A snow melting method for string photovoltaic inverters

Abstract

The present application discloses a snow melting method for a string-type photovoltaic inverter, comprising the following steps: setting all DC / DC circuits of the inverter as bidirectional flow circuits; upon receiving a snow melting instruction, the inverter determines the photovoltaic strings that have melted and the photovoltaic strings that have not melted according to the current snow-covered state of the photovoltaic strings; determining a snow melting scheme based on the number of photovoltaic strings that have melted; wherein the snow melting scheme is used to indicate that all photovoltaic strings are divided into three types: strings that have melted, strings that are currently melting, and strings that are waiting for snow melting; performing MPPT loop control on the bidirectional flow circuits corresponding to the strings that have melted, performing power limiting loop control on the bidirectional flow circuits corresponding to the strings that are currently melting, and placing the bidirectional flow circuits corresponding to the strings waiting for snow melting in a non-wave state. Beneficial effects of the present application: Compared with traditional solutions, the present application can achieve snow melting of photovoltaic strings without increasing costs and can maximize economic benefits.

Description

Technical Field

[0001] The present application relates to the field of new energy power generation technology, and in particular to a snow melting method for a string-type photovoltaic inverter. Background Art

[0002] Because existing string-type photovoltaic inverters transmit energy in a one-way manner, from photovoltaic panels to the grid, in cold weather, when photovoltaic panels are covered with snow, even if sunlight is restored, the photovoltaic panels need to rely on sunlight to naturally melt the snow. The snow takes a long time to melt, seriously affecting the utilization rate of photovoltaic power generation. Therefore, the existing technology has proposed active snow melting methods for photovoltaic panels. However, existing active snow melting solutions for photovoltaic panels have the following drawbacks:

[0003] (1) An additional snow melting device is required to achieve rapid active snow melting of photovoltaic modules. Since this solution adds an active snow melting device, it will bring additional equipment costs and maintenance costs.

[0004] (2) Active snow melting for centralized photovoltaic panels using power from the grid. However, this solution is only applicable to centralized photovoltaic inverter architectures with bidirectional energy flow capabilities. It cannot achieve active snow melting for string photovoltaic inverters. Moreover, the energy required for snow melting in this method comes entirely from the grid, which is not economical for users' electricity consumption. Summary of the Invention

[0005] One of the objectives of the present application is to provide a snow melting method for a string photovoltaic inverter that can solve at least one of the defects in the above-mentioned background technology.

[0006] In order to achieve at least one of the above-mentioned purposes, the technical solution adopted in the present application is: a snow melting method for a string-type photovoltaic inverter, comprising the following steps: setting all DC / DC circuits of the inverter as bidirectional flow circuits; when receiving a snow melting instruction, the inverter determines the photovoltaic strings that have melted and the photovoltaic strings that have not melted according to the current snow-covered status of the photovoltaic strings; determining a snow melting plan based on the number of photovoltaic strings that have melted; wherein the snow melting plan is used to indicate that all photovoltaic strings are divided into three types: strings that have melted, strings that are currently melting, and strings that are waiting for snow melting; performing MPPT loop control on the bidirectional flow circuits corresponding to the strings that have melted, performing power limiting loop control on the bidirectional flow circuits corresponding to the strings that are currently melting, and placing the bidirectional flow circuits corresponding to the strings that are waiting for snow melting in a non-wave state; obtaining a power increment and a loop selection flag, and switching the power limiting loop control to the MPPT loop control based on different values ​​of the loop selection flag, so as to realize the switching from the strings that are currently melting to the strings that have melted.

[0007] Preferably, the MPPT loop control process of the DC / DC circuit is: set the photovoltaic string voltage value v PV and the current value i PVIt is sent to the MPPT loop, and after passing through the voltage loop and current loop, the switch tube control signal PWM is obtained. DC / DC The power limiting loop control process of the DC / DC circuit is as follows: the PV string voltage value v PV , current value i PV And limit the target power target value P PV_ref The signal is sent to the power limiting loop, and then passes through the voltage loop and current loop to obtain the switch control signal PWM. DC / DC .

[0008] Preferably, the power increment ΔP and the loop selection flag flag are both used as inputs of the power loop; when the loop performs MPPT loop control, the loop selection flag flag = 0, so that the tracking target power is within the interval [P MPPT +ΔP,P MPPT -ΔP]; when the loop performs power limiting loop control, the loop selection flag flag = 1, so that the target power is limited to the interval [P PV_ref +ΔP,P PV_ref -ΔP].

[0009] Preferably, when switching from power limiting loop control to MPPT loop control on the right slope of the PV curve, if the output power P of the PV string is PV >P PV_ref +ΔP, voltage target value v given in loop control PV * =v PV -Δv1+flag×Δv2; if the output power of the photovoltaic string P PV <P PV_ref -ΔP, the voltage target value v given in the loop control PV * =v PV -Δv3-flag×Δv4; wherein Δv1, Δv2, Δv3 and Δv4 all represent positive voltage increments, and Δv2>Δv1.

[0010] Preferably, the diode in the DC / DC circuit is replaced with a switching device with bidirectional conduction capability, so that the DC / DC circuit is configured as a bidirectional flow circuit.

[0011] Preferably, the determination of the snow-covered state of the photovoltaic strings includes the following process: detecting the open-circuit voltage of all photovoltaic strings; if the detected open-circuit voltage is less than a set first threshold value, the photovoltaic string will be determined to be in an unmelted snow state; otherwise, the maximum power tracked during the detection period will be compared with a set second threshold value; if the tracked maximum power is greater than the second threshold value, the photovoltaic string will be determined to be in a melted snow state, otherwise the photovoltaic string will be determined to be in an unmelted snow state.

[0012] Preferably, when the power output capability of the photovoltaic strings is detected, except for the photovoltaic string currently performing the MPPT loop control, the remaining at least one photovoltaic string performs the bus voltage loop control.

[0013] Preferably, the snow melting of the photovoltaic strings is carried out in multiple rounds, and the photovoltaic string types are reclassified in each round of snow melting; in each round of type classification, based on the number of strings that have melted snow, it is determined that the number of strings for this snow melting is equal to the number of strings that have melted snow.

[0014] Preferably, the snow melting of the photovoltaic strings is carried out in multiple rounds, and the number of snow melting rounds of the photovoltaic strings and the time corresponding to each round of snow melting are predicted based on the number of strings that have melted snow; the number of snow melting rounds and the number of photovoltaic strings in each round of snow melting are adjusted according to the electricity sales price during each round of snow melting.

[0015] Preferably, a multi-round snowmelt total benefit prediction model is constructed based on the number of snowmelt rounds and the number of photovoltaic strings in each snowmelt round, and a snowmelt allocation plan with the highest total benefit is obtained by traversing the model algorithm.

[0016] Optimized, multi-round snowmelt total benefit prediction model E total The calculation formula is as follows:

[0017] ;

[0018] Among them, m represents the number of initial snowmelt strings, Q mppt represents the power generation of a single photovoltaic string when working in MPPT mode, k represents the total number of snow melting rounds, E j The unit electricity sales revenue of the j-th round of snow melting process, n j represents the number of photovoltaic strings corresponding to the jth round of snowmelt process, Q loss Indicates the amount of electricity required for snow melting by a single PV string.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] Compared to traditional solutions, this application achieves snowmelt for photovoltaic strings with virtually no additional cost and maximizes economic benefits. It also proposes a loop-switching compatible control method for DC / DC circuits, minimizing the impact of loop switching. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is a schematic diagram of the workflow of this application.

[0022] Figure 2 The figure is a schematic diagram of the hardware circuit structure of an existing string photovoltaic inverter.

[0023] Figure 3Schematic diagram of the inverter hardware circuit structure for the DC / DC circuit modification in this application.

[0024] Figure 4 This is a schematic diagram of the equivalent circuit structure of the photovoltaic string in this application.

[0025] Figure 5 Schematic diagram of the PV curve of the photovoltaic string in this application.

[0026] Figure 6 This is a loop control diagram of the DC / DC circuit and the DC / AC circuit when the inverter in this application is operating normally.

[0027] Figure 7 Schematic diagram of the PV curve of the photovoltaic string in this application when there is no light and low light.

[0028] Figure 8 This is a schematic diagram of the hardware circuit structure of the inverter for melting snow when there is no snow-melting string at the beginning of this application.

[0029] Figure 9 For this application Figure 8 Schematic diagram of the loop control corresponding to the first PV string in FIG.

[0030] Figure 10 This is a schematic diagram of the hardware circuit structure of the inverter with snow-melting strings for snow melting in this application.

[0031] Figure 11 For this application Figure 10 Schematic diagram of loop control corresponding to the first and second PV strings.

[0032] Figure 12 Schematic diagram of the process of self-checking the snow cover status for this application.

[0033] Figure 13 This is a schematic diagram of the loop control when performing self-test of snow-covered status for this application.

[0034] Figure 14 This is a schematic diagram of the PV curve after the power increment is introduced into this application.

[0035] Figure 15 This is a schematic diagram of switching between MPPT loop control and power limiting loop control for this application. DETAILED DESCRIPTION

[0036] Below, the present application is further described in conjunction with specific implementation methods. It should be noted that, in the description of this specification, the reference terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" and the like are intended to mean that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representation of the above terms should not be understood as necessarily referring to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification.

[0037] In the description of this application, it should be noted that for directional words, such as the terms "center", "horizontal", "longitudinal", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise" and so on, indicating the orientation and position relationship are based on the orientation or position relationship shown in the accompanying drawings, which is only for the convenience of describing this application and simplifying the description, and does not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and cannot be understood as limiting the specific scope of protection of this application.

[0038] It should be noted that the terms "first", "second", etc. in the description and claims of this application are used to distinguish similar objects, and are not necessarily used to describe a specific order or sequence.

[0039] In this application, unless otherwise expressly specified or limited, terms such as "mounted," "connected," "connect," and "fixed" should be understood in a broad sense. For example, they may refer to connection, detachable connection, or integration; mechanical connection or electrical connection; direct connection or indirect connection through an intermediate medium; and internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of the above terms in this application based on specific circumstances.

[0040] In this application, unless otherwise expressly specified or limited, a first feature being "above" or "below" a second feature may include the first and second features being in direct contact, or may include the first and second features being in contact not directly but through another feature between them. Moreover, a first feature being "above," "above," and "above" a second feature may include the first feature being directly above or obliquely above the second feature, or may simply mean that the first feature is higher in level than the second feature. A first feature being "below," "below," and "below" a second feature may include the first feature being directly below or obliquely below the second feature, or may simply mean that the first feature is lower in level than the second feature.

[0041] The terms "comprises" and "having" and any variations thereof in the specification and claims of this application are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus that includes a series of steps or units is not necessarily limited to those steps or units expressly listed, but may include other steps or units not expressly listed or inherent to such process, method, product or apparatus.

[0042] One of the preferred embodiments of this application is as follows: Figure 1 and Figure 3 As shown, a snow melting method for a string-type photovoltaic inverter includes the following steps: setting all DC / DC circuits of the inverter as bidirectional flow circuits. When receiving a snow melting instruction, the inverter determines the photovoltaic strings that have melted and the photovoltaic strings that have not melted according to the current snow-covered status of the photovoltaic strings. According to the number of photovoltaic strings that have melted, all photovoltaic strings are divided into three types: snow-melted strings, current snow-melting strings, and strings waiting for snow melting. MPPT loop control is performed on the bidirectional flow circuit corresponding to the snow-melted strings, power limiting loop control is performed on the bidirectional flow circuit corresponding to the current snow-melting strings, and the bidirectional flow circuit corresponding to the waiting snow-melting strings is in a non-wave state. Repeat the above process until all photovoltaic strings have melted or a snow-melting stop instruction is received, and the inverter enters a normal working state. Compared with traditional solutions, the present application can achieve snow melting of photovoltaic strings without basically increasing costs.

[0043] It should be known that if Figure 2The figure below shows the hardware circuit architecture of a conventional string-type photovoltaic inverter. Its structure primarily consists of multiple photovoltaic strings, each connected in parallel to the DC side of a DC / AC circuit via a series DC / DC circuit. The AC side of the DC / AC circuit is connected to the power grid. The multiple photovoltaic strings can be labeled PV#1, PV#2, ..., PV#N; and the DC / DC circuits corresponding to each photovoltaic string can be labeled DC / DC#1, DC / DC#2, ..., DC / DC#N. Each DC / DC circuit utilizes a boost circuit architecture, which involves bidirectionally conducting switching devices connected in parallel between the positive and negative busbars. A diode is connected in series with the positive busbar, rendering the conventional DC / DC circuit unidirectional.

[0044] Since snowmelt from photovoltaic strings requires power from the grid, existing string-type photovoltaic inverters only provide unidirectional energy flow and are unable to draw power from the grid to melt snow from the photovoltaic strings. Therefore, the string-type photovoltaic inverter's circuit structure must be modified to enable bidirectional conduction. There are various specific modification methods for achieving bidirectional conduction in string-type photovoltaic inverters. For ease of understanding, the following will provide a detailed explanation using a specific example.

[0045] Specifically, such as Figure 3 As shown, a traditional DC / DC circuit is transformed from a boost circuit to a buck / boost circuit; that is, the diodes in the traditional DC / DC circuit are replaced with bidirectional switching devices, making the DC / DC circuit a bidirectional circuit. Bidirectional switching devices come in a variety of types, such as IGBTs or MOSFETs, and can be selected based on the needs of those skilled in the art. Since the DC / AC circuit has four-quadrant operation and inherently enables bidirectional energy flow, no modification is required.

[0046] In order to more conveniently understand the technical solution of the present application, the specific working process of the modified string photovoltaic inverter can be described in detail below.

[0047] like Figure 4 The figure shows the equivalent circuit structure of a photovoltaic string. The photovoltaic string is actually a large-area planar diode. sc is the current stimulated by photons in the photovoltaic string, R sh is the bypass resistance, which is generally around 1kΩ; Rs is the internal resistance of the photovoltaic string, which is generally small. According to the equivalent circuit model of the photovoltaic string, the PV curve of the photovoltaic string under light regulation can be calculated as follows: Figure 5 As shown. Figure 5In the PV curve shown, the portion with power greater than 0 represents the PV string's external output power; the portion with power less than 0 represents the external device's input power to the PV string. Therefore, the technical solution of this application can divide the inverter's power control of the PV string into normal operating mode and snowmelt mode based on the operating characteristics of the PV string.

[0048] When the inverter is performing normal power generation, it controls the PV strings to operate in the upper half of the PV curve and track the maximum power point (MPPT). At this time, DC / DC#1 to DC / DC#N all operate in Boost mode to achieve MPPT control of the PV strings; at the same time, the DC / AC circuit operates in Inverter mode to achieve bus voltage control. Figure 6 As shown, the specific control method of the DC / DC circuit and the DC / AC circuit is consistent with that of the conventional string-type photovoltaic inverter. Since the technical solution of the present application is mainly to switch the DC / DC circuit, for the sake of ease of understanding, the loop control process of the DC / DC circuit during normal operation will be briefly described below.

[0049] like Figure 6 As shown in the figure, for the MPPT loop control of the DC / DC circuit, the loop includes the MPPT loop, voltage loop, current loop and PWM generation module. PV and the current value i PV It is sent to the MPPT loop, and then the corresponding voltage target value v can be given by the corresponding MPPT algorithm. PV * . Voltage target value v PV * In the voltage loop, the voltage value v PV By comparison, the corresponding current target value i can be obtained PV * ; Current target value i PV * In the current loop, the current value i PV By comparison, the corresponding duty cycle d can be obtained DC / DC , duty cycle d DC / DC The corresponding control signal PWM can be generated by the PWM generation module DC / DC .

[0050] When the inverter is melting snow, the inverter controls the PV strings to work in the lower half of the PV curve. At this time, the PV strings are blocked by snow and are in no light or low light mode. The corresponding PV curve is as follows: Figure 7 As shown, Figure 7 (1) is the PV curve in the no-light mode. Figure 7 Middle (2) is the PV curve in low light mode.

[0051] It is understandable that when categorizing PV strings into three types, there may be three possible situations: the first situation is that all PV strings are not covered with snow; the second situation is that all PV strings are covered with snow; and the third situation is that some PV strings are not covered with snow.

[0052] It should be noted that for the first case mentioned above, there is no need to melt snow from the photovoltaic strings. For the second case mentioned above, in order to reduce the energy consumption demand on the power grid, electricity can be taken from the power grid to melt snow from some photovoltaic strings, and then the photovoltaic strings that have melted snow can be used to supply power to the photovoltaic strings that have not melted snow to achieve snow melting for all photovoltaic strings. For the third case mentioned above, snow melting can be achieved by directly supplying power to the photovoltaic strings that are covered with snow from the photovoltaic strings that are not covered with snow; of course, if the number of photovoltaic strings that are not covered with snow is small, electricity can also be taken from the power grid to increase the snow melting efficiency of the photovoltaic strings. Since the snow melting process in the second case will go through the third case, the snow melting process in the second case will be described below.

[0053] Specifically, the snow melting process in the second case mentioned above can be divided into two stages. The first stage is the snow melting process without snow-melted strings, and the second stage is the snow melting process with snow-melted strings. For ease of understanding, the specific working processes of the two stages will be described below.

[0054] In the first stage, the snow melting of PV strings can only be achieved by relying on power from the grid. In order to reduce the energy consumption demanded by the grid, some PV strings will be melted by the grid first. The following will take the snow melting of PV strings corresponding to DC / DC#1 as an example. Figure 8 As shown, DC / DC#1 works in Buck mode, controlling the inverter to output target power P to PV string PV#1. PV1_ref To achieve snow melting for PV string PV#1, the loop control of DC / DC#1 is based on the MPPT loop control in the normal power generation state, and only the MPPT loop is adjusted to the power limiting loop.

[0055] like Figure 9 As shown, the specific control process is: set the PV string voltage value v PV , current value i PV And limit the target power target value P PV_ref By sending it into the power limiting loop, the voltage target value v of the photovoltaic string PV#1 can be obtained. PV1 * To track the target power P PV1_ref Then set the voltage value v PV1 and the voltage target value v PV1 * By comparison, the current target value i is obtained through the PI controller PV1* Then the current value i PV1 and the current target value i PV1 * By comparison, the control signal PWM of the switching device in DC / DC#1 is obtained after the PI controller and the PWM generation module. DC / DC#1 At this time, the DC / DC#2~DC / DC#N circuits are not working, and the DC / AC circuit has the same loop control as the normal working state, performing bus voltage control to achieve power balance.

[0056] In the second stage, the PV strings that have melted the snow generate electricity to melt the snow on the PV strings that have not melted the snow. Figure 10 As shown, the following example uses the case where the PV strings connected to the DC / DC #1 circuit that has completed snow melting generate electricity to melt snow for the PV strings connected to the DC / DC #2 circuit. Other cases can be expanded accordingly.

[0057] like Figure 11 As shown, DC / DC circuit #1 operates in Boost mode to achieve MPPT control, and its control is exactly the same as in normal operation. DC / DC circuit #2 operates in Buck mode, performing power limiting loop control. Its control method is the same as that of DC / DC circuit #1 in the first stage, so it will not be repeated here. At this point, the DC / AC circuit loop control is the same as in normal operation, performing bus voltage control to achieve power balancing.

[0058] In this embodiment, when determining the snow-covered status of PV strings, conventional methods for obtaining string snow-covered status information include having maintenance personnel or users notify the inverter via a host computer, or by installing sensors (temperature sensors and / or pressure sensors) at the PV strings to obtain string status. However, these methods increase costs. To minimize the cost of existing inverters, this application allows obtaining string snow-covered status information through inverter self-testing.

[0059] Specifically, such as Figure 12 As shown in the figure, the process of determining the snow-covered state of the photovoltaic strings based on the inverter self-test is as follows: the open circuit voltage v of all photovoltaic strings is PV_oc Perform detection; if the detected open circuit voltage v PV_oc Less than the set first threshold v PV_th , it will be determined that the PV string is in the unmelted snow state; otherwise, the power output capacity of the PV string will be tested, that is, through the detection period T m The maximum power P tracked MPPT With the second threshold value P set th For comparison. If the maximum power P tracked MPPT Greater than the second threshold P th, the PV string will be deemed to be in the snow-melted state, otherwise it will be deemed to be in the snow-unmelted state.

[0060] Understandably, during the initial self-test, the inverter assumes it has not yet reached the required grid-connected power. At this point, the DC / AC circuit is inoperative, and the self-test relies solely on the DC / DC circuit. Considering system power balance, during the power output capability test of the PV strings, in addition to the PV string currently executing MPPT loop control, at least one remaining PV string can execute bus voltage loop control to ensure system power balance.

[0061] For ease of understanding, the following example will be used to illustrate the situation where DC / DC#1 performs MPPT loop control and DC / DC#2 performs bus voltage loop control. Figure 13 As shown in the figure, the specific process of MPPT loop control performed by DC / DC#1 has been described in the previous content, so it will not be repeated here. For the bus voltage loop control of DC / DC#2, the bus voltage v bus and the given bus voltage target value v bus * Send it to the voltage loop for comparison, and you can get the current target value i PV2 * ; Then the current value i PV2 and the current target value i PV2 * The current loop is sent for comparison, and after passing through the PI controller and PWM generation module, the control signal PWM of the switching device in DC / DC#2 is obtained. DC / DC#2 .

[0062] It should be noted that the detection period T m The specific value of can be determined according to the speed of the MPPT algorithm.

[0063] In this embodiment, after determining the snow cover status information of each photovoltaic string, a snow melting scheme can be determined, thereby determining the corresponding loop control method. There are many ways to determine the snow melting scheme. For ease of understanding, the following will be described in detail using two specific examples.

[0064] Example 1: PV strings are melted in multiple rounds, with each round reclassifying the PV strings. During each round, based on the number of melted strings, the number of strings currently melting is determined to be equal to the number of melted strings. The remaining number of strings awaiting melting is calculated as the total number of strings minus the number of strings currently melting minus the number of strings already melting. This increases exponentially. If the calculated number of strings awaiting melting is ≤ 0, the number of strings currently melting is calculated as the total number of strings minus the number of strings already melting. The inverter then stops melting and returns to normal power generation.

[0065] Example 2: Snow melting of PV strings is carried out in multiple rounds. The number of snow melting rounds and the corresponding time of each snow melting round are predicted based on the number of PV strings that have melted. The number of snow melting rounds and the number of PV strings in each snow melting round are adjusted according to the electricity sales price during each snow melting round.

[0066] It is understandable that Example 1 melts snow slightly faster than Example 2. However, Example 1 does not consider electricity prices at different times of day, which may result in Example 1 being slightly less economically efficient than Example 2. Therefore, when determining a snow melting solution, this embodiment prefers Example 2 to maximize user economic benefits.

[0067] It should be known that there are multiple specific schemes for determining the number of snowmelt rounds and the number of photovoltaic strings for each snowmelt round based on the above-mentioned Example 2; for example, the first scheme may be based on the optimal economic benefits of each snowmelt round, and the second scheme may be based on the optimal economic efficiency of all snowmelt rounds.

[0068] Specifically, the optimal economic benefit of each round of snowmelt is considered; that is, the optimal economic benefit of the current round of snowmelt is predicted in order of the rounds to determine the number of photovoltaic strings for each round of snowmelt. The optimal economic efficiency of all snowmelt rounds is considered; that is, a multi-round snowmelt total benefit prediction model based on the number of snowmelt rounds and the number of photovoltaic strings in each round of snowmelt is constructed, and the snowmelt allocation plan with the highest total benefit is obtained by traversing the model algorithm. Taking into account the interference between the benefits of adjacent rounds of snowmelt, and the first snowmelt specific plan does not take into account the interference between the benefits of adjacent rounds of snowmelt, the final total benefit of the first snowmelt specific plan is slightly inferior to that of the second snowmelt specific plan. Therefore, in this embodiment, the second snowmelt specific plan is preferably used.

[0069] For ease of understanding, the following is a multi-round snowmelt total benefit prediction model E for the second snowmelt specific scheme mentioned above: total The construction process is described in detail.

[0070] When determining the current snowmelt plan, it is known that the number of initial snowmelt strings is m, the number of all snowmelt rounds is k, the number of first snowmelts is n1, and the number of remaining k-1 snowmelts is predicted to be n2, n3, ..., n respectively. k The number of snowmelts and the amount of snowmelt each time are adjusted according to the electricity price. The premise of this plan is that during each round of snowmelt, the power generation of a single photovoltaic string working in MPPT mode is approximately a constant value Q mppt The amount of electricity required for snow melting by a single photovoltaic string is approximately a constant value Q loss , then the calculation formula of the total benefit prediction model for each round of snowmelt is as follows:

[0071] .

[0072] Among them, E j The unit electricity sales revenue of the j-th round of snow melting process, n j Indicates the number of PV strings corresponding to the jth round of snowmelt process.

[0073] In this embodiment, in the snowmelt state, the loop controls corresponding to the DC / DC circuit and the DC / AC circuit need to be switched. The switching of the DC / AC circuit is the same as the conventional control method, so it will not be elaborated on here. For the loop control switching of the DC / DC circuit, it is necessary to switch between the power limiting loop control and the MPPT loop control. If two different loop controls are directly set to respectively achieve power limiting operation and MPPT tracking, this will increase the complexity of the loop control and reduce the response speed of the loop switching. Therefore, this embodiment can achieve compatibility between the switching of the MPPT loop control and the power limiting loop control by modifying the algorithm of the conventional MPPT loop control.

[0074] Specifically, such as Figure 14 As shown in the figure, both MPPT control and power limit control are realized by power tracking, and the target values ​​tracked are the maximum power point and the limit target power point. Now the limit target power P PV_ref Set a power increment ΔP to expand the target power point to the interval [P PV_ref +ΔP,P PV_ref -ΔP]. Wherein, the target power is limited to P PV_ref The power increment ΔP is determined according to the specific snow melting status of the PV strings.

[0075] like Figure 15 As shown in the figure, after the power increment ΔP is introduced, the loop selection flag is introduced again; at this time, only the voltage target value v of the photovoltaic string output in the MPPT control algorithm needs to be adjusted. PV *By slightly adjusting the calculation method, the switch between MPPT loop control and power limit loop control can be realized. That is, the power increment ΔP loop selection flag is used as the input of the power loop, so that when the loop performs MPPT loop control, the loop selection flag flag = 0, so that the tracking target power is within the interval [P MPPT +ΔP,P MPPT -ΔP]; when the loop performs power limiting loop control, the loop selection flag flag = 1, so that the target power is limited to the interval [P PV_ref +ΔP,P PV_ref -ΔP].

[0076] Specifically, in the conventional MPPT control algorithm, when the voltage of the photovoltaic string is Figure 13 When the right slope is PV * =v PV -Δv1; Therefore, when performing the compatible setting of MPPT loop control and power limit loop control, the voltage target value v of the photovoltaic string on the left side of the PV curve is maintained. PV * The calculation method of the right slope remains unchanged, and the calculation method of the right slope can be modified as follows:

[0077] If the output power of the PV string P PV >P PV_ref +ΔP, voltage target value v given in loop control PV * =v PV -Δv1+flag×Δv2. If the output power of the photovoltaic string P PV <P PV_ref -ΔP, the voltage target value v given in the loop control PV * =v PV -Δv3-flag×Δv4, where Δv1, Δv2, Δv3, and Δv4 all represent positive voltage increments, and Δv2>Δv1.

[0078] It can be understood that when MPPT loop control is executed, the corresponding limit target power P PV_ref That is the maximum power P MPPT .

[0079] The above describes the basic principles, main features, and advantages of the present application. Those skilled in the art should understand that the present application is not limited to the above-described embodiments. The above-described embodiments and the specification merely illustrate the principles of the present application. Various changes and improvements may be made to the present application without departing from the spirit and scope of the present application. These changes and improvements fall within the scope of the present application for which protection is sought. The scope of protection claimed by the present application is defined by the appended claims and their equivalents.

Claims

1. A snow melting method for a string photovoltaic inverter, characterized in that: The steps include: Set all DC / DC circuits of the inverter to bidirectional flow circuits; When receiving a snow melting instruction, the PV strings whose snow has melted and the PV strings whose snow has not melted are determined according to the current snow cover status of the PV strings; Determine a snowmelt plan based on the number of PV strings that have melted snow. The snowmelt plan is used to indicate the classification of all PV strings into three types: strings that have melted snow, strings that are currently melting snow, and strings waiting for snowmelt. Perform MPPT loop control on the bidirectional flow circuit corresponding to the snow-melting strings, and perform power limit loop control on the bidirectional flow circuit corresponding to the current snow-melting strings, and wait until the bidirectional flow circuit corresponding to the snow-melting strings is in a non-wave state; By modifying the MPPT loop control algorithm, the MPPT loop control and the power limiting loop control can be switched and compatible with each other; Obtain the power increment and loop selection flag, and switch the power limit loop control to the MPPT loop control based on the different values ​​of the loop selection flag, so as to switch the current snow-melting string to the snow-melted string; Snow melting of PV strings is carried out in multiple rounds; Reclassify the PV string types during each round of snowmelt. During each round of classification, based on the number of strings that have already melted, determine that the number of strings in this round of snowmelt is equal to the number of strings that have already melted. Alternatively, the number of snowmelt rounds for the PV strings and the time corresponding to each round of snowmelt are predicted based on the number of initial snowmelt strings; the number of snowmelt rounds and the number of PV strings in each round of snowmelt are adjusted according to the electricity sales price during each round of snowmelt.

2. The snow melting method for a string photovoltaic inverter according to claim 1, wherein: The MPPT loop control process of the DC / DC circuit is as follows: the PV string voltage value v PV and the current value i PV It is sent to the MPPT loop, and after passing through the voltage loop and current loop, the switch tube control signal PWM is obtained. DC / DC ; The power limiting loop control process of the DC / DC circuit is as follows: the PV string voltage value v PV , current value i PV And limit the target power target value P PV_ref The signal is sent to the power limiting loop, and then passes through the voltage loop and current loop to obtain the switch control signal PWM. DC / DC .

3. The snow melting method for a string photovoltaic inverter according to claim 2, wherein: The power increment ΔP and the loop selection flag are both used as inputs of the power loop; When the loop performs MPPT loop control, the loop selection flag flag = 0; When the loop is performing power limiting loop control, the loop selection flag flag = 1, so that the target power is limited in the interval [P PV_ref +ΔP,P PV_ref -ΔP].

4. The snow melting method for a string photovoltaic inverter according to claim 3, wherein: When switching from power limit loop control to MPPT loop control on the right slope of the PV curve, if the output power P of the PV string is PV >P PV_ref +ΔP, voltage target value v given in loop control PV * =v PV -Δv1+flag×Δv2, if the output power of the PV string P PV <P PV_ref -ΔP, the voltage target value v given in the loop control PV * =v PV -Δv3-flag×Δv4; Δv1, Δv2, Δv3 and Δv4 all represent positive voltage increments, and Δv2>Δv1.

5. The snow melting method for a string photovoltaic inverter according to claim 1, wherein: The diode in the DC / DC circuit is replaced with a switching device with bidirectional conduction capability, so that the DC / DC circuit is set as a bidirectional flow circuit.

6. The snow melting method for a string photovoltaic inverter according to claim 1, wherein: The determination of the snow-covered status of PV strings includes the following process: Detect the open circuit voltage of all photovoltaic strings; If the detected open-circuit voltage is less than the set first threshold, the PV string will be deemed to be in an unmelted snow state; otherwise, the maximum power tracked during the detection period will be compared with the set second threshold; If the tracked maximum power is greater than the second threshold, the photovoltaic string is determined to be in the snow-melted state; otherwise, the photovoltaic string is determined to be in the snow-unmelted state.

7. The snow melting method for a string photovoltaic inverter according to claim 6, wherein: When the power output capability of the photovoltaic strings is detected, at least one photovoltaic string other than the photovoltaic string currently performing the MPPT loop control performs the bus voltage loop control.

8. The snow melting method for a string photovoltaic inverter according to claim 1, wherein: A multi-round snowmelt total benefit prediction model is constructed based on the number of snowmelt rounds and the number of photovoltaic strings in each snowmelt round. The snowmelt allocation plan with the highest total benefit is obtained through the traversal model algorithm.

9. The snow melting method for a string photovoltaic inverter according to claim 8, characterized in that: The predicted total benefit E of multiple rounds of snowmelt total The calculation formula is as follows: ； Among them, m represents the number of initial snowmelt strings, Q mppt represents the power generation of a single photovoltaic string when working in MPPT mode, k represents the total number of snow melting rounds, E j The unit electricity sales revenue of the j-th round of snow melting process, n j represents the number of photovoltaic strings corresponding to the jth round of snowmelt process, Q loss Indicates the amount of electricity required for snow melting by a single PV string.

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