Phase-cut control method and string inverter

Abstract

The application provides a phase-cut control method and a string photovoltaic inverter. The phase-cut control method comprises the following steps: when the inverter is in a target working mode, determining a contribution degree parameter value of a DC / DC conversion branch based on the heat loss of the DC / DC conversion branch, and the target working mode comprises a light load or a limited power running mode; determining a target conversion branch, and the target conversion branch is a DC / DC conversion branch with the minimum contribution degree parameter value in a first set, and the first set comprises conversion branches in a running state in the DC / DC conversion branches; and performing a phase-cut operation on the target conversion branch. The technical scheme provided by the application can improve the phase-cut effect and the running stability of the system.

Description

Technical Field

[0001] This invention relates to the field of inverters, and in particular to a phase-cutting control method and a string photovoltaic inverter. Background Technology

[0002] String photovoltaic inverters typically have a two-stage topology consisting of multiple front-end DC / DC conversion branches and a rear-end DC / AC conversion circuit. The multiple DC / DC conversion branches achieve maximum power point tracking (MPPT), and their outputs are connected in parallel to a common DC bus. The outputs are then inverted into AC by the rear-end DC / AC conversion circuit and fed into the grid.

[0003] To improve system operating efficiency and grid-connected power quality, the industry has proposed a phase shedding control strategy, which involves actively disconnecting some parallel DC / DC converter branches under light load or power-limited conditions, so that the load rate of the remaining DC / DC converter branches can be increased to a high-efficiency range, thereby reducing the overall system loss.

[0004] However, the existing phase-cutting control schemes have poor phase-cutting performance. Summary of the Invention

[0005] This application provides a phase-cutting control method and a string photovoltaic inverter.

[0006] In a first aspect, this application provides a phase-cutting control method applied to a string photovoltaic inverter, the inverter including multiple DC / DC conversion branches, the method comprising: when the inverter is in a target operating mode, determining the contribution parameter value of the DC / DC conversion branch based on the heat loss of the DC / DC conversion branch, the target operating mode including light load or power-limited operation mode; determining a target conversion branch, the target conversion branch being the DC / DC conversion branch with the smallest contribution parameter value in a first set, the first set including the conversion branches in operation among the DC / DC conversion branches; and performing a phase-cutting operation on the target conversion branch.

[0007] In conjunction with the first aspect, in one possible implementation, the heat loss includes switching losses and / or conduction losses.

[0008] In conjunction with the first aspect, in one possible implementation, performing a phase-cutting operation on the target conversion branch includes: performing a temporary disconnection operation on the target conversion branch; detecting whether the operating status parameters of the inverter are within a target range, the operating status parameters including the bus voltage; if the operating status parameters exceed the target range, restoring the target conversion branch and stopping the phase-cutting operation; if the operating status parameters are within the target range, confirming the disconnection of the target conversion branch; wherein, after confirming the disconnection of the target conversion branch, the method further includes: updating the first set and the target conversion branch; performing the phase-cutting operation on the updated target conversion branch until the operating status parameters exceed the target range.

[0009] In conjunction with the first aspect, in one possible implementation, the method is executed periodically in units of control cycles. The method further includes: obtaining the contribution parameter value of the target transformation branch within N consecutive control cycles, where N is a positive integer greater than 1; the temporary removal operation on the target transformation branch includes: determining whether the contribution parameter value of the target transformation branch is within a preset removable range within M consecutive control cycles, where the M consecutive control cycles are included within the N consecutive control cycles; if so, the temporary removal operation is performed on the target transformation branch.

[0010] In conjunction with the first aspect, in one possible implementation, performing a temporary cut-off operation on the target transformation branch includes: determining whether the change in the contribution parameter value of the target transformation branch within a first preset time period is less than or equal to a first preset value; if so, performing the temporary cut-off operation on the target transformation branch.

[0011] In conjunction with the first aspect, in one possible implementation, the method is executed periodically in units of control cycles. The method further includes: generating a first sequence in ascending order of the contribution parameter values; the temporary removal operation on the target transformation branch includes: if the index of the target transformation branch in the first sequence is less than or equal to a second preset value for Q consecutive control cycles, and / or the number of changes of the index in W consecutive control cycles is less than or equal to a third preset value, then the temporary removal operation is performed on the target transformation branch, where Q and W are positive integers greater than 1.

[0012] In conjunction with the first aspect, in one possible implementation, detecting whether the inverter's operating status parameters are within the target range includes: after a second preset time delay from the completion time of the temporary disconnection operation, detecting whether the inverter's operating status parameters are within the target range.

[0013] In conjunction with the first aspect, in one possible implementation, the method further includes: when the inverter exits the target operating mode, sequentially restoring the DC / DC conversion branches that exited the operating state due to the execution of the phase-cutting operation, according to the contribution parameter values ​​in descending order.

[0014] In conjunction with the first aspect, in one possible implementation, the method is executed periodically in units of control cycles, and the method further includes: obtaining the contribution parameter value of each DC / DC converter branch within multiple control cycles; when the inverter exits the target operating mode, based on the order of the change in the contribution parameter value from large to small, sequentially restoring the DC / DC converter branches that exited the operating state due to the execution of the phase-cutting operation.

[0015] Secondly, this application also provides a string photovoltaic inverter, including: multiple DC / DC conversion branches and a controller, the controller being used to execute the phase-cutting control method as described in the first aspect or any possible implementation of the first aspect.

[0016] The technical solution provided in this application determines the contribution parameter value of each DC / DC conversion branch based on the heat loss of each branch when the inverter is in the target operating mode. It prioritizes cutting off the branch with the smallest contribution parameter value and ensures the safety of each cutting operation through a closed-loop verification mechanism of the bus voltage. If the bus voltage remains within the target range after cutting off a branch, the next branch is cut off; if the bus voltage exceeds the target range, the cut-off branch is restored, thus balancing system heat loss minimization and operational reliability. Simultaneously, an anti-oscillation control mechanism ensures the stability of phase-cutting decisions under fluctuating operating conditions. When exiting the target operating mode, the cut-off branches are restored according to the magnitude of the contribution parameter value or the change in value, allowing for a smooth transition between full-power operation and the target operating mode. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of a string photovoltaic inverter;

[0018] Figure 2 A schematic flowchart illustrating a phase-cutting control method provided in this application; Figure 3 A schematic flowchart illustrating another phase-cutting control method provided in this application. Detailed Implementation

[0019] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0020] Figure 1 This is a schematic diagram of a string photovoltaic inverter. The inverter employs a two-stage topology with multiple front-stage direct current-to-direct current (DC / DC) conversion branches and a rear-stage DC / alternating current (DC / AC) conversion circuit. The front-stage DC / DC conversion branches can utilize a boost topology, providing independent maximum power point tracking (MPPT) capability. The power switches in the DC / DC conversion branches can be silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs) to support higher switching frequencies and lower switching losses.

[0021] like Figure 1 As shown, the string photovoltaic inverter includes: P independent DC / DC conversion branches, each DC / DC conversion branch input corresponding to one photovoltaic string, and the outputs of all DC / DC conversion branches connected in parallel to the DC bus; the DC side of the subsequent DC-AC conversion circuit is connected to the DC bus, and the AC side is connected to the power grid, used to convert the electrical energy of the DC bus into power frequency AC energy to achieve grid-connected operation. In some embodiments, the input of each DC / DC conversion branch can correspond to multiple photovoltaic strings.

[0022] Each preceding DC / DC converter branch corresponds to a DC / DC controller, such that the i-th DC / DC converter branch corresponds to the i-th DC / DC controller. i is a positive integer less than or equal to P.

[0023] Each DC / DC controller supports two operating modes: Maximum Power Point Tracking (MPPT) mode and Bus Voltage mode. MPPT mode controls the maximum output power of the corresponding photovoltaic string; Bus Voltage mode controls the bus voltage, limiting the photovoltaic output power. For example, the DC / DC controller can sample the current ipv_i, voltage upv_i, and DC bus voltage Udc of the corresponding photovoltaic string, calculate and output the drive duty cycle di of the power switches in the corresponding DC / DC converter branch, thereby regulating the branch's operating state.

[0024] When an inverter operates under light load or power-limited conditions, keeping all DC / DC converter branches running continuously can lead to high system heat loss and low efficiency under light load. Therefore, the industry commonly employs a phase-shedding control strategy: in light load or power-limited operating modes, some DC / DC converter branches are actively disconnected, concentrating the load on the remaining DC / DC converter branches to increase their load factor and allow them to operate in a more efficient load range, thereby reducing overall system losses.

[0025] However, the phase-cutting effect and robustness of existing phase-cutting control schemes are insufficient to meet the operational requirements of various scenarios. For example, most existing phase-cutting control schemes rely on electrical parameters such as load current and output power, or offline loss models based on device parameters, as the basis for branch switching decisions, without incorporating the real-time thermal state of the branch power devices into the decision-making dimension. Especially for high-frequency boost branches using wide-bandgap devices such as silicon carbide (SiC) metal-oxide-semiconductor field-effect transistors (MOSFETs), the switching losses of SiC devices are much lower than those of silicon-based devices, supporting higher switching frequencies; however, under high-frequency operating conditions, the ratio of switching losses to conduction losses exhibits stronger nonlinear characteristics with load levels and device junction temperatures. It is difficult to accurately determine the actual loss contribution of each branch based solely on electrical parameters such as load current or voltage, resulting in inaccurate phase-cutting decisions. Furthermore, existing phase-cutting control schemes generally adopt open-loop control strategies, cutting off branches only based on preset thresholds, lacking a closed-loop verification mechanism for the system state after cutting off. If the DC bus voltage becomes unstable after a branch is disconnected, or if the output power of the remaining branch cannot meet the power-limited operation target, it will affect the safe and stable operation of the system.

[0026] In view of this, this application provides a phase-cutting control method and a string photovoltaic inverter. The technical solution of this application can be applied to string photovoltaic inverters operating under light load or power-limited scenarios. In the technical solution provided by this application, the contribution parameter value is calculated based on the heat loss of the DC / DC conversion branch. Since heat loss can directly quantify the true contribution of each conversion branch to the system, a better phase-cutting decision can be achieved. Furthermore, the solution of this application introduces a closed-loop verification mechanism during phase-cutting operation, which can further improve the phase-cutting effect and the operational stability of the system.

[0027] The following is combined with Figure 2 and Figure 3 This application provides a detailed description of the technical solution provided.

[0028] Figure 2 A schematic flowchart illustrating a phase-cutting control method provided in this application. Figure 2 As shown, the method includes S210, S220 and S230.

[0029] As an example, the method is executed by a control unit, which may be located within the inverter or independently of the inverter; this application is not limited in this regard. The controller may be a hardware and / or software module. For example, the controller may include one or more processors and a storage medium storing a computer program. When the computer program is executed by the processor, the various steps of the phase-cutting control method described below are implemented.

[0030] S210, when the inverter is in the target operating mode, determines the contribution parameter value of the DC / DC conversion branch based on the heat loss of the DC / DC conversion branch. The target operating mode includes light load or power-limited operation mode.

[0031] Light load operation mode refers to an optimized operating mode adopted by the inverter when its output power is less than or equal to a certain percentage of its rated output power, in order to reduce losses, standby power consumption, and noise. For example, if the actual output power is less than or equal to 30% of the rated output power, light load operation mode can be used.

[0032] Power-limited operation mode refers to a working mode in which the inverter's output power is limited to below its maximum output power according to external dispatch instructions or system control requirements. In some embodiments, power-limited operation mode may also be referred to as power reserve operation mode.

[0033] In this application, the controller can determine whether the inverter is in the target operating mode by detecting the ratio of the inverter's current output power to its rated power, or by receiving a power limiting command issued by the grid dispatch center. If the inverter is in the target operating mode, the contribution parameter values ​​of each DC / DC conversion branch are further determined. If the inverter is not in the target operating mode, it maintains normal MPPT mode operation.

[0034] In this application, the contribution parameter value is related to the heat loss of the DC / DC converter branch. For example, the contribution parameter value can be directly taken from the calculated heat loss value of the DC / DC converter branch.

[0035] Optionally, heat loss may include switching loss and / or conduction loss. When heat loss includes both switching loss and conduction loss, the following relationship exists between heat loss, switching loss, and conduction loss:

[0036] Among them, P loss For heat loss, P sw For switching losses, P cond This refers to conduction loss.

[0037] Taking wide-bandgap devices such as silicon carbide MOSFETs as an example, the switching loss of one DC / DC converter branch satisfies the following relationship:

[0038] Among them, E on The turn-on loss of the power switching transistors in the DC / DC converter branch is expressed in joules (E). off The turn-off loss of the power switch in the DC / DC converter branch is expressed in joules (f). sw E represents the switching frequency of the power transistors in the DC / DC converter branch, measured in Hertz (Hz). on and E off The measured drain-source voltage Vds, on-state current Id, and junction temperature T of the power switch transistor can be used as the basis for this calculation. j The switching loss curve can be determined by consulting the device datasheet or by using an online fitting algorithm.

[0039] Taking wide-bandgap devices such as silicon carbide MOSFETs as an example, the switching loss of one DC / DC converter branch satisfies the following relationship:

[0040] In the above formula, I rms R is the effective value of the output current of the DC / DC converter branch, in amperes. ds For the power switch in the DC / DC converter branch at the current junction temperature T j The on-resistance is measured in ohms. The on-resistance increases with increasing junction temperature, and its temperature characteristic can be corrected for by the temperature coefficient in the device datasheet. Optionally, a temperature detection module can be installed on the DC / DC converter branch to detect the device junction temperature.

[0041] Optionally, the controller can collect the operating parameters of each DC / DC converter branch, including but not limited to the output current, drain-source voltage, conduction current, switching frequency and junction temperature of each DC / DC converter branch, and calculate the contribution parameter value of each DC / DC converter branch based on the above operating parameters.

[0042] S220, determine the target conversion branch, which is the DC / DC conversion branch with the smallest contribution parameter value in the first set. The first set includes the DC / DC conversion branches that are in operation.

[0043] In this application, after calculating the contribution parameter values ​​of each DC / DC converter branch, the DC / DC converter branch with the smallest contribution parameter value is identified as the target converter branch, and it is preferentially disconnected. Since the branch with the smallest contribution parameter value contributes the least to the system's heat loss, disconnecting this branch has the least impact on the overall power output of the system, thereby improving the system's operational stability.

[0044] It should be noted that, considering that the contribution parameter of a DC / DC converter branch is zero when it is in a closed or offline state, the target converter branch is determined from the DC / DC converter branches that are in operation.

[0045] S230 performs a phase-cutting operation on the target transformation branch.

[0046] Phase-cutting operations include: stopping the output of drive signals to the power switches in the target conversion branch, causing the power switches in the target conversion branch to stop switching, thereby disconnecting the target conversion branch from the power conversion circuit; or, disconnecting the target conversion branch from the DC bus through a bypass switch, making it electrically isolated from the DC bus.

[0047] This application quantifies the true contribution of each DC / DC converter branch to system heat generation based on the heat loss of each branch, thereby enabling better phase-cutting decisions based on the nonlinear coupling characteristics of switching and conduction losses varying with operating conditions. It should be noted that in actual operation, due to differences in input power, device aging, and heat dissipation conditions among the DC / DC converter branches, even if the output current of each branch is the same, their heat losses may differ due to variations in the composition of switching losses or the temperature coefficient of the conduction resistance. Therefore, compared to using single electrical parameters such as current and voltage to determine the contribution of each branch, the contribution calculated in this application is more accurate.

[0048] This application eliminates the DC / DC converter branch with the smallest system contribution parameter value, allowing the remaining branches to operate in a higher efficiency range. This effectively reduces unnecessary switching and conduction losses, and lowers the overall system heat loss and heat dissipation burden.

[0049] Figure 3 A schematic flowchart illustrating another phase-cutting control method provided in this application. Figure 3 Yes Figure 2 Further details and explanations of the Chinese solution. For example... Figure 3 As shown, the method includes steps S310 to S350.

[0050] S310, when the inverter is in the target operating mode, determines the contribution parameter value of the DC / DC conversion branch based on the heat loss of the DC / DC conversion branch. The target operating mode includes light load or power-limited operation mode.

[0051] In this application, the specific implementation of S310 can be found in S210, and will not be repeated here.

[0052] Optionally, to mitigate the problem of reduced system stability caused by frequent switching of phase-cutting decisions due to factors such as rapid changes in illumination, the phase-cutting control method provided in this application can be executed periodically in units of control cycles. The control cycle can be set according to actual needs, and this application does not impose specific limitations on it.

[0053] For example, the controller can periodically calculate the contribution parameter value on a control cycle basis. Within each control cycle, operating parameters of each DC / DC converter branch are collected, and the contribution parameter value of each DC / DC converter branch is calculated based on these operating parameters.

[0054] S320, Determine the target conversion branch. The target conversion branch is the DC / DC conversion branch with the smallest contribution parameter value in the first set. The first set includes the DC / DC conversion branches that are in operation.

[0055] In this application, the specific implementation of S320 can be found in S220, and will not be repeated here.

[0056] Optionally, after calculating the contribution parameter values ​​of each DC / DC conversion branch, the controller can sort the branches in ascending or descending order of the contribution parameter values ​​and select the branch with the smallest contribution parameter value as the target conversion branch.

[0057] S330, perform a temporary cut-off operation on the target transformation branch.

[0058] In this embodiment, before performing a temporary cutoff operation, the stability of the target transformation branch can be assessed based on an anti-oscillation control mechanism. The temporary cutoff operation is only performed on the target transformation branch when the stability condition is met, thereby improving system stability. This approach avoids erroneous cutoffs caused by instantaneous illumination fluctuations and improves the robustness of phase-cutting decisions.

[0059] In one possible approach, the contribution parameter value of the target transformation branch within N consecutive control cycles can be obtained, where N is a positive integer greater than 1, and it can be determined whether the contribution parameter value of the target transformation branch is within a preset removable range within M consecutive control cycles, where the M consecutive control cycles are included within the N consecutive control cycles.

[0060] If the contribution parameter value of the target transformation branch is within the preset removable range for M consecutive control cycles, a temporary removal operation is performed on the target transformation branch. The specific implementation of the temporary removal operation can be found in S230 and will not be repeated here. For example, if N is 5 and M is 3, that is, if the contribution parameter value of the target transformation branch is within the preset removable range for 3 consecutive control cycles within 5 control cycles, then the state of the target switching branch is confirmed to be stable, and a temporary removal operation can be performed.

[0061] The preset resectable range can be set according to actual working conditions and is not specifically limited here. For example, the resectable range can be set when the contribution parameter value is below a certain threshold.

[0062] In one possible approach, the stability of the target switching branch can also be evaluated based on the change in the contribution parameter value of the target switching branch. For example, it can be determined whether the change in the contribution parameter value of the target switching branch within a first preset time period is less than or equal to a first preset value. If so, a temporary cutoff operation is performed on the target switching branch.

[0063] In this implementation, if the contribution parameter value of the target transformation branch changes very little over a period of time, it indicates that the operating state of the target transformation branch is relatively stable and a temporary cut-off operation can be performed.

[0064] The first preset value can be set according to the system's stability requirements, and is not specifically limited here. For example, the first preset value can be 5% of the contribution parameter value.

[0065] In one possible implementation, after the controller sorts the contribution parameters of each DC / DC conversion branch, the stability of the target conversion branch can also be determined based on the sequence number of the target conversion branch.

[0066] In this implementation, a first sequence can be generated according to the contribution parameter values ​​in ascending order, with each DC / DC conversion branch in the first sequence corresponding to a sequence number. For example, the first sequence S = {s1, s2, ..., sn}. Here, s1 is the branch with the smallest contribution, which can be identified as the target conversion branch. sn is the branch with the largest contribution.

[0067] If the index of the target transformation branch in the first sequence is less than or equal to the second preset value for Q consecutive control cycles, and / or the number of changes in the index of the target transformation branch in the first sequence for W consecutive control cycles is less than or equal to the third preset value, then a temporary cutoff operation is performed on the target transformation branch, where Q and W are positive integers greater than 1. The second and third preset values ​​can be set according to actual needs and are not restricted here.

[0068] In this embodiment, through the above-mentioned anti-oscillation mechanism, the stability and reliability of phase switching decisions can still be guaranteed under the conditions of rapid changes in photovoltaic illumination and large fluctuations in the output power of each DC / DC conversion branch, thus avoiding additional losses and switching stress caused by frequent branch switching in and out.

[0069] S340 checks whether the inverter's operating status parameters are within the target range.

[0070] In this application, after performing a temporary disconnection operation on the target conversion branch, it is necessary to detect whether the inverter's operating status parameters are within the target range, thereby verifying the impact of the temporary disconnection operation on the system's operational stability.

[0071] Optionally, operating status parameters include bus voltage. The target range can be determined based on the inverter's rated bus voltage and its permissible fluctuation range. For example, the target range could be ±1% of the rated bus voltage.

[0072] If the bus voltage is within the target range, the target transformer branch is confirmed to be disconnected; if the bus voltage exceeds the target range, the target transformer branch is restored and the phase-switching operation is stopped. Stopping the phase-switching operation can be understood as stopping the phase-switching operation within this control cycle. Considering that the target transformer branch is the branch with the smallest contribution parameter value, if the disconnection operation of the target transformer branch causes a large fluctuation in the bus voltage, it indicates that the target transformer branch is not disconnectable. Correspondingly, the remaining branches with contribution parameter values ​​greater than those of the target transformer branch cannot be disconnected either.

[0073] Optionally, restoring the target conversion branch includes: outputting a drive signal to the power switch in the target conversion branch, or disconnecting the bypass switch to connect the target conversion branch to the DC bus.

[0074] This application adds a closed-loop verification mechanism to verify the inverter's operating status parameters after the disconnection operation is performed, thereby reducing the impact of the disconnection operation on the stable operation of the system.

[0075] In one possible approach, the inverter's bus voltage can be detected after a second preset time delay from the completion of the temporary disconnection operation of the target switching branch, thereby reducing the impact of the transient process of the bus voltage on the detection results and improving the detection accuracy.

[0076] The second preset duration can be set according to actual needs and is not specifically limited here. For example, the second preset duration can be 100 milliseconds.

[0077] S350, update the first set and the target transformation branch.

[0078] In this application, after the target transformation branch is confirmed to be cut, the first set and the target transformation branch can be updated. For example, the target transformation branch that has undergone the cut operation is removed from the first set to obtain an updated first set, and the branch with the smallest contribution parameter value in the updated first set is determined as the new target transformation branch.

[0079] Optionally, after generating the first sequence S={s1,s2,...,sn} in ascending order of contribution parameter values, if the target transformation branch s1 is removed, branch s2 can be directly identified as the new target transformation branch.

[0080] In this application, after determining the new target conversion branch, S330 and S340 can be executed on the new target conversion branch until the inverter's operating state parameters exceed the target range.

[0081] The phase-cutting control method and string photovoltaic inverter provided in this application determine the contribution parameter value of each DC / DC conversion branch based on the heat loss of each branch when the inverter is in the target operating mode. The method prioritizes cutting off the branch with the smallest contribution parameter value and ensures the safety of each cutting operation through a closed-loop verification mechanism of the bus voltage. If the bus voltage remains within the target range after cutting off a branch, the next branch is cut off; if the bus voltage exceeds the target range, the cut-off branch is restored, thus balancing system heat loss minimization and operational reliability. Simultaneously, an anti-oscillation control mechanism ensures the stability of the phase-cutting decision under fluctuating operating conditions.

[0082] In one possible approach, when the inverter exits the target operating mode, the DC / DC conversion branch that exited the operating state due to the phase-switching operation can be restored until the system returns to the full branch operating state or the output power meets the requirements, thereby achieving a smooth transition between full-power operation and the target operating mode.

[0083] Optionally, DC / DC converter branches that have undergone phase-switching operations are restored sequentially according to their contribution parameter values, from largest to smallest. Prioritizing the restoration of the disconnected branches with the largest contribution parameter values ​​allows for the rapid improvement of the system's power output capability.

[0084] Optionally, when the phase-switching control method is executed periodically in units of control cycles, if the inverter exits the target operating mode, the contribution parameter values ​​of each DC / DC converter branch within multiple control cycles can be obtained. Based on the order of the magnitude of change in the contribution parameter values ​​from largest to smallest, the DC / DC converter branches that have undergone phase-switching operations are restored sequentially. Considering that DC / DC converter branches with large changes in contribution parameter values ​​have a greater impact on the stable operation of the system, the branches with the largest changes in contribution parameter values ​​within the control cycle can be restored first to quickly restore the system's output power balance.

[0085] The term "multiple" in this document refers to two or more. The character " / " generally indicates an "or" relationship between related objects; in formulas, " / " indicates a "division" relationship between related objects. Furthermore, it should be understood that in the description of this application, terms such as "first" and "second" are used only for descriptive purposes and should not be construed as indicating or implying relative importance or order.

[0086] It is understood that, in the embodiments of this application, the order of the above-mentioned process numbers does not imply the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of this application.

[0087] It should be emphasized that the above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A phase-cutting control method applied to a string photovoltaic inverter, the inverter comprising multiple DC / DC conversion branches, characterized in that, The method includes: When the inverter is in the target operating mode, the contribution parameter value of the DC / DC conversion branch is determined based on the heat loss of the DC / DC conversion branch. The target operating mode includes light load or power-limited operation mode. The target conversion branch is determined as the DC / DC conversion branch with the smallest contribution parameter value in the first set, and the first set includes the DC / DC conversion branches that are in operation. A phase-cutting operation is performed on the target transformation branch.

2. The phase-cutting control method according to claim 1, characterized in that, The heat loss includes switching losses and / or conduction losses.

3. The phase-cutting control method according to claim 1 or 2, characterized in that, The phase-cutting operation performed on the target transform branch includes: Perform a temporary cut-off operation on the target transformation branch; Detect whether the operating status parameters of the inverter are within the target range, the operating status parameters including the bus voltage; If the operating status parameters exceed the target range, then the target transformation branch is restored and the phase-cutting operation is stopped; If the operating status parameters are within the target range, then it is confirmed that the target transformation branch will be cut off; The method further includes, after confirming the removal of the target transformation branch: Update the first set and the target transformation branch; The phase-cutting operation is performed on the updated target transformation branch until the operating state parameters exceed the target range.

4. The phase-cutting control method according to claim 3, characterized in that, The method is executed periodically in units of control cycles, and the method further includes: Obtain the contribution parameter value of the target transformation branch within N consecutive control cycles, where N is a positive integer greater than 1; The temporary cut-off operation on the target transformation branch includes: Determine whether the contribution parameter value of the target transformation branch is within a preset removable range within M consecutive control cycles, wherein the M consecutive control cycles are included within the N consecutive control cycles; If so, the temporary cut-off operation is performed on the target transformation branch.

5. The phase-cutting control method according to claim 3, characterized in that, The temporary cut-off operation on the target transformation branch includes: Determine whether the change in the contribution parameter value of the target transformation branch within a first preset time period is less than or equal to a first preset value; If so, the temporary cut-off operation is performed on the target transformation branch.

6. The phase-cutting control method according to claim 3, characterized in that, The method is executed periodically in units of control cycles, and the method further includes: The first sequence is generated according to the contribution parameter values ​​in ascending order; The temporary cut-off operation on the target transformation branch includes: If the index of the target transformation branch in the first sequence is less than or equal to a second preset value for Q consecutive control cycles, and / or the number of changes of the index within W consecutive control cycles is less than or equal to a third preset value, then the temporary cut-off operation is performed on the target transformation branch, where Q and W are positive integers greater than 1.

7. The phase-cutting control method according to claim 3, characterized in that, The detection of whether the inverter's operating status parameters are within the target range includes: After a second preset time delay from the completion of the temporary disconnection operation, it is detected whether the operating status parameters of the inverter are within the target range.

8. The phase-cutting control method according to claim 1, characterized in that, The method further includes: When the inverter exits the target operating mode, the DC / DC conversion branches that exited the operating state due to the phase-cutting operation are restored in descending order of the contribution parameter values.

9. The phase-cutting control method according to claim 1, characterized in that, The method is executed periodically in units of control cycles, and the method further includes: Obtain the contribution parameter value of each DC / DC converter branch within multiple control cycles; When the inverter exits the target operating mode, the DC / DC conversion branches that exited the operating state due to the phase-cutting operation are restored sequentially based on the change in the contribution parameter value from largest to smallest.

10. A string photovoltaic inverter, characterized in that, include: Multiple DC / DC conversion branches and a controller, the controller being used to execute the phase-cutting control method as described in any one of claims 1 to 9.

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