Snow melting method for string type photovoltaic inverter

Abstract

The invention discloses a snow melting method for a string type photovoltaic inverter. The snow melting method comprises the following steps: setting all DC / DC circuits of the inverter as bidirectional flow circuits; when a snow melting instruction is received, the inverter determines the photovoltaic string with snow melting and the photovoltaic string without snow melting according to the snow covering state of the current photovoltaic string; determining a snow melting scheme based on the number of the photovoltaic strings subjected to snow melting; wherein the snow melting scheme is used for indicating that all the photovoltaic group strings are divided into three types, namely a snow-melted group string, a current snow melting group string and a snow-waiting group string; mPPT loop control is executed on the two-way flow circuit corresponding to the snow melting string, power limiting loop control is executed on the two-way flow circuit corresponding to the snow melting string, and the two-way flow circuit corresponding to the snow melting string is waited to be in a non-wave-sending state. The beneficial effects of the invention are that compared with a traditional scheme, the method can achieve the snow melting of the photovoltaic string basically without increasing the cost, and can achieve the maximization of economic benefits.

Description

Technical Field

[0001] This application relates to the technical field of new energy power generation, and particularly to a snow melting method for a string-type photovoltaic inverter. Background Art

[0002] Since the energy of the existing string-type photovoltaic inverter is unidirectionally transmitted from the photovoltaic to the power grid, in cold weather, when the photovoltaic modules are covered with snow, even when the light is restored, the photovoltaic modules need to rely on natural snow melting by light, and it takes a long time for the snow to melt, seriously affecting the utilization rate of photovoltaic power generation. Therefore, the prior art has proposed an active snow melting method for photovoltaic modules. However, the existing active snow melting schemes for photovoltaic modules have the following defects: (1) An additional snow melting device is required to achieve rapid active snow melting of the photovoltaic modules through the additional snow melting device. Since this scheme adds an active snow melting device, it will bring additional equipment costs and maintenance costs.

[0003] (2) The power grid is used to supply power for active snow melting of centralized photovoltaic modules, but this scheme is only applicable to the centralized photovoltaic inverter architecture with the ability of bidirectional energy flow, and cannot achieve active snow melting of string-type photovoltaic inverters. Moreover, the energy required for snow melting by this method completely comes from the power grid, which does not meet the user's electricity economy. Summary of the Invention

[0004] One of the purposes of this application is to provide a snow melting method for a string-type photovoltaic inverter that can solve at least one of the defects in the above background art.

[0005] To achieve at least one of the above purposes, the technical solution adopted in this application is: 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 been snow melted and the photovoltaic strings that have not been snow melted according to the snow covering state of the current photovoltaic strings; determining a snow melting plan based on the number of photovoltaic strings that have been snow melted; where the snow melting plan is used to indicate that all photovoltaic strings are divided into three types: snow-melted strings, strings to be snow melted this time, and strings waiting to be snow melted; performing MPPT loop control on the bidirectional flow circuits corresponding to the snow-melted strings, performing power limit loop control on the bidirectional flow circuits corresponding to the strings to be snow melted this time, and the bidirectional flow circuits corresponding to the strings waiting to be snow melted are in a non-wave-sending state; obtaining a power increment and a loop selection flag bit, and switching the power limit loop control to MPPT loop control based on different values of the loop selection flag bit to realize the switching of the strings to be snow melted this time to the snow-melted strings.

[0006] Preferably, the MPPT loop control process of the DC / DC circuit is: taking the photovoltaic string voltage value v PV and the current value i PVFeed into the MPPT loop, and obtain the switching tube control signal PWM after passing through the voltage loop and the current loop DC / DC ; The power limit loop control process of the DC / DC circuit is as follows: Feed the photovoltaic string voltage value v PV , current value i PV and the limit target power target value P PV_ref into the power limit loop, and obtain the switching tube control signal PWM after passing through the voltage loop and the current loop DC / DC .

[0007] Preferably, both the power increment ΔP and the loop selection flag bit flag are used as inputs to the power loop; when the loop performs MPPT loop control, the loop selection flag bit flag = 0, so that the tracking target power is in the interval [P MPPT +ΔP, P MPPT -ΔP] for tracking; when the loop performs power limit loop control, the loop selection flag bit flag = 1, so that the limit target power is in the interval [P PV_ref +ΔP, P PV_ref -ΔP] for tracking.

[0008] Preferably, 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 photovoltaic string PV > P PV_ref +ΔP, the given voltage target value v in the loop control PV * = v PV -Δv 1 + flag×Δv 2 ; if the output power P of the photovoltaic string PV < P PV_ref -ΔP, the given voltage target value v in the loop control PV * = v PV -Δv 3 - flag×Δv 4 ; where, Δv 1 , Δv 2 , Δv 3 and Δv 4 all represent positive voltage increments, and Δv 2 > Δv 1 .

[0009] Preferably, replace the diode in the DC / DC circuit with a switching device having bidirectional conduction ability, so that the DC / DC circuit is set as a bidirectional flow circuit.

[0010] Preferably, the determination of the snow-covered state of the photovoltaic string includes the following process: detecting the open-circuit voltage of all photovoltaic strings; if the detected open-circuit voltage is less than the set first threshold, it is determined that the photovoltaic string is in the non-melted snow state; otherwise, comparing the maximum power tracked during the detection period with the set second threshold; if the tracked maximum power is greater than the second threshold, it is determined that the photovoltaic string is in the melted snow state, otherwise it is determined that the photovoltaic string is in the non-melted snow state.

[0011] Preferably, when detecting the power output capacity of the photovoltaic string, except for the photovoltaic string currently performing the MPPT loop control, at least one of the remaining photovoltaic strings performs the bus voltage loop control.

[0012] Preferably, the snow melting of the photovoltaic string is carried out in multiple rounds, and the type of the photovoltaic string is re-divided during each round of snow melting; during the process of each round of type division, based on the number of the already melted snow strings, it is determined that the number of the snow melting strings in this round is equal to the number of the already melted snow strings.

[0013] Preferably, the snow melting of the photovoltaic string is carried out in multiple rounds, predicting the number of snow melting rounds of the photovoltaic string and the corresponding time for each round of snow melting based on the number of the already melted snow strings; adjusting the number of snow melting rounds and the number of photovoltaic strings for each round of snow melting according to the electricity selling price during each round of snow melting.

[0014] Preferably, a multi-round snow melting total revenue prediction model regarding the number of snow melting rounds and the number of photovoltaic strings for each round of snow melting is constructed, and the snow melting allocation scheme with the highest total revenue is obtained by traversing the model algorithm.

[0015] Preferably, the multi-round snow melting total revenue prediction model E total has the following calculation formula: ; wherein, m represents the number of the initially melted snow strings, Q mppt represents the power generation amount when a single-path photovoltaic string operates in the MPPT mode, k represents the total number of snow melting rounds, E j represents the unit electricity selling revenue during the j-th round of snow melting process, n j represents the number of photovoltaic strings corresponding to the j-th round of snow melting process, Q loss represents the electricity amount required for a single-path photovoltaic string to melt snow.

[0016] Compared with the prior art, the beneficial effects of the present application are as follows: Compared with the traditional scheme, the present application can realize the snow melting of the photovoltaic string with basically no additional cost, and can maximize the economic benefits. At the same time, a control method compatible with the loop switching of the DC / DC circuit is proposed, which can minimize the influence of the loop switching. Description of the Drawings

[0017] Figure 1 Schematic diagram of the workflow of this application.

[0018] Figure 2 Schematic diagram of the hardware circuit structure of an existing string-type photovoltaic inverter.

[0019] Figure 3 Schematic diagram of the hardware circuit structure of the inverter after the DC / DC circuit of this application is modified.

[0020] Figure 4 Schematic diagram of the equivalent circuit structure of the photovoltaic string in this application.

[0021] Figure 5 Schematic diagram of the PV curve of the photovoltaic string in this application.

[0022] Figure 6 Schematic diagram of the loop control of the DC / DC circuit and the DC / AC circuit when the inverter in this application is working properly.

[0023] Figure 7 Schematic diagram of the PV curve of the photovoltaic string when there is no light or low light in this application.

[0024] Figure 8 Schematic diagram of the hardware circuit structure of the inverter for melting snow on the initial non-melted snow photovoltaic string in this application.

[0025] Figure 9 This application Figure 8 Schematic diagram of the loop control corresponding to the first photovoltaic string.

[0026] Figure 10 Schematic diagram of the hardware circuit structure of the inverter for melting snow on the photovoltaic string with melted snow in this application.

[0027] Figure 11 This application Figure 10 Schematic diagram of the loop control corresponding to the first photovoltaic string and the second photovoltaic string.

[0028] Figure 12 Schematic diagram of the process for self-checking the snow-covered state in this application.

[0029] Figure 13 Schematic diagram of the loop control when self-checking the snow-covered state in this application.

[0030] Figure 14 Schematic diagram of the PV curve after introducing power increment in this application.

[0031] Figure 15 Schematic diagram of the switching between MPPT loop control and power limit loop control in this application. Detailed implementation manners

[0032] Next, in combination with specific embodiments, the present application will be further described. It should be noted that in the description of this specification, the descriptions with reference to terms such as "one embodiment", "some embodiments", "example", "specific example", or "some examples" mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic descriptions 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 can be combined in a suitable manner in any one or more embodiments or examples. In addition, those skilled in the art can combine and combine the different embodiments or examples described in this specification.

[0033] In the description of the present application, it should be noted that for orientation terms, if there are terms such as "center", "horizontal", "longitudinal", "length", "width", "thickness", "upper", "lower", "front", "rear", "left", "right", "vertical", "horizontal", "top", "bottom", "inner", "outer", "clockwise", "counterclockwise", etc., indicating the orientation and position relationship is based on the orientation or position relationship shown in the drawings. It is only for the convenience of describing the present application and simplifying the description, rather than indicating or implying that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and should not be understood as limiting the specific protection scope of the present application.

[0034] It should be noted that the terms "first", "second", etc. in the description and claims of the present application are used to distinguish similar objects and do not necessarily have to be used to describe a specific order or sequence.

[0035] In the present application, unless otherwise clearly specified and defined, terms such as "install", "connect", "connect", "fix", etc. should be understood in a broad sense. For example, it can be a connection, a detachable connection, or integrated; it can be a mechanical connection or an electrical connection; it can be directly connected or indirectly connected through an intermediate medium, and it can be the communication inside two elements or the interaction relationship between two elements. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0036] In this application, unless otherwise clearly specified and defined, the first feature being "on" or "under" the second feature may include direct contact between the first and second features, or may also include the situation where the first and second features are not in direct contact but in contact through additional features therebetween. Moreover, the first feature being "above", "over" and "on top of" the second feature includes the first feature being directly above and obliquely above the second feature, or simply indicating that the horizontal height of the first feature is higher than that of the second feature. The first feature being "under", "below" and "beneath" the second feature includes the first feature being directly below and obliquely below the second feature, or simply indicating that the horizontal height of the first feature is lower than that of the second feature.

[0037] The terms "comprise" and "have" and any variations thereof in the description and claims of this application are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that comprises a series of steps or units does not necessarily have to be limited to those steps or units clearly listed, but may include other steps or units not clearly listed or inherent to these processes, methods, products or devices.

[0038] One preferred embodiment of this application, as Figure 1 and Figure 3 shown, is a method for melting snow on a string-type photovoltaic inverter, which 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 been melted and the photovoltaic strings that have not been melted according to the snow-covered state of the current photovoltaic strings. All photovoltaic strings are divided into three types: melted strings, strings to be melted this time, and strings waiting to be melted according to the number of melted photovoltaic strings. Perform MPPT loop control on the bidirectional flow circuit corresponding to the melted strings, perform power limit loop control on the bidirectional flow circuit corresponding to the strings to be melted this time, and the bidirectional flow circuit corresponding to the strings waiting to be melted is in a non-wave-sending state. Repeat the above process until all photovoltaic strings have been melted or a stop snow melting instruction is received, and the inverter enters the normal working state. Compared with the traditional scheme, this application can achieve snow melting on photovoltaic strings with basically no additional cost.

[0039] It should be known that, as Figure 2As shown in the figure, it is a schematic diagram of the hardware circuit architecture of an existing string-type photovoltaic inverter; its structure mainly includes multiple photovoltaic strings. Each photovoltaic string is connected in parallel to the DC side of the DC / AC circuit through a series DC / DC circuit, and the AC side of the DC / AC circuit is connected to the power grid. The multiple photovoltaic strings can be labeled as PV#1, PV#2, ……, PV#N; the DC / DC circuits corresponding to each photovoltaic string can be correspondingly labeled as DC / DC#1, DC / DC#2, ……, DC / DC#N. The architecture of each DC / DC circuit adopts a Boost circuit; that is, a bidirectional conduction switch device is connected in parallel between the positive and negative busbars, and a diode is connected in series on the positive busbar at the same time, so that the existing DC / DC circuit only has the ability of unidirectional conduction.

[0040] Since the snow melting of the photovoltaic string needs to draw power from the power grid, and the energy of the existing string-type photovoltaic inverter can only flow unidirectionally, it is impossible to obtain electrical energy from the power grid to realize the snow melting of the photovoltaic string. Therefore, it is necessary to first transform the circuit structure of the string-type photovoltaic inverter so that the string-type photovoltaic inverter has the ability of bidirectional conduction. There are various specific transformation methods to realize the bidirectional conduction of the string-type photovoltaic inverter. For the convenience of understanding, a specific example will be described in detail below.

[0041] Specifically, as Figure 3 shown, the traditional DC / DC circuit is transformed from a Boost circuit to a Buck / Boost circuit; that is, the diode in the traditional DC / DC circuit is replaced with a switch device with bidirectional conduction ability, so that the DC / DC circuit is set as a bidirectional flow circuit. There are various specific types of switch devices with bidirectional conduction ability, such as IGBT or MOSFET, etc., and specific selection can be made according to the actual needs of those skilled in the art. Since the DC / AC circuit has the ability to operate in four quadrants and can realize the bidirectional flow of energy by itself, there is no need to make any changes.

[0042] For a more convenient understanding of the technical solution of this application, the specific working process of the transformed string-type photovoltaic inverter will be described in detail below.

[0043] As Figure 4 shown, it is a schematic diagram of the equivalent circuit structure of the photovoltaic string. The photovoltaic string is actually a large-area planar diode. i sc is the current excited by photons in the photovoltaic string, R sh is the shunt resistance, generally about 1 kΩ; Rs is the internal resistance of the photovoltaic string, and its value is generally small. According to the equivalent circuit model of the photovoltaic string, the PV curve of the photovoltaic string under light adjustment can be calculated as Figure 5 shown. At Figure 5In the PV curve shown, the part where the power is greater than 0 represents the power output of the photovoltaic string to the outside; the part where the power is less than 0 represents the power input of the external device to the photovoltaic string. Therefore, the technical solution of this application can divide the power control of the inverter for the photovoltaic string into a normal working state and a snow melting state according to the working characteristics of the photovoltaic string.

[0044] When the inverter is operating in normal power generation, the inverter controls the photovoltaic string to operate in the upper half of the PV curve and tracks the maximum power point MPPT. At this time, all of DC / DC#1 to DC / DC#N operate in the Boost mode to achieve the MPPT control of the photovoltaic string; at the same time, the DC / AC circuit operates in the inversion mode to achieve the bus voltage control. As Figure 6 shown, the specific control methods of the DC / DC circuit and the DC / AC circuit are the same as those of the conventional string-type photovoltaic inverter; since the technical solution of this application mainly performs switching control on the DC / DC circuit, for the convenience of understanding, the loop control process of the DC / DC circuit during normal operation will be briefly described below.

[0045] As Figure 6 shown, for the MPPT loop control of the DC / DC circuit, the loop includes an MPPT loop, a voltage loop, a current loop, and a PWM generation module. The voltage value v PV and the current value i PV output by the photovoltaic string are sent into the MPPT loop, and then the corresponding voltage target value v PV * can be given through the corresponding MPPT algorithm. The voltage target value v PV * is compared with the voltage value v PV in the voltage loop to obtain the corresponding current target value i PV * ; the current target value i PV * is compared with the current value i PV in the current loop to obtain the corresponding duty cycle d DC / DC , and the duty cycle d DC / DC generates the corresponding control signal PWM DC / DC through the PWM generation module.

[0046] When the inverter is performing snow melting, the inverter controls the photovoltaic string to operate in the lower half of the PV curve. At this time, the photovoltaic string is blocked by snow and is in a no-light or low-light mode. The corresponding PV curve is as Figure 7 shown, Figure 7 in which (1) is the PV curve in the no-light mode, Figure 7 and (2) is the PV curve in the low-light mode.

[0047] It can be understood that when classifying the three types of photovoltaic strings, the following three situations may exist. The first situation is that all photovoltaic strings are in a non-snow-covered state; the second situation is that all photovoltaic strings are in a snow-covered state; the third situation is that some photovoltaic strings are in a non-snow-covered state.

[0048] It should be known that for the first situation above, there is no need to melt the snow on the photovoltaic strings. For the second situation above, in order to reduce the power consumption demand on the power grid, power can be taken from the power grid first to melt the snow on some photovoltaic strings, and then the melted photovoltaic strings can supply power to the non-melted photovoltaic strings to achieve the melting of all photovoltaic strings. For the third situation above, the snow can be melted directly by the non-snow-covered photovoltaic strings supplying power to the snow-covered photovoltaic strings; of course, if the number of non-snow-covered photovoltaic strings is small, power 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 situation will go through the third situation, the snow melting process in the second situation will be described below.

[0049] Specifically, the snow melting process in the second situation above can be divided into two stages. The first stage is the snow melting process without melted strings, and the second stage is the snow melting process with melted strings; for the convenience of understanding, the specific working processes of the two stages will be described below.

[0050] In the first stage, only power supply from the power grid can be relied on to melt the snow on the photovoltaic strings; in order to reduce the power consumption demand on the power grid, the power grid will first melt the snow on some photovoltaic strings. Below, the snow melting of the photovoltaic strings corresponding to DC / DC#1 will be used as an example for illustration. As Figure 8 shown, DC / DC#1 operates in the Buck mode, and the inverter is controlled to output the target power P to the photovoltaic string PV#1 PV1_ref to achieve the snow melting of the photovoltaic string PV#1. At this time, 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 needs to be adjusted to a power limit loop.

[0051] As Figure 9 shown, the specific control process is as follows: the voltage value v of the photovoltaic string PV , the current value i PV and the target value P of the limited target power PV_ref are sent into the power limit loop, and the voltage target value v of the photovoltaic string PV#1 can be obtained PV1 * to track the target power P PV1_ref . Then the voltage value v PV1 is compared with the voltage target value v PV1 * , and the current target value i is obtained through the PI controller PV1* Then, the current value i PV1 is compared with the current target value i PV1 * After passing through the PI controller and the PWM generation module, the control signal PWM of the switching device in DC / DC#1 is obtained DC / DC#1 At this time, the DC / DC#2~DC / DC#N circuits do not work, and the DC / AC circuit has the same loop control as the normal working state, and bus voltage control is performed to achieve power balance.

[0052] In the second stage, the photovoltaic strings that have been de - snowed generate electricity to de - snow some of the photovoltaic strings that have not been de - snowed. As Figure 10 shown, taking the photovoltaic strings connected to the DC / DC#1 circuit that has completed de - snowing as an example to generate electricity for the photovoltaic strings connected to the DC / DC#2 circuit to de - snow, other situations can be extended according to this.

[0053] As Figure 11 shown, where the DC / DC#1 circuit operates in Boost mode to achieve MPPT control, and its control is exactly the same as the normal working state. The DC / DC#2 circuit operates in Buck mode and performs power - limit loop control. Its control method is the same as that of the DC / DC#1 circuit in the first stage, so it will not be described repeatedly here. At this time, the DC / AC circuit has the same loop control as the normal working state, and bus voltage control is performed to achieve power balance.

[0054] In this embodiment, when determining the snow - covered state of the photovoltaic strings, for the conventional acquisition of the snow - covered state information of the strings, it can be informed to the inverter by the maintenance personnel or the user using the upper computer, or the string state can be obtained by setting sensors (temperature sensors and / or pressure sensors) at the photovoltaic strings. However, these methods will all lead to an increase in cost. In order not to increase the cost on the basis of the existing inverter, this application can obtain the snow - covered state information of the strings through the self - inspection of the inverter.

[0055] Specifically, as Figure 12 shown, the process of determining the snow - covered state of the photovoltaic strings based on the self - inspection of the inverter is as follows: Detect the open - circuit voltage v PV_oc of all photovoltaic strings; If the detected open - circuit voltage v PV_oc is less than the set first threshold v PV_th , it is determined that the photovoltaic string is in the non - de - snowed state; Otherwise, the power output ability of the photovoltaic string will be detected, that is, by comparing the maximum power P m tracked within the detection period T MPPT with the set second threshold P th . If the tracked maximum power P MPPT is greater than the second threshold P th, it will be determined that the photovoltaic string is in the snow-melted state; otherwise, it will be determined that the photovoltaic string is in the non-snow-melted state.

[0056] It can be understood that when the self-check is first performed, it is defaulted that the inverter does not reach the power required for grid connection. At this time, the DC / AC circuit does not work, and only the DC / DC circuit can be relied on for self-check. Then, considering the power balance of the system, when detecting the power output ability of the photovoltaic string, in addition to the photovoltaic string currently performing the MPPT loop control, at least one other photovoltaic string can perform the bus voltage loop control to ensure that the power of the system can be kept balanced.

[0057] For the convenience of understanding, the following will take DC / DC#1 performing the MPPT loop control and DC / DC#2 performing the bus voltage loop control as an example for illustration. As Figure 13 shown, the specific process of the MPPT loop control performed by DC / DC#1 has been described in the foregoing 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 * are sent to the voltage loop for comparison, and the current target value i PV2 * can be obtained; then the current value i PV2 and the current target value i PV2 * are sent to the current loop for comparison. After passing through the PI controller and the PWM generation module, the control signal PWM DC / DC#2 of the switching device in DC / DC#2 is obtained.

[0058] It should be noted that the specific value of the detection period T m can be determined according to the speed of the MPPT algorithm.

[0059] In this embodiment, after determining the snow-covering state information of each photovoltaic string, the snow-melting scheme can be determined, and thus the corresponding loop control method can be determined. There are various ways to determine the snow-melting scheme. For the convenience of understanding, the following will be described in detail through two specific examples.

[0060] Example 1: The snow melting of the photovoltaic string is carried out in multiple rounds, and the type of the photovoltaic string is re-divided during each round of snow melting; during the process of each round of type division, based on the number of strings with snow melted, determine the number of strings to be melted in this round = the number of strings with snow melted. The remaining is the number of strings waiting to be melted = the total number of all strings - the number of strings to be melted in this round - the number of strings with snow melted, so that the number of strings to be melted will increase exponentially. When the calculated result of the number of strings waiting to be melted ≤ 0, then the number of strings to be melted in this round = the total number of all strings - the number of strings with snow melted, and the inverter stops snow melting and enters the normal power generation state.

[0061] Example 2: The snow melting of the photovoltaic string is carried out in multiple rounds, predict the number of snow melting rounds of the photovoltaic string and the corresponding time for each round of snow melting based on the number of strings with snow melted; adjust the number of snow melting rounds and the number of photovoltaic strings for each round of snow melting according to the electricity selling price at the time of each round of snow melting.

[0062] It can be understood that the snow melting speed of the above Example 1 is slightly faster than that of Example 2 above. However, the solution of Example 1 does not consider the electricity price situation in different time periods when in use, which may result in the economic benefit of the user of Example 1 being slightly worse than that of Example 2 above. Therefore, when determining the snow melting scheme, this embodiment preferably adopts Example 2 above in order to maximize the economic benefit of the user.

[0063] It should be known that there are various specific schemes for determining the number of snow melting rounds and the number of photovoltaic strings for each round of snow melting based on Example 2 above; for example, the first one can be considered based on the best economic benefit of each round of snow melting, and the second one can be considered based on the best economic efficiency of all snow melting rounds.

[0064] Specifically, considering the best economic benefit of each round of snow melting; that is, in the order of rounds, predict the current round's best economic benefit of snow melting for each round to determine the number of photovoltaic strings to be melted in each round. Considering the best economic efficiency of all snow melting rounds; that is, construct a multi-round snow melting total revenue prediction model regarding the number of snow melting rounds and the number of photovoltaic strings for each round of snow melting, and obtain the snow melting allocation scheme with the highest total revenue through traversing the model algorithm. Considering that there is interference between the revenues of adjacent rounds of snow melting, and the first specific snow melting scheme does not consider the revenue interference of adjacent rounds of snow melting, making the final total revenue of the first specific snow melting scheme slightly inferior to that of the second specific snow melting scheme. Therefore, in this embodiment, the second one above is preferably adopted for the specific snow melting scheme.

[0065] For the convenience of understanding, the construction process of the multi-round snow melting total revenue prediction model E total of the second specific snow melting scheme above can be described in detail below.

[0066] When determining the current snow melting scheme, it is known that the number of initially melted string groups is m, the total number of snow melting rounds is k, and the initial snow melting quantity is n 1 , and it is predicted that the snow melting quantities for the remaining k - 1 snow melting rounds are respectively n 2 、n 3 、...、n k . The number of snow melting times and the snow melting quantity each time are adjusted according to the electricity selling price. The prerequisite that this scheme needs to clarify is that during each round of snow melting process, the power generation of a single - path photovoltaic string working in the MPPT mode is approximately a constant value Q mppt , and the power required for snow melting of a single - path photovoltaic string is approximately a constant value Q loss . Then, the calculation formula of the total revenue prediction model for each round of snow melting is as follows: .

[0067] Among them, E j represents the revenue from selling electricity per unit of electricity during the j - th round of snow melting process, and n j represents the number of photovoltaic strings corresponding to the j - th round of snow melting process.

[0068] In this embodiment, in the snow melting state, the loop control corresponding to the DC / DC circuit and the DC / AC circuit needs to be switched. The switching of the DC / AC circuit is the same as the conventional control method, so it will not be elaborated in detail here. For the loop control switching of the DC / DC circuit, it is necessary to switch between the power - limited loop control and the MPPT loop control. If two different loop controls are directly set to achieve power - limited operation and MPPT tracking respectively, this will increase the complexity of the loop control and reduce the response speed of the loop switching. Therefore, in this embodiment, by modifying the algorithm of the conventional MPPT loop control, the mutual switching between the MPPT loop control and the power - limited loop control can be achieved and made compatible.

[0069] Specifically, as Figure 14 shown, for both the MPPT control and the power - limited control, they are both achieved through power tracking, and the target values of tracking are the maximum power point and the limited target power point respectively. Now, a power increment ΔP is set for the limited target power P PV_ref , so that the limited target power point is extended to the interval [P PV_ref +ΔP, P PV_ref -ΔP]. Among them, the limited target power P PV_ref and the power increment ΔP are determined according to the specific snow melting state of the photovoltaic string.

[0070] As Figure 15 shown, after introducing the power increment ΔP, a loop selection flag bit 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 processed PV* By slightly adjusting the calculation method, the switching between the MPPT loop control and the power limit loop control can be achieved. That is, the power increment ΔP loop selection flag bit flag is used as the input of the power loop. Thus, when the loop performs MPPT loop control, the loop selection flag bit flag = 0, so that the tracking target power is within the interval [P MPPT +ΔP, P MPPT -ΔP] for tracking; when the loop performs power limit loop control, the loop selection flag bit flag = 1, so that the restricted target power is within the interval [P PV_ref +ΔP, P PV_ref -ΔP] for tracking.

[0071] Specifically, in the conventional MPPT control algorithm, when the voltage of the photovoltaic string is at the Figure 13 right slope in, there is v PV * =v PV -Δv 1 ; therefore, when making the compatible setting of the MPPT loop control and the power limit loop control, keep the calculation method of the voltage target value v PV * on the left side of the PV curve of the photovoltaic string unchanged, and the calculation method of the right slope can be modified as follows: If the output power P PV >P PV_ref +ΔP, the given voltage target value v PV * =v PV -Δv 1 +flag×Δv 2 . If the output power P PV <P PV_ref -ΔP, the given voltage target value v PV * =v PV -Δv 3 -flag×Δv 4 . Among them, Δv 1 , Δv 2 , Δv 3 and Δv 4 all represent positive voltage increments, and Δv 2 >Δv 1 .

[0072] It can be understood that when performing MPPT loop control, the corresponding restricted target power P PV_ref is the maximum power P MPPT .

[0073] The basic principles, main features and advantages of the present application have been described above. Those skilled in the art should understand that the present application is not limited by the above embodiments. What is described in the above embodiments and the specification is only the principle of the present application. Without departing from the spirit and scope of the present application, there will be various changes and improvements to the present application, and these changes and improvements all fall within the scope of the present application claimed. The scope of protection required 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 as bidirectional flow circuits; When receiving a snow melting instruction, determining the photovoltaic strings whose snow has melted and the photovoltaic strings whose snow has not melted according to the current snow covering state of the photovoltaic strings; Determine a snowmelt plan based on the number of PV strings that have melted snow; wherein the snowmelt plan is used to indicate that all PV strings are divided into three types: strings that have melted snow, strings that are currently melting snow, and strings that are waiting for snowmelt; Perform MPPT loop control on the bidirectional flow circuit corresponding to the snow-melting string, perform power limit loop control on the bidirectional flow circuit corresponding to the current snow-melting string, and wait for the bidirectional flow circuit corresponding to the snow-melting string to be in a non-wave state; The power increment and loop selection flag are obtained, and based on different values ​​of the loop selection flag, the power limit loop control is switched to the MPPT loop control to realize the switching from the current snow-melting string to the snow-melted string.

2. The snow melting method for a string photovoltaic inverter according to claim 1, characterized in that: The MPPT loop control process of the DC / DC circuit is as follows: 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: Set 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 the switch control signal PWM is obtained after passing through the voltage loop and current loop. DC / DC .

3. The snow melting method for a string photovoltaic inverter according to claim 2, characterized in that: 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 performs power limiting loop control, the loop selection flag bit 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, characterized in that: 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 photovoltaic string is P PV <P PV_ref -ΔP, voltage target value v given in loop control PV * =v PV -Δv3-flag×Δv4; Among them, 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, characterized in that: 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, characterized in that: The determination of the snow-covered state 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 PV string is deemed to be in a snow-melted state; otherwise, the PV string is deemed to be in an unmelted state.

7. The snow melting method for a string photovoltaic inverter according to claim 6, characterized in that: When the power output capacity of the photovoltaic strings is detected, at least one photovoltaic string other than the photovoltaic string currently executing the MPPT loop control executes the bus voltage loop control.

8. The snow melting method for a string photovoltaic inverter according to any one of claims 1 to 7, characterized in that: Snow melting of PV strings is carried out in multiple rounds; Reclassify the types of photovoltaic strings in each round of snowmelt, and in each round of classification, determine that the number of strings for this snowmelt is equal to the number of strings for which snowmelt has already melted, based on the number of strings for which snowmelt has already melted; Alternatively, the number of snowmelt rounds of the photovoltaic strings and the time corresponding to each round of snowmelt are predicted based on the number of snowmelt strings that have initially melted; The number of snowmelt rounds and the number of photovoltaic strings in each round of snowmelt are adjusted according to the electricity sales price during each round of snowmelt.

9. The snow melting method for a string photovoltaic inverter according to claim 8, characterized in that: 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 round, and the snowmelt allocation plan with the highest total benefit is obtained by traversing the model algorithm.

10. The snow melting method for a string photovoltaic inverter according to claim 9, characterized in that: The predicted total benefit E of multiple rounds of snow melting total The calculation formula is as follows: ； Where 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 number of rounds of total snow melting, E j represents the unit electricity sales revenue in the jth 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 a single PV string to melt snow.

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