boundary. If it is met, step S3 is executed; if it is not met, an alarm is issued.

[0056] In this embodiment, after detecting a frequency drop, the time scale ratio is first calculated based on the thermal time constant of the photovoltaic module junction temperature and the current virtual inertia response time window, and then compared with a safety ratio threshold. When the time scale ratio is greater than the safety ratio threshold, the assumption of approximately constant junction temperature is considered valid, allowing short-term over-maximum power point (OPP) active power enhancement and initiating the virtual inertia response. When the time scale ratio is less than or equal to the safety ratio threshold, the impact of junction temperature change on the OPP position is considered non-negligible, and short-term OPP active power enhancement is not performed to avoid control instability or device overstress due to operating point offset; in this case, an alarm is issued, requiring relevant personnel intervention.

[0057] The formula for calculating the time scale ratio is: in, This is a time-scale ratio, dimensionless; This is the thermal time constant of the photovoltaic module junction temperature, in seconds; This represents the virtual inertia response time window, measured in seconds. When the value exceeds a preset safety ratio threshold, the assumption that the junction temperature is approximately constant within the response window period is considered valid, allowing short-term super-maximum power point active power generation. In this embodiment, the safety ratio threshold is 15.

[0058] In this embodiment, the relationship between the output power of the photovoltaic module and its junction temperature is quantified using a power-temperature model. The power-temperature model describes the relationship between the maximum power point power of the photovoltaic module and the junction temperature. in, The junction temperature is Maximum power point power at that time; The maximum power point power at the reference temperature; This is the power temperature coefficient, expressed in degrees Celsius. This refers to the actual junction temperature of the photovoltaic module; For reference temperature, 25 degrees Celsius is typically used. In this embodiment, the power temperature coefficient... The temperature range is -0.38% / ℃ to -0.50% / ℃, covering the power temperature characteristics of mainstream crystalline silicon photovoltaic cells. The junction temperature is estimated by placing temperature sensors on the surface of the photovoltaic module backsheet and combining them with the irradiance correction relationship; or it can be estimated based on the linear relationship between the measured open-circuit voltage of the photovoltaic module and the junction temperature.

[0059] The power temperature model in this invention serves the following purpose: During the virtual inertia response window, since the junction temperature is approximately constant, the output value of the power temperature model remains unchanged, thus confirming that the maximum power point position does not drift within the response window. This provides a quantitative basis for the time-scale decoupling of transient virtual inertia control and steady-state maximum power point tracking control. Using a power temperature coefficient of -0.45% / ℃, a reference power of 60 kW, and a junction temperature change of 2℃, the power change at the maximum power point is approximately 540 watts, accounting for only 0.9% of the rated power, and its impact on the maximum power point position is negligible.

[0060] Let's illustrate this with a specific operating scenario. A string photovoltaic (PV) power generation system operates under the following conditions: irradiance of 800 watts per square meter and junction temperature of 55 degrees Celsius. The maximum power point voltage (MPP) is 580 volts, the MPP current is 82 amperes, the short-circuit current is 88 amperes, and the MPP power is 47,560 watts. The thermal time constant of the PV module junction temperature is 90 seconds, and the virtual inertia response time window is 1 second.

[0061] The calculated time scale ratio is The value is greater than the safety ratio threshold of 15, and the assumption of approximately constant junction temperature holds. The maximum limit for active power generation is calculated as follows: watt Assuming the target active power increase calculated from the virtual inertia equation is 3000 watts, the actual active power increase command after limiting is... Watts. The corresponding current increment command is: The operating current of the photovoltaic module has been increased from 82 amps to 85.6 amps, which does not exceed the short-circuit current of 88 amps, ensuring safe operation.

[0062] In another operating scenario, the photovoltaic modules operate under conditions of 1000 watts per square meter of irradiance and a junction temperature of 45 degrees Celsius. The maximum power point voltage is 600 volts, the maximum power point current is 100 amperes, the short-circuit current is 108 amperes, and the maximum power point power is 60,000 watts. The thermal time constant of the photovoltaic module junction temperature is 150 seconds, and the virtual inertia response time window is 0.5 seconds. The timescale ratio is... This is far greater than the safety ratio threshold of 15. The maximum limit for active power generation is: Assuming the active power increase target calculated by the virtual inertia equation is 2000 watts, which does not exceed the maximum limit, the actual active power increase command is 2000 watts. The corresponding current increment command is... The operating current of the photovoltaic module has been increased from 100 amps to 103.33 amps, ensuring safe operation.

[0063] Example 4 In this embodiment, a virtual inertia grid-connected control system for a photovoltaic power generation system is also provided, and the technical solution adopted is as follows: including: The frequency detection module is used to detect the grid connection frequency in real time. The frequency drop detection module is used to calculate the frequency deviation based on the grid connection frequency and the rated frequency. When the frequency deviation is less than the preset frequency drop detection threshold, it determines that a grid frequency drop has occurred. It calculates the time scale ratio based on the thermal time constant of the photovoltaic module junction temperature and the current virtual inertia response time window. When the time scale ratio is greater than the safety ratio threshold, the virtual inertia response is activated. The virtual inertia response module is used to perform virtual inertia response using closed-loop control based on short-time super-maximum power point active power enhancement, including: Calculate the grid frequency change rate based on the grid connection frequency, and then calculate the target value of active power generation required for the virtual inertial response. The target value of active power generation is limited by the maximum limit value of active power generation, and then the current increment command is calculated based on the limited target value of active power generation. The actual operating current of the photovoltaic module is adjusted according to the current increment command by the current closed-loop controller; the actual operating current of the photovoltaic module is temporarily increased by superimposing the operating current increment command to the maximum power point current reference value. During the virtual inertia response, the grid connection frequency is measured in real time. When the grid connection frequency recovers to the preset frequency recovery range, the virtual inertia response is determined to be over. After the virtual inertia response ends, the photovoltaic power generation system is controlled to seamlessly return to maximum power point tracking steady-state operation.

[0064] The specific working process of each module is as follows: This system is applied to photovoltaic power generation systems. Generally, photovoltaic modules are string structures composed of multiple photovoltaic panels connected in series. The DC output terminal of the photovoltaic module is electrically connected to the DC input terminal of the inverter. The inverter controls the operating voltage and current of the photovoltaic module by adjusting the duty cycle of the switching transistors to achieve maximum power point tracking.

[0065] The frequency detection module is deployed at the grid connection point of the photovoltaic power generation system to detect the grid connection frequency in real time and send it to the frequency drop judgment module.

[0066] The frequency drop detection module receives real-time measurements of the grid connection frequency from the frequency detection module. It then calculates the difference between the grid connection frequency and the rated frequency (50 Hz) to obtain the frequency deviation. When the frequency deviation is less than a preset frequency drop detection threshold, a grid frequency drop event is determined to have occurred.

[0067] After determining that a frequency drop has occurred, the frequency drop determination module obtains the junction temperature information (thermal time constant) of the photovoltaic module and the preset value of the current virtual inertia response time window (e.g., 2 seconds), and calculates the time scale ratio. When the time scale ratio is greater than the safety ratio threshold, a start command is sent to the virtual inertia response module to start the virtual inertia response.

[0068] Junction temperature information can be obtained in the following two ways: Direct measurement method: A temperature sensor is installed on the surface of the photovoltaic module's backsheet to measure the backsheet temperature in real time. The junction temperature is then estimated based on an empirical relationship between the backsheet temperature and the junction temperature. The empirical relationship is as follows: in, This refers to the actual junction temperature of the photovoltaic module; The measured value is from the back panel temperature sensor; This is the current irradiance; For reference irradiance; This represents the temperature difference offset between the junction temperature and the backplane temperature under standard test conditions.

[0069] Indirect estimation method: Obtain the open-circuit voltage measurement value of the photovoltaic module through the inverter, and estimate the junction temperature using the linear relationship between the open-circuit voltage and the junction temperature. The linear relationship is as follows: in, This is the actual measured open-circuit voltage; For reference temperature; The open-circuit voltage at the reference temperature; This represents the open-circuit voltage temperature coefficient. This indirect estimation method does not require the installation of additional temperature sensors and is suitable for already deployed and operating photovoltaic power generation systems.

[0070] The corresponding thermal time constant is determined based on the estimated actual junction temperature of the photovoltaic module.

[0071] The virtual inertia response module is connected to the frequency drop judgment module, the frequency detection module and the inverter. After receiving the start command, it uses closed-loop control based on short-time super-maximum power point active power generation to perform virtual inertia response, and determines whether the virtual inertia response has ended based on the real-time grid connection frequency.

[0072] The virtual inertia response module calculates the grid frequency change rate based on multiple samples of the grid connection frequency, and then calculates the target value of active power increase required for the virtual inertia response based on the grid frequency change rate. Next, it calculates the maximum limit value of the active power increase for the photovoltaic modules; the smaller value between the target value and the maximum limit value is used to calculate the current increment command. Finally, the current increment command is sent to the current closed-loop controller.

[0073] The inverter's current closed-loop controller superimposes the incremental operating current command onto the maximum power point current reference value to obtain the target reference current. This enables the actual operating current of the photovoltaic module to quickly track the target reference current, thereby temporarily increasing the actual operating current of the photovoltaic module and achieving short-term active power generation exceeding the maximum power point.

[0074] During the virtual inertia response period, the virtual inertia response module acquires the grid connection frequency measured by the grid connection frequency in real time. When the grid connection frequency recovers to the preset frequency recovery range, the virtual inertia response is determined to end; and the photovoltaic power generation system is controlled to seamlessly return to maximum power point tracking steady-state operation.

[0075] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A virtual inertia grid-connected control method for a photovoltaic power generation system, characterized in that, include: S1: Real-time detection of grid connection frequency; S2: Calculate the frequency deviation based on the grid connection frequency and the rated frequency. When the frequency deviation is less than the frequency drop judgment threshold, it is determined that a grid frequency drop has occurred, and the virtual inertia response is initiated. S3: Virtual inertial response is achieved using closed-loop control based on short-time supermaximum power point active power generation, including: Calculate the grid frequency change rate based on the grid connection frequency, and then calculate the target value of active power generation required for the virtual inertial response. Calculate the current increment command based on the active power generation target value; The actual operating current of the photovoltaic module is adjusted according to the current increment command through a current closed-loop controller. S4: After the virtual inertia response ends, control the photovoltaic power generation system to seamlessly return to maximum power point tracking steady-state operation.

2. The virtual inertia grid-connected control method for a photovoltaic power generation system as described in claim 1, characterized in that, The target value of active power generation is limited by the maximum limit value of active power generation.

3. The virtual inertia grid-connected control method for a photovoltaic power generation system as described in claim 2, characterized in that, The formula for calculating the maximum limit of active power generation is: in, This is the maximum limit value for active power generation. This represents the short-circuit current of the photovoltaic module under the current irradiance and junction temperature conditions. The maximum power point current, The voltage at the maximum power point. This is the safety margin coefficient.

4. The virtual inertia grid-connected control method for a photovoltaic power generation system as described in claim 2, characterized in that, The formula for limiting the amplitude is expressed as follows: in, This represents the target value for the increased active power generation after the limit is applied. To obtain the minimum value, This is the maximum limit value for active power generation. This is the target value for active power generation.

5. The virtual inertia grid-connected control method for a photovoltaic power generation system as described in claim 1, characterized in that, After determining that a grid frequency drop has occurred, the time scale ratio is calculated based on the thermal time constant of the photovoltaic module junction temperature and the current virtual inertia response time window; when the time scale ratio is greater than the safety ratio threshold, the virtual inertia response is initiated.

6. The virtual inertia grid-connected control method for a photovoltaic power generation system as described in claim 1, characterized in that, The time scale ratio is the ratio of the thermal time constant of the photovoltaic module junction temperature to the virtual inertia response time window.

7. The virtual inertia grid-connected control method for a photovoltaic power generation system as described in claim 1, characterized in that, In step S4, the active power generation target value is gradually reduced with a preset slope until the operating point of the photovoltaic module returns to the maximum power point.

8. The virtual inertia grid-connected control method for a photovoltaic power generation system as described in claim 1, characterized in that, During the virtual inertia response, the grid connection frequency is measured in real time. When the grid connection frequency recovers to the preset frequency recovery range, the virtual inertia response is considered to have ended.

9. The virtual inertia grid-connected control method for a photovoltaic power generation system as described in claim 1, characterized in that, In step S3, the current closed-loop controller adopts a proportional-integral controller, whose input is the difference between the current increment command and the actual current increment, and whose output is the duty cycle adjustment of the inverter.

10. A virtual inertia grid-connected control system for a photovoltaic power generation system, characterized in that, A virtual inertia grid-connected control method for a photovoltaic power generation system as described in any one of claims 1 to 9, comprising: The frequency detection module is used to detect the grid connection frequency in real time. The frequency drop determination module is used to calculate the frequency deviation based on the grid connection frequency and the rated frequency. When the frequency deviation is less than the frequency drop determination threshold, it is determined that a grid frequency drop has occurred and the virtual inertia response is initiated. The virtual inertia response module is used to perform virtual inertia response using closed-loop control based on short-time super-maximum power point active power enhancement, including: Calculate the grid frequency change rate based on the grid connection frequency, and then calculate the target value of active power generation required for the virtual inertial response. Calculate the current increment command based on the active power generation target value; The actual operating current of the photovoltaic module is adjusted according to the current increment command through a current closed-loop controller. After the virtual inertia response ends, the photovoltaic power generation system is controlled to seamlessly return to maximum power point tracking steady-state operation.

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