Centralized photovoltaic power generation system and control method therefor

Abstract

Provided are a centralized photovoltaic power generation system and a control method therefor. The system comprises an inverter and a plurality of combiner boxes; the inverter comprises an inverter control unit, a second direct current switch, an inverter module, and an alternating current switch; each combiner box comprises a combiner box control unit, a first direct current switch, a voltage measurement unit, and a first branch current measurement unit; the voltage measurement unit is configured to measure the output voltage at the other end of the first direct current switch; the first branch current measurement unit is configured to measure the branch current of the combiner box; and the combiner box control unit is separately communicationally connected to the voltage measurement unit, the first branch current measurement unit, the inverter control unit, and the first direct current switch, and is configured to determine a short-circuit fault on the basis of the output voltage and the branch current of each combiner box, and when a short-circuit fault occurs, to turn off the first direct current switch, control the inverter module to stop working, and turn off the second direct current switch and the alternating current switch. By detecting arcing and short-circuit faults, protection is provided, thereby improving system safety.

Description

A centralized photovoltaic power generation system and its control method

[0001] Cross-reference to related applications

[0002] This disclosure claims priority to Chinese patent application CN202410999539.6, filed on July 24, 2024, entitled “A centralized photovoltaic power generation system and its control method,” the entire contents of which are incorporated herein by reference. Technical Field

[0003] This disclosure mainly relates to the field of photovoltaic power generation technology, specifically to a centralized photovoltaic power generation system and its control method. Background Technology

[0004] In centralized photovoltaic power generation systems, the transmission lines between the DC combiner box and the inverter distribution unit are long, have many connection points, and operate at high voltage. Loose connections, poor contact, aging insulation materials, damp or corroded wires, and damaged insulation materials are common occurrences in these lines, easily leading to DC arcing, open flames, and even fires. This presents a technical problem of poor safety in centralized photovoltaic power generation systems.

[0005] Summary of the Invention

[0006] To address the technical problems in certain situations, this disclosure provides a low-cost, safe, and reliable centralized photovoltaic power generation system and its control method.

[0007] To solve the above-mentioned technical problems, the technical solution proposed in this disclosure is as follows: a centralized photovoltaic power generation system, including a centralized photovoltaic inverter and multiple combiner boxes; the centralized photovoltaic inverter includes an inverter control unit, a second DC switch, an inverter module, and an AC switch; the second DC switch, the inverter module, and the AC switch are connected in series; the inverter control unit is communicatively connected to the second DC switch, the inverter module, and the AC switch; each combiner box includes a combiner box control unit, a first DC switch, a voltage detection unit, and a first branch current detection unit; one end of the first DC switch is connected to the output end of the photovoltaic module, and the other end of the first DC switch is connected to the second DC switch through a DC transmission line. The system includes: a voltage detection unit configured to detect the output voltage at the other end of the first DC switch and send it to the combiner box control unit; a first branch current detection unit configured to detect the current in each combiner box branch and send it to the combiner box control unit; and a combiner box control unit communicatively connected to the voltage detection unit, the first branch current detection unit, the inverter control unit, and the first DC switch, configured to determine a short-circuit fault in the DC transmission line based on the output voltage and the current in each combiner box branch, and to disconnect the first DC switch and send a corresponding fault signal to the inverter control unit when a short-circuit fault occurs in the DC transmission line, thereby controlling the inverter module to stop working and disconnecting the second DC switch and the AC switch.

[0008] In an exemplary embodiment, the centralized photovoltaic inverter further includes a second branch current detection unit, which is connected to the inverter control unit and configured to detect the branch current signal at the input terminal of the centralized photovoltaic inverter and send it to the inverter control unit. When the inverter control unit determines that there is a reverse current in the branch at the input terminal of the inverter and the reverse current value is greater than a preset standard current value, it determines that a short circuit fault has occurred in the DC transmission line.

[0009] In an exemplary embodiment, both the first branch current detection unit and the second branch current detection unit are Hall sensors.

[0010] In one exemplary embodiment, a ripple detection unit is further included, configured to detect voltage ripple at the other end of the first DC switch and send it to the combiner box control unit.

[0011] In one exemplary embodiment, the ripple detection unit is a current transformer.

[0012] In one exemplary embodiment, the combiner box further includes a switching power supply with energy storage function. The input terminal of the switching power supply is connected to the other end of the first DC switch, and the output terminal is connected to the combiner box control unit and the first DC switch, respectively. It is configured to provide the required power when a short circuit occurs in the line and the voltage drops to zero volts.

[0013] In one exemplary embodiment, the centralized photovoltaic inverter further includes a filter module connected in series between the inverter module and the AC switch.

[0014] In one exemplary embodiment, both the first DC switch and the second DC switch are DC switches with shunt trip.

[0015] In one exemplary embodiment, the communication between each combiner box control unit, and between the combiner box control unit and the inverter control unit, is via CAN bus, RS485, PLC power line carrier, ZigBee, or LoRa communication.

[0016] This disclosure also discloses a control method based on the above-mentioned centralized photovoltaic power generation system, including a short-circuit fault detection and protection method: the combiner box control unit obtains the output voltage of the combiner box through a voltage detection unit and obtains the branch current of each combiner box through a first branch current detection unit; when the output voltage of the combiner box is below the lower limit of the normal operating voltage of the inverter module, and the sum of the branch currents of each combiner box is greater than the current preset value, it is determined that a short-circuit fault has occurred in the DC transmission line on the output side of the combiner box; when a short-circuit fault is determined in the DC transmission line, the combiner box control unit disconnects the first DC switch and sends a corresponding fault signal to the inverter control unit, the inverter control unit controls the inverter module to stop working and disconnects the second DC switch and the AC switch; at the same time, the inverter control unit feeds back the fault information to other combiner box control units, and after receiving the fault information, the other combiner box control units disconnect the corresponding first DC switch.

[0017] In an exemplary embodiment, the branch current signal at the input terminal of the centralized photovoltaic inverter is obtained through the second branch current detection unit; when the inverter control unit determines that there is a reverse current in the branch at the input terminal of the inverter and the reverse current value is greater than the preset standard current value, it determines that a short circuit fault has occurred in the DC transmission line.

[0018] In one exemplary embodiment, the method further includes an arcing fault detection and protection method: the combiner box control unit acquires the voltage ripple value and output current ripple value of the combiner box; when the output voltage ripple value is greater than a preset standard voltage ripple value and the output current ripple value is greater than a preset standard current ripple value, an arcing fault is determined to have occurred in the DC transmission line; when an arcing fault is determined in the DC transmission line, the combiner box control unit disconnects the first DC switch and sends a corresponding fault signal to the inverter control unit, which controls the inverter module to stop working and disconnects the second DC switch and the AC switch; simultaneously, the inverter control unit feeds back fault information to other combiner box control units, which disconnect the corresponding first DC switch after receiving the fault information.

[0019] In one exemplary embodiment, the preset standard voltage ripple value is twice the DC voltage ripple value under normal conditions of the combiner box.

[0020] In one exemplary embodiment, the preset standard current ripple value is twice the DC current ripple value under normal conditions of the combiner box. Attached Figure Description

[0021] Figure 1 is a topology diagram of the DC combiner box in an embodiment of this disclosure.

[0022] Figure 2 is a topology diagram of the centralized inverter in this disclosure in an embodiment.

[0023] Figure 3 is a topology diagram of the centralized photovoltaic system in an embodiment of this disclosure.

[0024] Figure 4 is a flowchart of one of the embodiments of the short-circuit fault detection and protection method of this disclosure.

[0025] Figure 5 is a schematic diagram of a short-circuit fault in this disclosure.

[0026] Figure 6 is a flowchart of the short-circuit fault detection and protection method in this disclosure in one of the embodiments.

[0027] Figure 7 is a flowchart of the arc fault detection and protection method of this disclosure in an embodiment. Detailed Implementation

[0028] The present disclosure will be further described below with reference to the accompanying drawings and specific embodiments.

[0029] In centralized photovoltaic (PV) power generation systems, the transmission lines between the DC combiner box and the inverter distribution unit are long, have numerous connection points, and operate at high voltage. Loose connections, poor contact, aging insulation, dampness, corrosion, and damaged insulation are common occurrences in these lines, easily leading to DC arcing, open flames, and even fires. This presents a technical challenge regarding the poor safety of the DC side in centralized PV power generation systems. Therefore, improving the safety of the DC side in centralized PV power generation systems is an urgent problem to be solved.

[0030] Existing technical solutions for improving the DC-side safety of centralized photovoltaic power generation systems include:

[0031] Patent application CN 111740392A discloses a fault protection method, an intelligent combiner box, and a photovoltaic inverter. This fault protection method first collects the branch current and output voltage of the intelligent combiner box through a control unit in the intelligent combiner box, and determines whether an arcing fault or short-circuit fault has occurred on the DC side of the intelligent combiner box based on the collection results. If an arcing fault or short-circuit fault occurs on the DC side of the intelligent combiner box, the control unit in the intelligent combiner box controls the DC switch on the output side of the intelligent combiner box to disconnect, and sends the fault information to the control unit in the inverter. The control unit in the inverter, based on the fault information, controls the inverter to shut down and disconnects the DC switch and AC switch in the inverter. Therefore, in the event of a short-circuit fault or arcing fault, the interlocking protection of the intelligent combiner box and the inverter can avoid the serious arcing or even fire problems that can easily occur in existing photovoltaic power generation systems when the DC voltage is too high.

[0032] If the output voltage of the intelligent combiner box drops to a preset range including zero voltage within a preset time, and the branch current increases to a preset range including the component short-circuit current within a preset time, then a short-circuit fault is determined to have occurred on the DC side of the intelligent combiner box. If the output voltage fluctuates back and forth at least twice within a preset range including open-circuit voltage and a preset range including zero voltage, and the branch current fluctuates back and forth at least twice within a preset range including the component short-circuit current and a preset range including zero voltage, then an arcing fault is determined to have occurred on the DC side of the intelligent combiner box.

[0033] This scheme uses the back-and-forth fluctuations of the combiner box output voltage and branch current as the basis for the occurrence of arcing faults on the DC side. However, the working principle of MPPT requires disturbance of DC voltage. In addition, the influence of external environment such as cloudy weather can cause fluctuations in DC voltage and current. Therefore, this method is prone to misjudgment.

[0034] In addition, in centralized photovoltaic power generation systems, multiple combiner box outputs are combined through the DC busbar on the inverter side. When an arcing short circuit fault occurs in a single combiner line, only the faulty combiner box and the inverter switch are disconnected, while the switches of other combiner boxes remain open, and the safety risk is not eliminated.

[0035] Patent application CN218997681U discloses a DC arc-cutting system for fire prevention in new energy equipment. The system includes an arc-light sensor, a combiner box, an inverter, and a DC circuit breaker. The combiner box is connected to the inverter, and both the combiner box and the inverter are equipped with arc-light sensors and DC circuit breakers. The DC circuit breaker has a shunt trip unit. This system can promptly cut off the upstream DC power supply, extinguishing the arc and preventing equipment burnout.

[0036] This solution identifies DC arcs by installing arc light sensors inside the combiner box and inverter. However, the detection range is limited to the combiner box and inverter equipment and cannot detect faults in the cable connection between the combiner box and the inverter.

[0037] In addition, the solution has the following shortcomings in some situations:

[0038] 1. Photovoltaic modules have small short-circuit currents, typically less than 1.1 times the module's maximum operating current. However, the DC circuit breakers in combiner boxes are usually set with larger currents to prevent malfunctions. When a short-circuit fault occurs in the combiner box output circuit, the DC circuit breaker cannot trip to cut off the circuit, thus causing the combiner box to continuously output DC energy to the short-circuit point, maintaining the arc combustion.

[0039] 2. Although a DC arc generates significant voltage and current changes, it does not cause a current overload. Therefore, traditional protection devices cannot detect and protect against DC arc faults.

[0040] 3. In traditional centralized photovoltaic power generation systems, the combiner box mainly serves to combine and transmit DC current in the branches of the photovoltaic module strings, and to detect and transmit DC current and voltage signals in the branches. The circuit breaker in the combiner box does not have a disconnect function, and there is no safety protection linkage mechanism between it and the inverter, so it cannot effectively isolate DC hazards from the DC transmission line.

[0041] As shown in Figure 3, the centralized photovoltaic power generation system of this disclosure includes a centralized photovoltaic inverter and multiple combiner boxes.

[0042] The centralized photovoltaic inverter includes an inverter control unit, a second DC branch fuse (Fuin1+, Fuin1-...FuinN+, FuinN-), a second DC switch (DC switch S2 with shunt trip), an inverter module (DC / AC inverter), a filter module (filter), and an AC switch S3; the second DC branch fuse, the second DC switch, the inverter module, the filter, and the AC switch are connected in series; the inverter control unit is communicatively connected to the second DC switch, the inverter module, and the AC switch, as shown in Figure 2.

[0043] Each combiner box includes a combiner box control unit, a first DC branch fuse (Fu1+, Fu1-...FuN+, FuN-), a first branch current detection unit, a first DC switch (with shunt trip DC switch S1), and a voltage detection unit; one end of the first DC switch is connected to the output terminal of the photovoltaic module through the first DC branch fuse, and the other end of the first DC switch is connected to the second DC switch through a DC transmission line, as shown in Figure 1; the combiner box control units of each combiner box are interconnected.

[0044] The voltage detection unit is configured to detect the output voltage at the other end of the first DC switch (i.e., the combiner box output voltage) and send it to the combiner box control unit.

[0045] The first branch current detection unit (including branch current sensors Hall11, Hall12...Hall1N) is configured to detect the branch current of each combiner box and send it to the combiner box control unit.

[0046] The combiner box control unit is communicatively connected to the voltage detection unit, the first branch current detection unit, the inverter control unit, and the first DC switch. It is configured to determine the DC transmission line short circuit fault based on the combiner box output voltage and branch current signal (the specific judgment process is described in the method). When the DC transmission line short circuit fault occurs, it disconnects the first DC switch and sends a corresponding fault signal to the inverter control unit to control the inverter module to stop working and disconnect the second DC switch and the AC switch.

[0047] At the same time, the inverter control unit feeds back fault information to other combiner box control units. After receiving the fault information, the other combiner box control units disconnect the corresponding first DC switch.

[0048] In one exemplary embodiment, the centralized photovoltaic inverter further includes a second branch current detection unit (including branch current sensors Hall21, Hall22...Hall2N). The second branch current detection unit is connected to the inverter control unit and configured to detect the branch reverse current signal at the input of the centralized photovoltaic inverter and send it to the inverter control unit. In one exemplary embodiment, as shown in Figures 5 and 6, when a short-circuit fault occurs in a DC combiner box to the inverter power distribution transmission line, the combiner box outputs continuous DC energy to the short-circuit point, causing an electric arc. The arc may result in an open circuit in section ab while section cd remains short-circuited. At this time, the current from other combiner branches of the inverter's DC collection line flows in reverse to the short-circuit point. In this situation, the second branch current detection unit detects the branch current signal at the input of the centralized photovoltaic inverter and sends it to the inverter control unit. When the inverter control unit determines that there is a reverse current in the inverter's branch and that the reverse current value is greater than a preset standard current value, it determines that a short-circuit fault has occurred in the DC transmission line. At this time, the inverter control unit controls the inverter to stop by outputting a PWM signal, then disconnects the DC switch S2 and the AC switch S3, and the inverter control unit feeds back the fault information to the DC combiner box through the communication line. After receiving the fault information, the control unit of the combiner box connected to the combiner line disconnects the DC switch S1.

[0049] The standard current value mentioned above can be taken as 10 times the zero-drift value of the second branch current detection unit (inverter current sensor). Of course, the specific value can be determined according to the actual situation, and can be selected from 8-12 times the zero-drift value.

[0050] In another exemplary embodiment, a ripple detection unit is further included, configured to detect the voltage ripple at the other end of the first DC switch and send it to the combiner box control unit for subsequent arcing determination (see the subsequent method description for details). The ripple detection unit uses a current transformer CT1, as shown in Figure 3. The current transformer operates on the principle of electromagnetic induction, blocking DC while transmitting AC, and is configured for AC measurement. Compared to normal operating conditions, when arcing occurs, there is ripple in the DC current. Determining arcing only requires detecting the ripple shunt, not the DC component. Therefore, using a current transformer on the DC side, detecting only the AC component, is ideal for ripple measurement, resulting in a simple structure and easy operation.

[0051] In one exemplary embodiment, considering that the DC voltage may drop to zero volts when a short circuit occurs in the line, the power supply of the combiner box control unit adopts a switching power supply with energy storage function. The input terminal of the switching power supply is connected to the other end of the first DC switch, and the output terminal is connected to the combiner box control unit and the first DC switch respectively, so as to ensure that the power required for communication of the combiner box control unit and the operation of the DC switch trip unit can be provided for a certain period of time in the event of a short circuit in the DC line.

[0052] In one exemplary embodiment, the communication methods between the combiner box control units and between the combiner box control units and the inverter control unit are not limited, and can be any one of the following communication methods: CAN bus, RS485, PLC power line carrier, ZigBee, LoRa, or other communication methods. The communication mechanism between the combiner box control units and the inverter control unit is not limited, and can be any one of the following communication mechanisms: broadcast, polling, or other communication mechanisms.

[0053] In the aforementioned centralized photovoltaic power generation system, there is no limit to the number of lines experiencing short circuits or arcing faults in the DC combiner box to inverter power distribution transmission lines, and all of them are within the scope of protection of this disclosure.

[0054] Based on the condition that the inverter stops working and the line operating current is zero when the DC voltage value is below the lower limit of the normal operating voltage, this disclosure uses the combiner box control unit to collect and determine that when the combiner box output voltage is below the lower limit of the inverter's normal operating voltage and the sum of the combiner box branch currents is greater than a preset current value, it determines that a short circuit fault has occurred in the DC transmission line and disconnects the combiner box switch. At the same time, the inverter control unit determines that the reverse current value of the inverter's DC input branch is greater than a preset value, determines that a short circuit fault has occurred in the DC transmission line, controls the inverter to stop and disconnects the inverter switch, thus ensuring the safety of the system.

[0055] This disclosure discloses a combiner box circuit breaker switch and inverter DC and AC switches equipped with shunt trip units, developing a safety linkage mechanism between the inverter and the connected combiner box. In the event of a line arcing or short-circuit fault, fault information is transmitted via the communication link between the combiner box control unit and the inverter control unit, disconnecting the inverter switch and the combiner box switch connected to the DC bus line, effectively isolating the fault. Through the safety linkage between the combiner box and the inverter, in the event of a line arcing or short-circuit fault, the inverter control unit controls the inverter to shut down and disconnects the inverter switch, while the combiner box control unit controls the combiner box switch connected to the DC bus line.

[0056] This disclosure identifies faults in the combiner box and inverter, as well as electrical circuit faults between them, based on the analysis of DC side voltage and current signals of the combiner box and inverter. It has a wide identification range and does not require the installation of an arc detection device, thus reducing costs. Through the safety linkage between the combiner box and inverter, once an arcing or short-circuit fault occurs, the inverter control unit controls the inverter to stop and disconnect the inverter switch, and the combiner box control unit controls the switch connected to the combiner box of the DC collection line, making the protection mechanism more comprehensive.

[0057] The control method based on the centralized photovoltaic power generation system described above in this disclosure includes a short-circuit fault detection and protection method and an arcing fault detection and protection method.

[0058] As shown in Figure 4, the specific process of short-circuit fault detection and protection is as follows: The short-circuit current of photovoltaic modules is small, generally within 1.1 times the maximum operating current of the modules, while the DC circuit breaker switch of the combiner box output is generally set with a large current to prevent malfunction. When a short-circuit fault occurs in the output circuit of the combiner box, the DC circuit breaker cannot trip to cut off the circuit, thus causing the combiner box to output continuous DC energy to the short-circuit point to maintain the arc burning.

[0059] Photovoltaic inverters can only operate within the normal DC voltage range. For example, in a DC 1500V system with an AC output voltage of 600V, the normal DC operating range of the inverter is 875V to 1500V. Below the lower limit of the DC operating voltage of 875V, the inverter cannot operate normally, and the operating current will be zero.

[0060] If a short circuit occurs between the positive and negative poles of the power transmission line from the DC combiner box to the inverter, the instantaneous voltage at both ends of the line will drop to zero when the short circuit point is closed. When a discharge gap occurs, the voltage at both ends of the line will fluctuate with the length of the gap, and its value is generally maintained at tens of volts, which is much lower than the lower limit of the inverter's DC operating voltage.

[0061] In one exemplary embodiment, as shown in FIG4, the combiner box output voltage and the current signal of each combiner box branch are acquired by the combiner box control unit;

[0062] If the combiner box output voltage is below the lower limit of the inverter's normal operating voltage, and the sum of the combiner box branch currents is greater than the preset current value, then a short circuit fault is determined to have occurred in the DC transmission line on the output side of the combiner box. At this time, the combiner box control unit controls the first DC switch S1 of the combiner box to open, and feeds back the fault to the inverter control unit through the communication line. After receiving the fault information, the inverter control unit controls the inverter to stop through the PWM signal output, and then opens the second DC switch S2 and the AC switch S3.

[0063] At the same time, the inverter control unit feeds back the fault information to the other combiner box control units connected to the DC combiner line through the communication line. After receiving the fault information, the other combiner box control units disconnect the first DC switch S1.

[0064] When the DC operating voltage is below the lower limit of 875V, the inverter does not work and the operating current is zero. The output current can be preset to 10 times the zero drift value of the output current transformer.

[0065] Considering that the DC voltage may drop to zero volts when a short circuit occurs in the line, the power supply of the combiner box control unit adopts a switching power supply with energy storage function. The input terminal of the switching power supply is connected to the other end of the first DC switch S1, and the output terminal is connected to the combiner box control unit and the first DC switch S1 respectively, so as to ensure that the power required for communication of the combiner box control unit and the operation of the DC switch trip unit can be provided for a certain period of time in the event of a short circuit in the DC line.

[0066] In a switching power supply with energy storage function, twice the communication cycle between the combiner box control unit and the inverter control unit can be taken as the energy storage energizing time to ensure communication reliability when a short circuit occurs in the line.

[0067] In an exemplary embodiment, as shown in Figure 5, when a short-circuit fault occurs in a DC combiner box to inverter power distribution transmission line, the combiner box continuously outputs DC energy to the short-circuit point, causing an electric arc. The arc may result in an open circuit in section ab while section cd remains short-circuited. At this time, the current from other combiner branches of the inverter's DC collection line flows in reverse to the short-circuit point. In this situation, the second branch current detection unit detects the reverse current signal of the branch at the input of the centralized photovoltaic inverter and sends it to the inverter control unit. When the inverter control unit determines that the inverter current signal (DC input branch reverse current value) is greater than a preset standard current value, it determines that a short-circuit fault has occurred in the DC transmission line. At this time, the inverter control unit controls the inverter to shut down via a PWM signal output, then disconnects the second DC switch S2 and the AC switch S3. The inverter control unit also feeds back fault information to the DC collection line combiner box via a communication line. Upon receiving the fault information, the control unit of the combiner box connected to the collection line disconnects the first DC switch S1.

[0068] The standard current value mentioned above can be taken as 10 times the zero-drift value of the second branch current detection unit (inverter current sensor). Of course, the specific value can be determined according to the actual situation, and can be selected from 8-12 times the zero-drift value.

[0069] As shown in Figure 7, the specific process of arc fault detection and protection is as follows: During the generation of DC arc, although there will be obvious changes in voltage and current, especially the current fluctuation amplitude is larger than that when there is no fault, and the ripple amplitude increases significantly, but there will be no current overload phenomenon. Therefore, traditional protection devices cannot play a role in detecting and protecting DC arc faults.

[0070] The combiner box control unit of this disclosure obtains the voltage ripple value by detecting the output voltage through the voltage detection unit, and collects the current ripple through the ripple detection unit (current transformer CT1). Arc faults are determined based on the voltage ripple and current ripple. In an exemplary embodiment, as shown in FIG7, the combiner box control unit of this disclosure directly collects the AC component of the combiner box output current and calculates the current ripple value.

[0071] When the output voltage ripple value exceeds the preset standard voltage ripple value, and the output current ripple value also exceeds the preset standard current ripple value, an arcing fault is determined to have occurred in the DC transmission line. At this time, the combiner box control unit controls the first DC switch S1 to open and feeds back the fault to the inverter control unit via the communication line. After receiving the fault information, the inverter control unit controls the inverter to stop via a PWM signal output, then opens the second DC switch S2 and the AC switch S3, and feeds back the fault information to other combiner box control units on the DC side of the DC bus line via the communication line. Upon receiving the fault information, the other combiner box control units open the corresponding first DC switch S1.

[0072] In one exemplary embodiment, during the generation of a DC arc, significant changes in voltage and current occur, particularly in the current fluctuation amplitude, which is larger than during fault-free conditions, and the ripple amplitude increases substantially. For ease of detection, a typical 10k-100k arc current spectrum can be detected and the current ripple value calculated, with a signal sampling rate of 250k or higher configured.

[0073] In an exemplary embodiment, under normal operating conditions, no arcing fault occurs, and the inverter MPPT control output disturbance voltage will generate certain DC voltage ripple and current ripple. Therefore, the preset standard voltage ripple value for judging an arcing fault can be taken as twice the DC voltage ripple value under normal operating conditions, and the preset standard current ripple value can be taken as twice the DC current ripple value under normal operating conditions.

[0074] By using both voltage ripple and current ripple values, the accuracy of arc signal identification is improved.

[0075] The above-mentioned methods for extracting fault arc current are not limited, such as Fourier transform, wavelet transform, Wigner-Ville and other intelligent extraction methods.

[0076] This disclosure uses the combiner box control unit to collect the combiner box output voltage and combiner box branch current, and then uses the inverter control unit to collect the inverter DC distribution branch reverse current signal. The abnormal output operating voltage value of the combiner box, the combiner box branch current signal, and the inverter input branch reverse current value are used as the basis for judging the DC transmission line short circuit fault. The ripple value of the combiner box output voltage and the AC component value of the output current are used as the basis for judging the DC transmission line arcing fault. Through analysis and judgment, if the DC transmission line experiences DC arcing or short circuit fault, the fault information is transmitted through the communication link between the combiner box control unit and the inverter control unit. The inverter control unit controls the inverter to shut down and disconnects the inverter DC switch and AC switch, and the combiner box control unit disconnects the combiner switch, thereby improving system safety.

[0077] Because the frequency domain characteristics of DC voltage and DC current signals differ significantly under arc fault conditions, this disclosure discloses a combiner box control unit that directly acquires the output voltage and acquires the output current through a current transformer. Arc faults are determined based on the frequency domain characteristics of the combiner box output voltage and current signals. By identifying DC voltage ripple and current ripple components as the basis for DC arcing, system safety and the accuracy of arcing detection are improved.

[0078] Compared to some other situations, the advantages of this disclosure are as follows: This disclosure collects the combiner box output voltage and combiner box branch current through the combiner box control unit, and then collects the reverse current signal of the inverter DC distribution branch through the inverter control unit. The abnormal output operating voltage value of the combiner box, the combiner box branch current signal, and the reverse current value of the inverter input branch are used as the basis for judging the DC transmission line short circuit fault; the ripple value of the combiner box output voltage and the AC component value of the output current are used as the basis for judging the DC transmission line arcing fault; through analysis and judgment, if a DC arcing or short circuit fault occurs in the DC transmission line, the fault information is transmitted through the communication link between the combiner box control unit and the inverter control unit. The inverter control unit controls the inverter to stop and disconnects the inverter DC switch and AC switch, and the combiner box control unit disconnects the combiner switch, thereby improving system safety. This disclosure discloses a combiner box control unit that directly acquires the output voltage and acquires the output current through a current transformer. It then uses the frequency domain characteristics of the combiner box output voltage and current signals to determine arc faults. By identifying DC voltage ripple and current ripple components as the basis for DC arcing detection, it improves system safety and the accuracy of arcing detection. This disclosure has advantages such as low cost, high safety and reliability, and effective detection and protection against line short circuits and arcing.

[0079] The above are merely preferred embodiments of this disclosure. The scope of protection of this disclosure is not limited to the above embodiments. All technical solutions falling within the scope of this disclosure are protected. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of this disclosure should be considered within the scope of protection of this disclosure.

Claims

1. A centralized photovoltaic power generation system, comprising a centralized photovoltaic inverter and a plurality of combiner boxes; The centralized photovoltaic inverter comprises an inverter control unit, a second DC switch, an inverter module and an AC switch;The second DC switch, the inverter module and the AC switch are connected in series;The inverter control unit is connected in communication with the second DC switch, the inverter module and the AC switch; Each of the combiner boxes comprises a combiner box control unit, a first DC switch, a voltage detection unit and a first branch current detection unit;One end of the first DC switch is connected to the output end of the photovoltaic module, and the other end of the first DC switch is connected to the second DC switch through a DC transmission line; The voltage detection unit is configured to detect the output voltage of the other end of the first DC switch and send it to the combiner box control unit; The first branch current detection unit is configured to detect the branch current of each combiner box and send it to the combiner box control unit; The combiner box control unit is connected in communication with the voltage detection unit, the first branch current detection unit, the inverter control unit and the first DC switch, respectively, and is configured to determine the short-circuit fault of the DC transmission line according to the output voltage and the branch current of each combiner box, and when the DC transmission line has a short-circuit fault, the first DC switch is disconnected, and a corresponding fault signal is sent to the inverter control unit to control the inverter module to stop working and the second DC switch and the AC switch to be disconnected.

2. The centralized photovoltaic power generation system according to claim 1, wherein, The centralized photovoltaic inverter further comprises a second branch current detection unit connected to the inverter control unit, configured to detect the branch current signal of the input end of the centralized photovoltaic inverter and send it to the inverter control unit, and the inverter control unit determines that the DC transmission line has a short-circuit fault when the inverter input end has a reverse current and the reverse current value is greater than a preset standard current value.

3. The concentrated photovoltaic power system of claim 2, wherein, The first branch current detection unit and the second branch current detection unit are both Hall sensors.

4. The concentrated photovoltaic power system of claim 1 or 2 or 3, wherein, It further comprises a ripple detection unit configured to detect the voltage ripple of the other end of the first DC switch and send it to the combiner box control unit.

5. The concentrated photovoltaic power system of claim 4, wherein, The ripple detection unit is a current transformer.

6. The concentrated photovoltaic power system of claim 1 or 2 or 3, wherein, The combiner box further comprises a switching power supply with energy storage function, the input end of the switching power supply is connected to the other end of the first DC switch, the output end is connected to the combiner box control unit and the first DC switch, respectively, and is configured to provide the required power supply when the line has a short-circuit and the voltage drops to zero volts.

7. The concentrated photovoltaic power system of claim 1 or 2 or 3, wherein, The centralized photovoltaic inverter further comprises a filter module connected between the inverter module and the AC switch.

8. The concentrated photovoltaic power system of claim 1 or 2 or 3, wherein, The first DC switch and the second DC switch are both DC switches with split excitation tripping.

9. The concentrated photovoltaic power system of claim 1 or 2 or 3, wherein, The communication between the combiner box control units and between the combiner box control unit and the inverter control unit is CAN bus, RS485, PLC power carrier, ZigBee or Lora communication mode. 10.A control method of the centralized photovoltaic power generation system according to any one of claims 1-9, comprising a short-circuit fault detection and protection method: The output voltage of the combiner box is obtained by the voltage detection unit, and the branch current of each combiner box is obtained by the first branch current detection unit; When the output voltage of the combiner box is below the lower limit of the normal working voltage of the inverter module, and the sum of the branch currents of each combiner box is greater than the preset current value, it is judged that a short circuit fault occurs in the DC transmission line on the output side of the combiner box; When the short circuit fault of the DC transmission line is judged, the combiner box control unit disconnects the first DC switch, and sends a corresponding fault signal to the inverter control unit, and the inverter control unit controls the inverter module to stop working and disconnects the second DC switch and the AC switch; At the same time, the inverter control unit feeds back the fault information to other combiner box control units, and after receiving the fault information, the other combiner box control units disconnect the corresponding first DC switch.

11. The control method of a centralized photovoltaic power generation system according to claim 10, wherein The branch current signal of the input end of the centralized photovoltaic inverter is obtained by the second branch current detection unit; When the inverter control unit judges that there is a reverse current at the input end of the inverter and the value of the reverse current is greater than the preset standard current value, it is judged that a short circuit fault occurs in the DC transmission line.

12. The control method of a centralized photovoltaic power generation system according to claim 10 or 11, wherein, It also includes an arc fault detection and protection method: The voltage ripple value and the output current ripple value of the combiner box are obtained by the combiner box control unit; When the output voltage ripple value is greater than the preset standard voltage ripple value, and the output current ripple value is greater than the preset standard current ripple value, it is judged that an arc fault occurs in the DC transmission line; When the arc fault of the DC transmission line is judged, the combiner box control unit disconnects the first DC switch, and sends a corresponding fault signal to the inverter control unit, and the inverter control unit controls the inverter module to stop working and disconnects the second DC switch and the AC switch; At the same time, the inverter control unit feeds back the fault information to other combiner box control units, and after receiving the fault information, the other combiner box control units disconnect the corresponding first DC switch.

13. The control method of a centralized photovoltaic power generation system according to claim 12, wherein, The preset standard voltage ripple value is twice the DC voltage ripple value under normal conditions of the combiner box.

14. The control method of a centralized photovoltaic power generation system according to claim 12, wherein, The preset standard current ripple value is twice the DC current ripple value under normal conditions of the combiner box.

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