A detection system and detection method for reverse connection of parallel strings in photovoltaic systems

Abstract

The present invention discloses a detection system and method for reverse connection of parallel strings in a photovoltaic system. The detection system includes a current sampling unit, a voltage sampling unit, and an MPPT device. The current sampling unit is arranged on the branch where the string is located, and the voltage sampling unit is connected to the main line formed by the parallel strings. The MPPT device is communicatively connected with the current sampling unit and the voltage sampling unit. The MPPT device can adjust the output power of the inverter according to the circuit state of the string-parallel connection. If the current sampling unit detects that the current flow direction of all branches is forward, the MPPT device controls the inverter to connect to the grid and output. If the current sampling unit detects a branch with reverse current and the voltage detected by the voltage sampling unit is less than a set threshold, the MPPT device controls the inverter to stop connecting to the grid and output. The detection system and detection method provided by the present invention distinguish the string voltage mismatch operating condition, thereby accurately determining whether it is reverse connection, prompting the user to make corrections, improving operation and maintenance efficiency, and improving the benefits of photovoltaic power generation.

Description

Technical Field

[0001] The present invention relates to the technical field of photovoltaic power generation, and in particular to a detection system and method for reverse connection of parallel strings of a photovoltaic system. Background Art

[0002] In existing photovoltaic systems, the current reverse polarity protection detection method detects reverse current flow in the strings. The presence of reverse current indicates reverse polarity. However, existing photovoltaic power generation technologies often utilize multiple strings connected in parallel to a single MPPT device. When the voltages of the parallel strings are mismatched, the high-voltage string will output power to the low-voltage string, resulting in reverse current flow. Reverse current flow in parallel strings is not always caused by reverse polarity, so a systematic method is needed to detect reverse polarity in parallel strings. Summary of the Invention

[0003] In order to overcome the shortcomings of the prior art, the present invention provides a detection system and method for reverse connection of parallel strings in a photovoltaic system. The specific technical solution is as follows:

[0004] On the one hand, a detection system for reverse connection of parallel strings of a photovoltaic system is provided. The photovoltaic system includes a photovoltaic panel assembly and an inverter. Multiple photovoltaic panel assemblies are connected in series to form a string. The multiple strings are connected in parallel through the inverter for grid-connected output. The detection system includes a current sampling unit, a voltage sampling unit, and an MPPT device. The current sampling unit is provided on a branch where the string is located to obtain the current flow direction of the branch. The voltage sampling unit is connected to a main line formed by the parallel strings. The MPPT device is communicatively connected to the current sampling unit and the voltage sampling unit. The MPPT device can adjust the output power of the inverter according to the circuit status of the parallel strings.

[0005] If the current sampling unit detects that the current flow directions of all branches are forward, the MPPT device controls the inverter to output power in a grid-connected manner;

[0006] If the current sampling unit detects a branch with reverse current, and the voltage detected by the voltage sampling unit is less than a set threshold, the MPPT device controls the inverter to stop grid-connected output;

[0007] Both ends of the photovoltaic panel assembly on the branch are connected in parallel with a first diode, and the set threshold is greater than the maximum value of the sum of the conduction voltage drops of the corresponding first diodes on all branches, and is less than or equal to the startup voltage of the inverter.

[0008] Furthermore, if the current sampling unit detects a branch with current reverse flow, and the voltage detected by the voltage sampling unit is greater than or equal to a set threshold, the MPPT device controls the inverter to output in grid connection and issues a voltage mismatch prompt.

[0009] Furthermore, each branch has the same number of photovoltaic panel assemblies connected in series, the conduction voltage drop of the first diode connected in parallel on each photovoltaic panel assembly is the same, and the set threshold is greater than the product of the conduction voltage drop of a single first diode and the number of first diodes on a single branch.

[0010] Furthermore, the first diode is directly connected to both ends of the corresponding photovoltaic panel assembly.

[0011] Furthermore, the detection system further includes a second diode, which is reverse-connected and connected in parallel at both ends of the MPPT device, and the voltage sampling unit is simultaneously connected in parallel at both ends of the second diode.

[0012] Furthermore, if n strings are connected in parallel, where n is a positive integer greater than 1, the current sampling unit is provided on each branch, or the current sampling unit is provided on n-1 branches respectively and also on the main line.

[0013] Furthermore, the detection system also includes a shutdown device, which is connected in series on each branch. If the current sampling unit detects a branch where current reverse occurs, and the voltage detected by the voltage sampling unit is less than a set threshold, the inverter controls the shutdown device on the branch where current reverse occurs to start, so as to disconnect the branch where current reverse occurs.

[0014] In another aspect, a detection method of the detection system is provided, comprising:

[0015] S1. Determine whether a branch with reverse current exists based on the current data collected by the current sampling unit. If a branch with reverse current exists, execute step S2; otherwise, execute step S3.

[0016] S2, the MPPT device controls the inverter to output the grid;

[0017] S3. Determine whether the sampled voltage exceeds a set threshold value based on the voltage data collected by the voltage sampling unit. If it is less than the set threshold value, execute step S4; otherwise, execute step S5.

[0018] S4, the MPPT device controls the inverter to stop grid-connected output and issues an alarm;

[0019] S5. The MPPT device controls the inverter to output the grid and provides a voltage mismatch prompt.

[0020] Furthermore, in step S4, after the grid-connected output is stopped, only the branch where the current reverse occurs is disconnected, and step S1 is restarted.

[0021] Furthermore, in step S5, the voltage mismatch prompt includes information about the branch where current reversal occurs, and the operation and maintenance personnel increase, decrease or repair the photovoltaic panel components on the corresponding branch according to the voltage mismatch prompt.

[0022] The present invention has the following advantages: it can simply and efficiently distinguish between the operating conditions of string reverse connection and string voltage mismatch without increasing hardware costs, thereby reducing power generation benefit losses caused by misjudgment and improving operation and maintenance efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 1. A schematic diagram of the hardware structure topology of a judgment mechanism in a detection system for reverse connection of parallel strings of a photovoltaic system provided by an embodiment of the present invention;

[0024] Figure 2 Schematic diagram of reverse current flow direction of n channels in a reverse connection detection system for parallel strings of a photovoltaic system provided by an embodiment of the present invention;

[0025] Figure 3 Schematic diagram of n-way reverse current flow in a detection system for reverse connection of parallel strings of a photovoltaic system provided by an embodiment of the present invention;

[0026] Figure 4 1. A schematic diagram of a circuit topology for adding a string shutdown function to a detection system for reverse connection of parallel strings of a photovoltaic system provided by an embodiment of the present invention;

[0027] Figure 5 2. A schematic diagram of a curve showing the effect of a parallel mismatch on the operating voltage of a component in a parallel string reverse connection detection system for a photovoltaic system provided by an embodiment of the present invention;

[0028] Figure 6 This is a schematic diagram of the judgment process of the judgment mechanism in the detection method provided by an embodiment of the present invention;

[0029] Figure 7 It is a schematic diagram of the judgment flow of the judgment mechanism for adding a string shut-off device in the detection method provided in an embodiment of the present invention.

[0030] The reference numerals are as follows: 1-photovoltaic panel assembly, 2-current sampling unit, 3-voltage sampling unit, 4-second diode. DETAILED DESCRIPTION

[0031] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0032] It should be noted that the terms "first," "second," and the like in the description and claims of the present invention and the accompanying drawings are used to distinguish similar items and are not necessarily used to describe a particular order or precedence. It should be understood that the terms used in this manner are interchangeable where appropriate, such that the embodiments of the present invention described herein can be implemented in sequences other than those illustrated or described herein. Furthermore, the terms "including," "having," and any variations thereof are intended to cover non-exclusive inclusions.

[0033] In one embodiment of the present invention, a detection system for reverse connection of parallel strings of a photovoltaic system is provided. The photovoltaic system includes a photovoltaic panel assembly 1 and an inverter. Multiple photovoltaic panel assemblies are connected in series to form a string. The multiple strings are connected in parallel through the inverter for grid-connected output. The detection system includes a current sampling unit 2, a voltage sampling unit 3, and an MPPT device. The current sampling unit is provided on a branch where the string is located to obtain the current flow direction of the branch. The voltage sampling unit is connected to a main line formed by the parallel strings. The MPPT device is communicatively connected to the current sampling unit and the voltage sampling unit. The MPPT device is also connected to the main line and can adjust the output power of the inverter according to the circuit state of the parallel strings.

[0034] If the current sampling unit detects that the current flow directions of all branches are forward, the MPPT device controls the inverter to output power in a grid-connected manner;

[0035] If the current sampling unit detects a branch with reverse current, and the voltage detected by the voltage sampling unit is less than a set threshold, the MPPT device controls the inverter to stop grid-connected output;

[0036] If the current sampling unit detects a branch with reverse current, and the voltage detected by the voltage sampling unit is greater than or equal to a set threshold, the MPPT device controls the inverter to output to the grid and issues a voltage mismatch prompt;

[0037] A first diode (not shown) is connected in parallel at both ends of the photovoltaic panel assembly on the branch, and the first diode is directly connected to the two ends of the corresponding photovoltaic panel assembly. The set threshold is greater than the maximum value of the sum of the conduction voltage drops of the corresponding first diodes on all branches, and is less than or equal to the start-up voltage of the inverter.

[0038] It should be noted that, assuming that reverse current occurs in a branch, the sum of the forward voltage drops of all the first diodes on the branch is compared with the sum of the forward voltage drops of all the first diodes on other branches. Since the number and forward voltage drops of the first diodes on each branch may be the same or different, the sum of the forward voltage drops on each branch may be the same or different. In this implementation, the maximum value of the sum of the forward voltage drops in all branches is taken, and the set threshold is greater than the maximum value of the sum of the forward voltage drops.

[0039] The current sampling unit has at least the following configurations:

[0040] The first method: if n groups are connected in parallel, n is a positive integer greater than 1, and the current sampling unit is set on each branch, the current direction can be determined by the pointer deflection direction of the current sampling unit on the corresponding branch or the measured positive or negative value.

[0041] The second method: if n groups are connected in parallel, n is a positive integer greater than 1, then the current sampling unit is set on the first n-1 branches and the current sampling unit is also set on the main line, such as Figure 1 As shown, the current sampling unit collects the currents of the first n-1 branches and the total input current of the main line. The nth current is obtained by subtracting the sum of the first n-1 currents from the total input current, so as to determine whether there is a reverse current in all branch currents.

[0042] In one embodiment of the present invention, each branch has the same number of photovoltaic panels connected in series, resulting in the same number of first diodes on each branch. In this case, the set threshold is greater than the product of the forward voltage drop of a single first diode and the number of first diodes on a single branch, and is less than or equal to the inverter's startup voltage. The detection system also includes a second diode 4, which is reverse-connected in parallel across the MPPT device. The voltage sampling unit is also connected in parallel across the second diode. The forward voltage drop of the second diode is equal to the forward voltage drop of the first diode.

[0043] Determine whether the reverse current is caused by reverse string connection or another reason. When determining that reverse current is occurring in a parallel branch, further investigation is required to determine the cause. This can be caused by reverse string connection or string voltage mismatch. This requires determining the MPPT device input voltage, which is the voltage data collected by the voltage sampling unit.

[0044] If the strings are reversed, the modules will operate near the short-circuit point, which can be divided into the following two situations:

[0045] In the first case, see Figure 2 When reverse connection occurs on less than n strings, the collected voltage data represents the sum of the forward voltage drops of the diodes connected in parallel in the reversed-connected string. This creates a loop between the forward-connected and reversed strings, potentially leading to module failure. For example, in a typical 1500V PV system, a single string typically has 30 PV panels, and the maximum forward voltage drop of a single diode is 1.5V. In this case, the value is approximately 45V.

[0046] In the second case, see Figure 3 When all n strings are reverse-connected, the collected voltage data is the conduction voltage drop of the MPPT device's second anti-parallel diode, which is generally a maximum of 1.5 V. At this time, all current reverse branches form a loop with the second diode, making it impossible to adapt the output to the inverter.

[0047] When the string voltages are mismatched, the strings are connected in parallel, the voltage across each string is always equal, and the total current of the circuit is equal to the sum of the currents of each string. Figure 4 , the open circuit voltage of a single string 1 is V oc1 Less than the open circuit voltage V of a single string 2 oc2 , I sc1 and I sc2 The short-circuit current values of strings 1 and 2 are respectively. The open-circuit voltage of strings 1 and 2 after being connected in parallel is between the two above. The voltage increase of the string with lower voltage actually reduces the open-circuit voltage of the string with higher voltage. At the same time, when the circuit is open, the string with higher voltage has a reverse current to the string with lower voltage, i.e. Figure 4 The intersection of the middle curve. Therefore, a voltage mismatch in parallel strings will manifest simultaneously as: a) reverse current in the mismatched low-voltage string; b) the total open-circuit voltage of the mismatched strings is no less than the open-circuit voltage of the mismatched lowest-voltage string. Voltage mismatch reduces the overall output efficiency of the PV system. For conventional on-site installation, the voltage of conventional components is greater than the inverter startup voltage. A threshold point can be configured based on the inverter startup voltage. Based on these two points, the threshold can be set to the inverter startup voltage to determine whether the input voltage is less than the set threshold. If it is less than the threshold, it is judged that the string is reversely connected. If it is greater than or equal to the threshold, it is judged that the string voltage mismatch occurs, thereby distinguishing between string voltage mismatch conditions.

[0048] In one embodiment of the present invention, the detection system further comprises a shut-off device, see Figure 5, each branch is connected in series with the shutdown device, and all the shutdown devices are communicatively connected to the inverter. If the current sampling unit detects a branch with current reverse, and the voltage detected by the voltage sampling unit is less than the set threshold, the inverter controls the shutdown device on the branch with current reverse to start, so as to disconnect the branch with current reverse.

[0049] In one embodiment of the present invention, a detection method based on the detection system is provided, comprising:

[0050] S1. Determine whether a branch with reverse current exists based on the current data collected by the current sampling unit. If a branch with reverse current exists, execute step S2; otherwise, execute step S3.

[0051] S2, the MPPT device controls the inverter to output the grid;

[0052] S3. Determine whether the sampled voltage exceeds a set threshold value based on the voltage data collected by the voltage sampling unit. If it is less than the set threshold value, execute step S4; otherwise, execute step S5.

[0053] S4, the MPPT device controls the inverter to stop grid-connected output and issues an alarm;

[0054] S5. The MPPT device controls the inverter to output the grid and provides a voltage mismatch prompt.

[0055] Among them, if the reverse-connected photovoltaic panel components in the string continue to operate near the short-circuit point, it may cause component failure, requiring a serious warning and the inverter to shut down and alarm; if the string voltage is mismatched, the MPPT device of that line can still work normally, but the mismatch will affect the component efficiency and still require a prompt, and the inverter needs to report it online.

[0056] In one embodiment of the present invention, see Figure 6 , the MPPT device stops power output, collects current values of all branches, and determines whether there is reverse current in all branches. If there is no current reverse, it is determined that the string is normal and the inverter is normally connected to the grid; if current reverse occurs, the reverse branch number is read, and on this basis, the input voltage and the set threshold are further determined. If the input voltage is less than the set threshold, it is determined that the string is reversed, the inverter stops and alarms, and the operation and maintenance personnel adjust the access direction of the photovoltaic panel components on the corresponding branch; if the input voltage is greater than or equal to the set threshold, it is determined that the string voltage is mismatched, the inverter is connected to the grid and a network report prompt is issued, wherein the voltage mismatch prompt includes the branch information where current reverse occurs, and the operation and maintenance personnel increase, decrease or repair the photovoltaic panel components on the corresponding branch according to the voltage mismatch prompt.

[0057] In one embodiment of the present invention, the alarm information includes the serial number of the reverse-connected string. After the grid-connected output is stopped, only the branch with reverse current is disconnected and step S1 is restarted. Figure 7 , which is suitable for detection systems with added shutdown devices. The inverter can shut down the reverse-connected strings through the communication system. After the reverse-connected strings are shut down, the remaining strings can work normally. At the same time, it also needs to alarm for reverse connection to facilitate maintenance.

[0058] The existing reverse connection judgment mechanism only detects branch current to determine whether the branch current is reversed. If the current reverses, it is judged as string reverse connection and an operation and maintenance prompt is issued. It does not judge the component voltage, ignores the component mismatch condition, causes misjudgment, leads to unnecessary downtime, causes unnecessary losses to users, and reduces the efficiency of photovoltaic power generation. The detection system and detection method for parallel string reverse connection of photovoltaic systems provided by the present invention can judge the parallel string voltage mismatch condition by detecting branch current and boost voltage, distinguish the string voltage mismatch condition, and thus accurately judge whether it is reversed, prompt the user to make corrections, improve operation and maintenance efficiency, and improve photovoltaic power generation efficiency.

[0059] It should be noted that the peak and valley periods of electricity consumption vary with the charging and discharging of energy storage stations, maintaining a dynamic and stable electricity consumption, essentially balancing supply and demand. The aforementioned charging and discharging method coordinates peak-shaving and valley-filling for the power grid, resulting in higher economic benefits and a win-win situation for both parties.

[0060] The present invention provides a simple and easy way to control the temperature of photovoltaic cells in a reverse-connected parallel string manner, thereby ensuring the accuracy of the control and the health of the battery.

[0061] The above description is only a preferred embodiment of the present invention and does not limit the scope of the patent. Any equivalent structure or equivalent process transformation made by using the contents of the description and drawings of the present invention, directly or indirectly applied in other related technical fields, is also included in the scope of patent protection of the present invention.

Claims

1. A detection system for reverse connection of parallel strings in a photovoltaic system, wherein the photovoltaic system comprises a photovoltaic panel assembly and an inverter, wherein a plurality of photovoltaic panel assemblies are connected in series to form a string, and the plurality of strings are connected in parallel through the inverter for grid-connected output, characterized in that: The detection system includes a current sampling unit, a voltage sampling unit, and an MPPT device. The current sampling unit is provided on the branch where the string is located to obtain the current flow direction of the branch. The voltage sampling unit is connected to the trunk line formed by the parallel strings. The MPPT device is communicatively connected with the current sampling unit and the voltage sampling unit. The MPPT device can adjust the output power of the inverter according to the circuit status of the parallel strings. If the current sampling unit detects that the current flow directions of all branches are forward, the MPPT device controls the inverter to output power in a grid-connected manner; If the current sampling unit detects a branch with reverse current, and the voltage detected by the voltage sampling unit is less than a set threshold, the MPPT device controls the inverter to stop grid-connected output; Both ends of the photovoltaic panel assembly on the branch are connected in parallel with a first diode, and the set threshold is greater than the maximum value of the sum of the conduction voltage drops of the corresponding first diodes on all branches, and is less than or equal to the startup voltage of the inverter.

2. The detection system for reverse connection of parallel strings in a photovoltaic system according to claim 1, characterized in that: If the current sampling unit detects a branch with current reverse, and the voltage detected by the voltage sampling unit is greater than or equal to a set threshold, the MPPT device controls the inverter to output in grid connection and issues a voltage mismatch prompt.

3. The detection system for reverse connection of parallel strings in a photovoltaic system according to claim 1, characterized in that: Each branch has the same number of photovoltaic panel assemblies connected in series, and the first diodes connected in parallel on each photovoltaic panel assembly have the same conduction voltage drop. The set threshold is greater than the product of the conduction voltage drop of a single first diode and the number of first diodes on a single branch.

4. The detection system for reverse connection of parallel strings in a photovoltaic system according to claim 1, characterized in that: The first diode is directly connected to both ends of the corresponding photovoltaic panel assembly.

5. The detection system for reverse connection of parallel strings in a photovoltaic system according to claim 1, characterized in that: It also includes a second diode, which is reverse-connected and connected in parallel at both ends of the MPPT device, and the voltage sampling unit is simultaneously connected in parallel at both ends of the second diode.

6. The parallel string reverse connection detection system for a photovoltaic system according to claim 1, characterized in that: If n strings are connected in parallel, where n is a positive integer greater than 1, the current sampling unit is provided on each branch, or the current sampling unit is provided on n-1 branches respectively and also on the main line.

7. The parallel string reverse connection detection system for a photovoltaic system according to claim 1, characterized in that: It also includes a shut-off device, which is connected in series on each branch. If the current sampling unit detects a branch with current reverse and the voltage detected by the voltage sampling unit is less than a set threshold, the inverter controls the shut-off device on the branch with current reverse to start, so as to disconnect the branch with current reverse.

8. A detection method based on the detection system according to any one of claims 1 to 7, characterized in that: include: S1. Determine whether a branch with reverse current exists based on the current data collected by the current sampling unit. If no branch with reverse current exists, execute step S2; otherwise, execute step S3. S2, the MPPT device controls the inverter to output the grid; S3. Determine whether the sampled voltage exceeds a set threshold value based on the voltage data collected by the voltage sampling unit. If it is less than the set threshold value, execute step S4; otherwise, execute step S5. S4, the MPPT device controls the inverter to stop grid-connected output and issues an alarm; S5. The MPPT device controls the inverter to output the grid and provides a voltage mismatch prompt.

9. The detection method according to claim 8, characterized in that In step S4, after the grid-connected output is stopped, only the branch where the current reverse occurs is disconnected, and step S1 is restarted.

10. The detection method according to claim 8, characterized in that In step S5, the voltage mismatch prompt includes information about the branch where current reversal occurs, and the operation and maintenance personnel increase, decrease or repair the photovoltaic panel components on the corresponding branch according to the voltage mismatch prompt.

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