A method for scanning IV curve

Abstract

The present application discloses an IV curve scanning method, comprising the following steps: dividing multiple photovoltaic strings into multiple groups according to a set number, giving each photovoltaic string a power target value of an initial state, and the sum of the power target values ​​of the multiple photovoltaic strings in each group is an equal fixed value; in each scanning cycle of the IV curve, increasing the power target value of at least one photovoltaic string in each group, and reducing the power target values ​​of the remaining photovoltaic strings, so that the sum of the power target values ​​of the photovoltaic strings in the group remains unchanged. The beneficial effects of the present application: by grouping photovoltaic strings and limiting power changes, it can be ensured that when a single photovoltaic string undergoes power changes during IV curve scanning, the total output power of all photovoltaic strings remains unchanged, thereby effectively avoiding or reducing the fluctuation of the total output power of the photovoltaic power generation system to improve the working stability of the power grid.

Description

Technical Field

[0001] The present application relates to the field of renewable energy power generation, and in particular to an IV curve scanning method. Background Art

[0002] PV strings are formed by multiple PV modules connected in series (in parallel) to match the rated voltage and current required by the system. As the use time increases, the performance of the PV strings may gradually degrade, such as reduced PV efficiency, increased internal resistance, etc., and there may be faults in the PV modules, such as broken cells, damaged connecting wires, pollution, etc. These performance changes and component failures will directly affect the shape and position of the IV curve, resulting in abnormal IV curve shape. By regularly scanning the IV curve, the performance degradation of the PV strings can be monitored. By comparing the actual measured IV curve with the theoretical curve or historical data, the problem can be quickly identified and located, and timely measures can be taken for maintenance or replacement.

[0003] The current online IV curve scanning method needs to continuously change the output voltage of the photovoltaic strings during the IV scanning of the photovoltaic strings, which will cause the output power of the photovoltaic strings to fluctuate over a large range, thereby causing the output power of the inverter connected to the photovoltaic strings to fluctuate over a large range, thereby having a great impact on the grid-connected power quality of the photovoltaic power generation system. Summary of the invention

[0004] One of the objectives of the present application is to provide an IV curve scanning method that can solve at least one of the defects in the above-mentioned background technology.

[0005] In order to achieve at least one of the above-mentioned purposes, the technical solution adopted in the present application is: an IV curve scanning method, comprising the following steps: dividing multiple photovoltaic strings into multiple groups according to a set number, giving each photovoltaic string a power target value in an initial state, and the sum of the power target values ​​of the multiple photovoltaic strings in each group is an equal fixed value; in each scanning cycle of the IV curve, increasing the power target value of at least one photovoltaic string in each group, and reducing the power target values ​​of the remaining photovoltaic strings, so that the sum of the power target values ​​of the photovoltaic strings in the group remains unchanged.

[0006] Preferably, the number of photovoltaic strings corresponding to each group is equal, and the number of photovoltaic strings in each group is an even number; in each scanning cycle, half of the photovoltaic strings increase their target power values, and the other half of the photovoltaic strings decrease their target power values.

[0007] Preferably, the power target value of each photovoltaic string is increased or decreased according to the set first power difference.

[0008] Preferably, the static operating point of the PV curve corresponding to the IV curve is uniformly set according to the set second power difference, wherein the midpoint of the multiple static operating points is located at the maximum power value point of the PV curve; in each group of multi-channel photovoltaic strings, the power target values ​​of the initial state given to half of the photovoltaic strings correspond to the static operating points of the left slope of the PV curve, and the power target values ​​of the initial state given to the other half of the photovoltaic strings correspond to the static operating points of the right slope of the PV curve; the power target values ​​of all photovoltaic strings are suitable for changing along the same direction of the PV curve when scanning the IV curve.

[0009] Preferably, assuming that the first power difference is ΔP´, the second power difference is ΔP, ΔP´=k·ΔP; wherein k represents a coefficient, and the value of k is an integer greater than or equal to 1.

[0010] Preferably, in a plurality of consecutive scanning cycles, the power target value of each photovoltaic string is suitable for being changed multiple times in a set number of stages simultaneously along the clockwise or counterclockwise direction of the PV curve.

[0011] Preferably, the static operating points of the PV curve include the maximum voltage point and the zero voltage point; when the power target value of the photovoltaic string changes clockwise through the maximum voltage point and the zero voltage point, the photovoltaic string is in an open circuit state in the first half of the scanning cycle and in a disconnected state in the second half; when the power target value of the photovoltaic string changes counterclockwise through the zero voltage point and the maximum voltage point, the photovoltaic string is in a disconnected state in the first half of the scanning cycle and in an open circuit state in the second half.

[0012] Preferably, each photovoltaic string is connected to a corresponding DC / DC unit, and the control loop corresponding to the DC / DC unit works in the MPPT mode when the IV curve is not scanned; when the IV curve is scanned, the control loop is switched from the MPPT mode to the IV curve scanning control mode; in the IV curve scanning control mode of the control loop, the photovoltaic string is regularly changed according to the static working point to the power target value P * as loop input.

[0013] Preferably, the control loop includes a power tracking unit, a voltage outer loop and a current inner loop; in the IV curve scanning control mode, the power tracking unit is based on the power target value P of the photovoltaic string. * 、Output voltage v real and the output current i real As input, according to the power target value P of the photovoltaic string * Perform left or right slope power tracking at the position in the PV curve to output voltage command v * ; The voltage outer loop converts the voltage command v * and the output voltage v real As input, the current command i *The output of the current inner loop is the current instruction i * and the output current i real As input, a control signal for driving the DC / DC unit is output.

[0014] Preferably, the output voltage v of the photovoltaic string real and the output current i real It is collected through a sampling circuit.

[0015] Compared with the prior art, the beneficial effects of this application are:

[0016] By grouping PV strings and limiting power changes, it is possible to ensure that when a single PV string changes power during IV curve scanning, the total output power of all PV strings remains unchanged, thereby effectively avoiding or reducing the total output power fluctuation of the PV power generation system to improve the working stability of the power grid. At the same time, by synchronizing the power changes of multiple groups of PV strings, rapid IV curve scanning can be achieved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 The following is a schematic diagram of the workflow of this application.

[0018] Figure 2 Schematic diagram of the output characteristic curve of the photovoltaic string in this application.

[0019] Figure 3 This is a schematic diagram of the IV curve scanning workflow based on the control loop of this application.

[0020] Figure 4 This is a schematic diagram of the static operating point division according to the PV curve and the power target value position of a given initial state in this application.

[0021] Figure 5 This is a schematic diagram of the power target value changing step by step clockwise in this application.

[0022] Figure 6 A schematic diagram of setting a power target value in an initial state for one of the specific examples of the present application.

[0023] Figure 7 For this application Figure 5 The example is a schematic diagram of the first clockwise change of the power target value.

[0024] Figure 8 For this application Figure 5 The example is a schematic diagram of the second clockwise change of the power target value.

[0025] Fig. 9 Schematic diagram of the control architecture of the photovoltaic system in this application.

[0026] Fig.10 This is a schematic diagram of the control architecture corresponding to one of the photovoltaic strings in this application.

[0027] Fig.11 Schematic diagram of the total output power change curve of the photovoltaic string during the IV scanning process of the present application. DETAILED DESCRIPTION

[0028] 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 description with reference to the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples" means 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 being directed 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.

[0029] In the description of the present application, it should be noted that directional words, such as the terms "center", "lateral", "longitudinal", "length", "width", "thickness", "up", "down", "front", "back", "left", "right", "vertical", "horizontal", "top", "bottom", "inside", "outside", "clockwise", "counterclockwise", etc., indicating directions and positional relationships are based on the directions or positional relationships shown in the accompanying drawings, and are only for the convenience of narrating the present application and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific direction, be constructed and operated in a specific direction, and cannot be understood as limiting the specific scope of protection of the present application.

[0030] 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 are not necessarily used to describe a specific order or sequence.

[0031] In this application, unless otherwise clearly specified and limited, the terms "installed", "connected", "connected", "fixed" and the like should be understood in a broad sense, for example, it can be connected, detachably connected, or integrated; it can be mechanically connected or electrically connected; it can be directly connected or indirectly connected through an intermediate medium, it can be the internal connection of two elements or the interaction relationship between two elements. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0032] In the present application, unless otherwise clearly specified and limited, a first feature being "above" or "below" a second feature may include that the first and second features are in direct contact, or may include that the first and second features are not in direct contact but are in contact through another feature between them. Moreover, a first feature being "above", "above" and "above" a second feature includes that the first feature is directly above and obliquely above the second feature, or simply indicates that the first feature is higher in level than the second feature. A first feature being "below", "below" and "below" a second feature includes that the first feature is directly below and obliquely below the second feature, or simply indicates that the first feature is lower in level than the second feature.

[0033] The terms "including" and "having" and any variations thereof in the specification and claims of the present application are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products or apparatuses.

[0034] One of the preferred embodiments of the present application is as follows: Figure 1 As shown, an IV curve scanning method includes the following steps: dividing multiple photovoltaic strings into multiple groups according to a set number; giving each photovoltaic string a power target value in an initial state, and the sum of the power target values ​​of the multiple photovoltaic strings in each group is an equal fixed value. In each scanning cycle of the IV curve, the power target value of at least one photovoltaic string in each group is increased, and the power target values ​​of the remaining photovoltaic strings are reduced, so that the sum of the power target values ​​of the photovoltaic strings in the group remains unchanged.

[0035] It is understandable that, since the output power of the photovoltaic strings needs to be adjusted during the IV curve scanning process, this will cause the output power of the entire photovoltaic power generation system to change, which will cause a large fluctuation in the grid-connected power and affect the stability of the power grid. Therefore, before performing the IV curve scanning, the present embodiment can group all the photovoltaic strings, and give the power target value to the grouped photovoltaic strings. Thus, when performing the IV curve scanning, the power target value of each group of photovoltaic strings can be changed, and the power target values ​​of different photovoltaic strings can be increased and decreased respectively, so as to ensure that the total output power of each group of photovoltaic strings remains unchanged before and after the power target value changes. In this way, the change in the power target value for a single photovoltaic string can ensure the stable implementation of the IV curve scanning, while the total output power of each group of photovoltaic strings or all photovoltaic strings remains unchanged, so as to ensure that the grid-connected power of the photovoltaic power generation system will not fluctuate, or the fluctuation is small, which basically will not affect the working stability of the power grid.

[0036] In layman's terms, the total increase and total decrease of the power target values ​​of different photovoltaic strings in the same group are equal, and can be complementary to each other so that the total output power of each photovoltaic string remains unchanged. Of course, another implementation of this embodiment can be that in two adjacent groups of photovoltaic strings, the photovoltaic strings of one group all increase their power target values, and the photovoltaic strings of the other group all decrease their power target values, and the total increase and total decrease of the power target values ​​of the two groups of photovoltaic strings are equal to achieve complementarity, which can also ensure that the total output power of the entire photovoltaic power generation system remains basically constant.

[0037] At the same time, in order to reduce the impact of power grid fluctuations, the traditional IV curve scanning method generally reduces the total output power fluctuation by reducing the number of power changes of the photovoltaic strings; that is, the output power of all photovoltaic strings is changed multiple times. In this embodiment, since the total output power of each group of photovoltaic strings remains unchanged, the output power of all photovoltaic strings can be changed synchronously. Compared with the traditional method, this application can effectively improve the efficiency of IV curve scanning.

[0038] It should be known that the number of photovoltaic strings and the specific number of photovoltaic strings in each group can be selected according to the actual needs of those skilled in the art, and it is only necessary to ensure that the total output power of each photovoltaic string group remains unchanged before and after the power target value is changed. For example, all photovoltaic strings can be divided into multiple groups according to the set number; for example, 20 photovoltaic strings can be divided into 5 groups with 4 strings in a group. Of course, all photovoltaic strings can also be divided into multiple groups according to different numbers of photovoltaic strings in each group; for example, 20 photovoltaic strings can be divided into 5 groups with 2, 3, 4, 5 and 6 strings respectively. In order to ensure the power balance of each group of photovoltaic strings and facilitate the change of the power target value, it is preferred to divide the multi-way photovoltaic strings into equal groups according to the same number.

[0039] Generally speaking, the total number of photovoltaic strings in a photovoltaic power generation system is an even number, and can be equally divided into at least two photovoltaic strings as a group. The number of photovoltaic strings in each group can be an odd number or an even number; for an odd number of photovoltaic strings, the number of photovoltaic strings with increased power target values ​​and the number of photovoltaic strings with reduced power target values ​​must be different. In order to facilitate power changes, the number of photovoltaic strings in each group in this embodiment is preferably an even number. In this way, when performing an IV curve scan, the power target value of half of the photovoltaic strings in each group can be increased, and the power target value of the other half can be reduced.

[0040] In this embodiment, when the power target value of each photovoltaic string in each group is changed, the change amount of the power target value of each photovoltaic string may not be the same; taking each group containing four photovoltaic strings as an example, assuming that the power target value of one photovoltaic string is increased by 5%, and the power target value of another photovoltaic string is increased by 3%, then it is only necessary to ensure that the power target values ​​of the other two photovoltaic strings are reduced by 8% in total. Of course, the power target value of each photovoltaic string can also be increased or decreased according to the set first power difference; taking each group containing four photovoltaic strings as an example, assuming that the first power difference is 4% of the power target value, then the power target values ​​of two photovoltaic strings are increased by 4%, and the power target values ​​of the remaining two photovoltaic strings are reduced by 4%. In order to improve the efficiency of the power target value change of the photovoltaic strings, in this embodiment, the power target value change of the photovoltaic strings of each group is preferably increased or decreased according to the set first power difference.

[0041] In this embodiment, in order to further improve the scanning efficiency of the IV curve, as shown in FIG. Figure 3 and Figure 4 As shown, the PV curve corresponding to the IV curve is uniformly set to a static operating point according to the set second power difference, wherein the midpoint of the multi-level static operating points is located at the maximum power value point of the PV curve. In each group of multi-channel photovoltaic strings, the power target value of the initial state given to half of the photovoltaic strings corresponds to the static operating point of the left slope of the PV curve, and the power target value of the initial state given to the other half of the photovoltaic strings corresponds to the static operating point of the right slope of the PV curve, so that when the IV curve is scanned, the power target values ​​of all photovoltaic strings can be changed in the same direction of the PV curve.

[0042] It is understandable that when performing IV curve scanning, since it is only necessary to ensure that the total output power of each group of photovoltaic strings remains unchanged, that is, the changes in the power target values ​​of each group of photovoltaic strings do not affect each other, this leads to disordered changes in the power target values ​​of each group of photovoltaic strings, which is more difficult for the algorithm control of IV curve scanning. In this embodiment, if Figure 2As shown in the figure, the output characteristic curve of the photovoltaic string includes the corresponding IV curve and PV curve; it can be seen from the PV curve of the photovoltaic string that the output power of the photovoltaic string gradually increases on the left side of the highest point of the PV curve, and gradually decreases on the right side of the highest point of the PV curve. The left side of the highest point of the PV curve is defined as the left slope, and the right side of the highest point is defined as the right slope; then the power target value of half of the photovoltaic strings in each group is set to the left slope, and the power target value of the other half of the photovoltaic strings is set to the right slope, then it is only necessary to transition all the photovoltaic strings in one direction of the PV curve to ensure that the power target value of half of the photovoltaic strings in each group is increasing, and the power target value of the other half of the photovoltaic strings is decreasing, and according to the setting rules of the static working point, the total increase and total decrease of the power target value of each photovoltaic string in each group are equal.

[0043] In simple terms, this embodiment sets the power target value of each group of photovoltaic strings on the left slope and the right slope of the PV curve respectively, and then gives the change direction of the power target value of all photovoltaic strings according to the extension direction of the PV curve, so that the change direction of the power target value of the photovoltaic strings is changed from disordered to ordered, which can effectively improve the scanning efficiency of the IV curve.

[0044] It should be noted that, assuming that the first power difference is ΔP´ and the second power difference is ΔP; in order to ensure that the power target value of the photovoltaic string can be changed smoothly, it is necessary to ensure that ΔP´=k·ΔP; where k represents a coefficient, and the value of k is an integer greater than or equal to 1. That is, the position change of the static working point corresponding to each change of the power target value of the photovoltaic string is k levels, and the specific value of k can be selected according to the actual needs of those skilled in the art.

[0045] In this embodiment, Figure 5 As shown, the extension direction of the PV curve mainly includes the clockwise direction and the counterclockwise direction. Therefore, in multiple consecutive scanning cycles, the power target value of each PV string can be changed multiple times along the clockwise or counterclockwise direction of the PV curve at the same time.

[0046] For ease of understanding, the following will be described in detail step by step.

[0047] (1) Set the static operating point of the IV scan.

[0048] like Figure 4 As shown in the figure, taking one photovoltaic string as an example, the IV curve scanning points and the maximum power value P of the photovoltaic string are obtained. max , according to the maximum power value P max 2n+1 static operating points are preset. That is, the maximum power value point P max As the center, n static working points are evenly divided on both sides of the PV curve according to the second power difference ΔP, then the second power difference ΔP=Pmax / n.

[0049] (2) Set the power target value of the PV string in the initial state.

[0050] like Figure 4 As shown in the figure, assuming that the current photovoltaic power generation system includes 2m photovoltaic strings, in order to balance the power during the scanning process, the power target value of the initial state corresponding to each photovoltaic string group needs to be evenly distributed on the left slope and the right slope respectively, then the left slope and the right slope of the PV curve need to be allocated to m photovoltaic strings. The specific allocation process is as follows:

[0051] The power target values ​​of the initial state of the m photovoltaic strings corresponding to the left slope are set at the static working points of the n different positions of each. The power target values ​​of the initial state of the m photovoltaic strings corresponding to the right slope are also set at the static working points of the n different positions of each. The power target values ​​of the initial state of the right slope and the left slope correspond one to one, and the sum of the power target value coefficients of each photovoltaic string group on the left slope and the right slope is 1, so that the total output power of each photovoltaic string group is K·P max , K represents the number of PV strings in each group.

[0052] Specifically, take each group including two photovoltaic strings as an example, that is, the value of K is 2. For one group of photovoltaic strings, if the power target value point of one photovoltaic string in the initial state is located at the xth static working point on the left slope, then the power target value of the photovoltaic string in the initial state is [2 (x-1) / (total number of static working points - 1)] · P max , then the power target value of the other PV string on the right slope in the initial state is [(total number of static working points - 1) / 2-(x-1)][2 / (total number of static working points - 1)]·P max The sum of the power target coefficients of the photovoltaic string group on the left slope and the right slope is [2 (x-1) / (total number of static working points - 1)] + [(total number of static working points - 1) / 2- (x-1)] [2 / (total number of static working points - 1)] = 1, that is, the total output power of the photovoltaic string group is P max , the total output power of the entire photovoltaic power generation system is mP max .

[0053] More specifically, Figure 4As shown, the m photovoltaic strings located on the right slope are sorted as #1, #2, ..., #m; the power target value points of the initial state given by the m photovoltaic strings are arranged step by step from a static working point closest to the maximum power value point of the PV curve to the right. The m photovoltaic strings located on the left slope are sorted as #m+1, #m+2, ..., #2m; the power target value points of the initial state given by the m photovoltaic strings are arranged step by step from a static working point closest to the zero point of the PV curve to the right. Then the #1 photovoltaic string and the #m+1 photovoltaic string are a group corresponding to the power target value, the #2 photovoltaic string and the #m+2 photovoltaic string are a group corresponding to the power target value, and so on until the #m photovoltaic string and the #2m photovoltaic string are a group corresponding to the power target value.

[0054] The preset initial power target values ​​of the m photovoltaic strings on the right slope are: The initial power target value of the #1 photovoltaic string is (n-1)P max / n, the initial power target value of the 2nd PV string (n-2)P max / n, ..., the initial power target value of the #mth string is (nm)P max / n. The preset initial state power target values ​​of the m photovoltaic strings on the left slope are: The initial state power target value of the #m+1 photovoltaic string is P max / n, the initial power target value of the #m+2 PV string is 2P max / n, ..., the initial power target value of the #2m string is mP max / n.

[0055] It should be noted that the number of static working points n on the left slope and the right slope is greater than or equal to the number of PV strings m.

[0056] (3) The power target value of each photovoltaic string is changed step by step to achieve IV curve scanning.

[0057] like Figure 4 and Figure 5 As shown, it is assumed that the IV curve is scanned i times, and the power target value of each photovoltaic string is changed along the clockwise direction of the PV curve as an example.

[0058] When the power target value of each PV string changes for the first time:

[0059] The power target values ​​of the m photovoltaic strings on the left slope are: The initial power target value of the #m+1 photovoltaic string is 2P max / n, the initial power target value of the #m+2 PV string is 3P max / n, ..., the initial power target value of the #2m-th string group is (m+1)Pmax / n. The power target values ​​of the m photovoltaic strings on the right slope are: the initial state power target value of the #1 photovoltaic string is (n-2)P max / n, the initial power target value of the 2nd PV string (n-3)P max / n, ..., the initial power target value of the #mth string is (nm-1)P max / n.

[0060] When the power target value of each PV string changes for the second time:

[0061] The power target values ​​of the m photovoltaic strings on the left slope are: The initial power target value of the #m+1 photovoltaic string is 3P max / n, the initial power target value of the #m+2 PV string is 4P max / n, ..., the initial power target value of the #2m-th string is (m+2)P max / n. The power target values ​​of the m photovoltaic strings on the right slope are: the initial state power target value of the #1 photovoltaic string is (n-3)P max / n, the initial power target value of the 2nd PV string (n-4)P max / n, ..., the initial power target value of the #mth string is (nm-2)P max / n.

[0062] Similarly, when the power target value of each photovoltaic string changes for the i-th time:

[0063] The power target values ​​of the m photovoltaic strings on the left slope are: The initial state power target value of the #m+1 photovoltaic string is (i+1)P max / n, the initial power target value of the #m+2 PV string (i+2) P max / n, ..., the initial power target value of the #2m-th string group is (i+m+1)P max / n. The power target values ​​of the m photovoltaic strings on the right slope are: the initial state power target value of the #1 photovoltaic string is (ni-1)P max / n, the initial power target value of the 2nd PV string (ni-2)P max / n, ..., the initial power target value of the #mth string is (nim)P max / n.

[0064] It should be noted that the static operating points of the PV curve include the maximum voltage point and the zero voltage point. When the change process of the power target value of the PV string passes through the maximum voltage point or the zero voltage point, the static operating point corresponding to the power target value of the PV string will move from the maximum voltage point to the zero voltage point or from the zero voltage point to the maximum voltage point within one scanning cycle.

[0065] Specifically, when the power target value of the photovoltaic string changes clockwise through the maximum voltage point and the zero voltage point, the photovoltaic string is in an open circuit state in the first half of the scanning cycle, so that the power target value is at the maximum voltage point; the photovoltaic string is in a disconnected state in the second half of the scanning cycle, so that the power target value is at the zero voltage point. When the power target value of the photovoltaic string changes counterclockwise through the zero voltage point and the maximum voltage point, the photovoltaic string is in a disconnected state in the first half of the scanning cycle, so that the power target value is at the zero voltage point; the photovoltaic string is in an open circuit state in the second half of the scanning cycle, so that the power target value is at the maximum voltage point.

[0066] For easier understanding, a specific example will be used below to provide a detailed description.

[0067] like Figure 6 As shown, it is assumed that the number of photovoltaic strings in the photovoltaic power generation system is 8, namely photovoltaic string A1, photovoltaic string A2, photovoltaic string A3, photovoltaic string A4, photovoltaic string B1, photovoltaic string B2, photovoltaic string B3 and photovoltaic string B4. Among them, photovoltaic components A1 to B4 are a group, photovoltaic components A2 to B3 are a group, photovoltaic components A3 to B2 are a group, and photovoltaic components A1 to B4 are a group. Photovoltaic strings A1 to A4 are assigned to the left slope of the PV curve, and starting from photovoltaic string A4, they are arranged step by step from the static operating point closest to the maximum power value point to the left; photovoltaic strings B1 to B4 are assigned to the right slope of the PV curve, and starting from photovoltaic string B1, they are arranged from the static operating point closest to the maximum voltage point V oc The nearest static working points are arranged step by step to the left.

[0068] When the power target value of the 8-way PV string changes for the first time, it is assumed that the power target value of the PV string changes by ΔP each time; Figure 7 As shown in the figure, the power target value of PV string A4 is located at the maximum power value point, and the power target values ​​of PV strings A3 to A1 are arranged in sequence on the left side of the maximum power value point. The power target value of PV string B1 is located at the maximum voltage point V in the first half of the scanning cycle. oc Position, in the second half of the scanning cycle is at the zero voltage point position, that is Figure 7 The power target value of PV strings B2 to B4 is at the maximum voltage point V oc Arranged in order on the left.

[0069] When the power target value of the 8-way PV string changes for the second time, Figure 8 As shown in the figure, the power target value of PV string A4 passes the maximum power value point and reaches the static operating point position closest to the maximum power value point on the right slope. The power target value of PV string A3 is located at the maximum power value point. The power target values ​​of PV strings A2 and A1 are arranged in sequence on the left side of the maximum power value point. The power target value of PV string B1 moves from the zero voltage point to the static operating point closest to the zero voltage point on the left slope. The power target value of PV module B2 is located at the maximum voltage point V in the first half of the scanning cycle. oc Position, in the second half of the scanning cycle is at the zero voltage point position, that is Figure 8 The power target value of PV strings B3 and B4 is at the maximum voltage point V oc Arranged in order on the left.

[0070] In this embodiment, Figure 3 and Fig. 9 As shown in the figure, each photovoltaic string is connected to a corresponding DC / DC unit, and the control loop corresponding to the DC / DC unit works in MPPT mode when IV curve scanning is not performed. When IV curve scanning is performed, the control loop switches from MPPT mode to IV curve scanning control mode. In the IV curve scanning control mode of the control loop, the photovoltaic string is regularly changed according to the static working point. * as loop input.

[0071] It should be known that each photovoltaic string can be composed of multiple photovoltaic modules connected in series, or multiple groups of photovoltaic modules connected in series in parallel; the photovoltaic power generation system includes multiple photovoltaic strings, each photovoltaic string is connected to the DC side of the same DC / AC unit through a corresponding DC / DC unit, and the AC side of the DC / AC unit is connected to the power grid and the load; each DC / DC unit is driven and controlled by a corresponding control loop.

[0072] It is understandable that, when IV curve scanning is not performed, in order to ensure that the photovoltaic power generation system can be in the best working state, the control loop can control the DC / DC unit to perform MPPT tracking so that the DC / DC unit works at the maximum power value point of the photovoltaic string. When performing IV curve scanning, it can be seen from the above content that the output power of the photovoltaic string needs to be re-given, and the power target value of the photovoltaic string needs to be changed during the IV scanning process. Therefore, the target of the control loop will no longer be the MPPT point of the photovoltaic string, but needs to be adaptively tracked according to the position and change law of the power target value of the photovoltaic string. Therefore, in this embodiment, two working modes are set for the control loop, namely the MPPT mode and the IV curve scanning control mode. Among them, the specific working process of the MPPT mode is well known to those skilled in the art, so it will not be elaborated in detail here; the IV curve scanning control mode will be described in detail below.

[0073] Specifically, Fig.10 As shown in Figure 1, the control loop includes a power tracking unit, a voltage outer loop, and a current inner loop. In the IV curve scanning control mode, the power tracking unit uses the power target value P of the photovoltaic string as the control loop. * , output voltage v real and the output current i real As input, according to the power target value P of the photovoltaic string * Perform left or right slope power tracking at the position in the PV curve to output voltage command v * The voltage outer loop converts the voltage command v * and the output voltage v real As input, the current command i * The output of the current inner loop is the current instruction i * and the output current i real As input, a control signal for driving the DC / DC unit is output.

[0074] It is understandable that the power tracking unit tracks the power according to whether the static operating point is on the left or right slope of the PV curve. * Located on the left slope of the PV curve, and the power target value P * The voltage command v * It is also necessary to increase the voltage value compared to the previous moment. * Located on the right slope of the PV curve, and the power target value P * The voltage command v * The voltage value needs to be reduced compared to the previous moment.

[0075] It should be known that the output voltage v of the photovoltaic stringreal and the output current i real The data is collected by a sampling circuit. The specific structure and working principle of the sampling circuit are well known to those skilled in the art. Common sampling circuits mainly include voltage sensors and current sensors. The voltage sensors and current sensors are connected to the output end of the photovoltaic string to monitor the corresponding electrical parameters.

[0076] In this embodiment, in order to facilitate understanding of the control process of the control loop on the DC / DC unit, the specific working process of the control loop will be described in detail below by taking one scanning cycle as an example.

[0077] like Figure 3 and Fig.11 As shown, during the normal operation of the photovoltaic power generation system, that is, Fig.11 Medium T 1 In the stage before the moment, each DC / DC unit of the photovoltaic power generation system is in the MPPT tracking state under the MPPT mode of the control loop. At this time, the total output power of the photovoltaic power generation system is P O .

[0078] In T 1 Always ready to perform IV curve scanning, the control loop starts to switch to IV curve scanning control mode. 1 To T 2 Within the time, the host computer evenly sets the static working point according to the maximum power value point of each photovoltaic string and the IV curve scanning point, and sets the power target value of each photovoltaic string in the initial state. Figure 6 to Figure 7 As an example, assume that the maximum power value point of each photovoltaic string is P max , then in T 1 The total output power P of the photovoltaic power generation system before time O =8P max , in T 2 The total output power P of the photovoltaic power generation system at the moment O ´=4P max , that is, P O = P O / 2.

[0079] It should be noted that in practice, the maximum power values ​​of each photovoltaic string are different, but each time the power target value is changed, the total output power value of the output power of each photovoltaic string is basically maintained at 4P max , that is, fluctuations above and below Po / 2 can ensure that the total output power fluctuation is very small during the IV curve scanning process.

[0080] In T 2 The IV curve scanning control mode of the control loop is switched at the moment T2 To T 3 Within this time, the power target value of each photovoltaic string will be changed once to complete an IV curve scanning process. 3 When the IV curve scan is completed at T, the control loop will switch from the IV curve scan control mode to the MPPT mode, and the photovoltaic power generation system will re-perform MPPT tracking until T 4 At this moment, each DC / DC unit of the photovoltaic power generation system returns to the MPPT state.

[0081] 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 by the above embodiments, and the above embodiments and the specification only describe the principles of the present application. The present application may have various changes and improvements without departing from the spirit and scope of the present application, and 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 attached claims and their equivalents.

Claims

1. An IV curve scanning method, characterized in that: The steps include: Divide multiple photovoltaic strings into multiple groups according to a set number, give each photovoltaic string a power target value in an initial state, and the sum of the power target values ​​of the multiple photovoltaic strings in each group is an equal fixed value; In each scanning cycle of the IV curve, the power target value of at least one photovoltaic string in each group is increased, and the power target values ​​of the remaining photovoltaic strings are reduced, so that the sum of the power target values ​​of the photovoltaic strings in the group remains unchanged; The number of photovoltaic strings corresponding to each group is equal, and the number of photovoltaic strings in each group is an even number; in each scanning cycle, half of the photovoltaic strings increase their power target values, and the other half of the photovoltaic strings decrease their power target values; The PV curve corresponding to the IV curve is evenly set to a static operating point according to the set second power difference, wherein the midpoint of the multi-level static operating points is located at the maximum power value point of the PV curve; In each group of multiple photovoltaic strings, the power target value of the initial state given to half of the photovoltaic strings corresponds to the static operating point of the left slope of the PV curve, and the power target value of the initial state given to the other half of the photovoltaic strings corresponds to the static operating point of the right slope of the PV curve; The power target values ​​of all PV strings are suitable for changing along the same direction of the PV curve when performing IV curve scanning; The static operating points of the PV curve include the maximum voltage point and the zero voltage point; When the power target value of the photovoltaic string changes clockwise through the maximum voltage point and the zero voltage point, the photovoltaic string is in an open circuit state in the first half of the scanning cycle and in a disconnected circuit state in the second half of the scanning cycle; When the power target value of the PV string changes counterclockwise through the zero voltage point and the maximum voltage point, the PV string is in a disconnected state in the first half of the scanning cycle and in an open circuit state in the second half of the scanning cycle.

2. The IV curve scanning method according to claim 1, characterized in that: The power target value of each photovoltaic string is increased or decreased according to the set first power difference.

3. The IV curve scanning method according to claim 2, characterized in that: Assume the first power difference is , the second power difference is , =k· ; Wherein, k represents a coefficient, and the value of k is an integer greater than or equal to 1.

4. The IV curve scanning method according to claim 1, characterized in that: In a plurality of consecutive scanning cycles, the power target value of each photovoltaic string is suitable for performing a plurality of changes of a set number of levels simultaneously along the clockwise or counterclockwise direction of the PV curve.

5. The IV curve scanning method according to any one of claims 1 to 4, characterized in that: Each photovoltaic string is connected to a corresponding DC / DC unit, and the control loop corresponding to the DC / DC unit works in MPPT mode when IV curve scanning is not performed; When the IV curve is scanned, the control loop switches from MPPT mode to IV curve scanning control mode; In the IV curve scanning control mode of the control loop, the power target value P of the photovoltaic string is changed regularly according to the static working point. * as loop input.

6. The IV curve scanning method according to claim 5, characterized in that: The control loop includes a power tracking unit, a voltage outer loop, and a current inner loop; In the IV curve scanning control mode, the power tracking unit uses the power target value P of the PV string as the power target value P * , output voltage v real and the output current i real As input, according to the power target value P of the photovoltaic string * Perform left or right slope power tracking at the position in the PV curve to output voltage command v * ; The voltage outer loop converts the voltage command v * and the output voltage v real As input, the current command i * Output: The current inner loop converts the current command i * and the output current i real As input, a control signal for driving the DC / DC unit is output.

7. The IV curve scanning method according to claim 6, characterized in that: The output voltage of the photovoltaic string is v real and the output current i real It is collected through a sampling circuit.

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