Photovoltaic inverter and derating control method for a photovoltaic inverter

Abstract

The embodiment of the present application provides a photovoltaic inverter and a derating control method of the photovoltaic inverter. The photovoltaic inverter comprises an inverter circuit, a plurality of direct-current conversion circuits and a controller. The plurality of direct-current conversion circuits are used for outputting direct-current electricity from a photovoltaic module to the inverter circuit after direct-current conversion. The plurality of direct-current conversion circuits comprise a first direct-current conversion circuit and a second direct-current conversion circuit. The inverter circuit is used for converting the direct-current electricity from the plurality of direct-current conversion circuits into alternating-current electricity. When the photovoltaic inverter is in a derating mode, the input voltage of the first direct-current conversion circuit is increased to reduce the output power of the first direct-current conversion circuit, so that the output power of the inverter circuit is reduced, wherein the input voltage of the first direct-current conversion circuit is smaller than the input voltage of the second direct-current conversion circuit.

Description

Technical Field

[0001] This application relates to the field of power electronics, and more particularly to a photovoltaic inverter and a derating control method for a photovoltaic inverter. Background Technology

[0002] In photovoltaic (PV) systems equipped with PV inverters, when the DC bus voltage of the PV inverter is too high, the difference between the grid voltage and the DC bus voltage is too large, or the internal temperature of the PV inverter is too high, the PV inverter usually needs to be derated. This means reducing the output power of the PV inverter to ensure stable operation under special conditions and extend the service life of the equipment. In actual derating, the voltage on the DC bus of the PV inverter can easily rise. This increased bus voltage leads to increased stress on the switching transistors in the PV inverter, resulting in increased heat loss on the switching transistors. Summary of the Invention

[0003] This application provides a photovoltaic inverter and a derating control method for the photovoltaic inverter, which can reduce the fluctuation range of the bus voltage in the photovoltaic inverter while meeting the derating requirements of the photovoltaic inverter.

[0004] In a first aspect, this application provides a photovoltaic inverter, which includes an inverter circuit, multiple DC-DC conversion circuits, and a controller. The multiple DC-DC conversion circuits convert DC power from photovoltaic modules into AC power and output it to the inverter circuit. The multiple DC-DC conversion circuits include a first DC-DC conversion circuit and a second DC-DC conversion circuit. The inverter circuit converts the DC power from the multiple DC-DC conversion circuits into AC power. When the photovoltaic inverter is in derating mode, the controller controls the input voltage of the first DC-DC conversion circuit to increase, thereby reducing the output power of the first DC-DC conversion circuit, resulting in a decrease in the output power of the inverter circuit. The input voltage of the first DC-DC conversion circuit is less than the input voltage of the second DC-DC conversion circuit.

[0005] When a photovoltaic (PV) inverter is operating normally, to ensure high power generation, the DC-DC converter typically controls the corresponding PV string to operate at its maximum power point. The PV string consists of multiple PV modules connected in series and electrically connected to the DC-DC converter via PV terminals. When the PV inverter overheats due to a fault, to ensure its safe operation, the inverter's output power needs to be reduced, i.e., it enters derating mode. It's important to note that the PV inverter's output power equals the product of the grid voltage and the PV inverter's output current. Since the grid voltage remains relatively stable, the derating mode of the PV inverter is usually achieved by reducing the PV inverter's output current. This is reflected on the PV string's PV curve, where the operating point of the PV string shifts left or right from the maximum power point. When the operating point shifts left, the PV string's output voltage is lower than the voltage corresponding to the maximum power point; when the operating point shifts right, the PV string's output voltage is higher than the voltage corresponding to the maximum power point. Currently, the industry typically reduces the output power of a photovoltaic (PV) inverter by shifting the operating point of the PV string to the right, i.e., increasing the output voltage of the PV string (equivalent to the input voltage of the DC-DC converter). Since a PV inverter comprises multiple DC-DC converters, and each converter connects to different PV strings, the PV curves for each PV string corresponding to a DC-DC converter are usually not identical. Because the magnitude of the DC bus voltage in a PV inverter is directly related to the maximum input voltage of the multiple DC-DC converters, PV strings with lower maximum power point voltages, compared to those with higher maximum power point voltages, can achieve greater power derating by shifting the power point of PV strings with lower maximum power point voltages to the right, while maintaining a stable DC bus voltage.

[0006] Based on this, the photovoltaic inverter provided in this application, when entering derating mode, prioritizes controlling the photovoltaic strings with lower voltages corresponding to the maximum power point to shift the power point to the right, while keeping the output voltage of the photovoltaic strings less than or equal to the maximum input voltage among the current multiple DC-DC converter circuits. This enables more power derating while maintaining a stable DC bus voltage and avoids overstressing of the switching transistors in the inverter circuit. The DC bus is used to electrically connect multiple DC-DC converter circuits and the inverter circuit. It is worth mentioning that, for ease of understanding, the above description is from the perspective of the photovoltaic inverter switching from the normal state, i.e., the corresponding photovoltaic string operating at the maximum power point, to the derating mode for the first time. In actual applications, there may be situations where the photovoltaic inverter still overheats after one derating. In this case, it may be necessary to increase the output voltage of the photovoltaic string that has already undergone one rightward shift of the operating point, that is, to shift the operating point of the corresponding photovoltaic string further to the right. Accordingly, the photovoltaic string with the lower current output voltage is still prioritized to shift the operating point to the right, and the output voltage of the photovoltaic string is kept less than or equal to the maximum input voltage among the current multiple DC-DC converter circuits. Under the condition of maintaining a stable DC bus voltage, the photovoltaic string with the lower output voltage can achieve more power derating, avoiding overstressing of the switching transistors in the inverter circuit.

[0007] In one possible implementation, the plurality of DC-DC converter circuits includes at least one third DC-DC converter circuit, the input voltage of which is greater than the input voltage of the first DC-DC converter circuit, and the input voltage of the first DC-DC converter circuit is less than or equal to the average input voltage of the plurality of DC-DC converter circuits. Since the input voltage of the first DC-DC converter circuit is in a lower range, prioritizing the boosting of the input voltage of the first DC-DC converter circuit, compared to prioritizing the boosting of DC-DC converter circuits with input voltages higher than the average input voltage, allows the photovoltaic inverter to achieve greater power derating while maintaining a stable DC bus.

[0008] In one possible implementation, the first DC-DC converter circuit is the one with the lowest input voltage among multiple DC-DC converter circuits. Because the first DC-DC converter circuit has the lowest input voltage, the difference between its input voltage and the highest input voltage among the multiple DC-DC converter circuits is the largest compared to boosting the input voltages of the other DC-DC converter circuits. Prioritizing the boosting of the first DC-DC converter circuit's input voltage allows for maximum power derating while maintaining a stable DC bus. Furthermore, during the process of controlling the increase in the input voltage of the first DC-DC converter circuit, the input voltages of one or more other DC-DC converter circuits can be simultaneously controlled to increase, thereby improving the derating efficiency of the photovoltaic inverter.

[0009] In one possible implementation, the photovoltaic inverter includes a DC bus, and the outputs of multiple DC-DC converter circuits are connected in parallel and then connected to the input of the inverter circuit via the DC bus. When the temperature of the photovoltaic inverter is higher than or equal to a first threshold, or the DC bus voltage is higher than or equal to a second threshold, or the difference between the grid voltage and the DC bus voltage is higher than or equal to a third threshold, the photovoltaic inverter enters derating mode. That is, when overheating or internal overvoltage is detected in the photovoltaic inverter, the controller controls the photovoltaic inverter to enter derating mode to ensure the safety of the photovoltaic inverter during operation.

[0010] In one possible implementation, when the controller controls the input voltage of the first DC-DC converter to increase, it controls the input voltage of the second DC-DC converter to remain unchanged, so as to prevent the input voltage of the second DC-DC converter from exceeding the maximum input voltage among the multiple DC-DC converters, thereby preventing the bus voltage from increasing.

[0011] In one possible implementation, the controller controls the input voltage of the first DC-DC converter circuit to increase from the maximum power point of the corresponding connected photovoltaic (PV) string, thereby increasing the output voltage of the PV string connected to the first DC-DC converter circuit from the voltage corresponding to the maximum power point. If the output voltage of the PV string is controlled to decrease for derating, the output voltage of the PV string may drop to the shutdown voltage value of the PV inverter, leading to undervoltage at the PV inverter input and triggering shutdown, affecting the normal operation of the PV inverter. Therefore, controlling the output voltage of the PV string to increase from the voltage corresponding to the maximum power point can achieve power derating while avoiding undervoltage at the PV inverter input.

[0012] In one possible implementation, the controller is used to determine that the current output power of the photovoltaic inverter has reached the derating requirement when the output power of the photovoltaic inverter is less than a set threshold during the process of controlling the input voltage of the first DC-DC converter circuit to rise, and then control the photovoltaic inverter to exit the derating mode.

[0013] In one possible implementation, the controller, during the process of controlling the input voltage of the first DC-DC converter to increase, when the input voltage of the first DC-DC converter reaches the maximum input voltage among multiple DC-DC converters, and the output power of the photovoltaic inverter is greater than or equal to a set threshold, controls the input voltage of the first DC-DC converter to remain unchanged to avoid an increase in the bus voltage. Since the current output power of the photovoltaic inverter does not meet the derating requirement, controlling the input voltage of the second DC-DC converter to increase enables the photovoltaic inverter to achieve more power derating.

[0014] In one possible implementation, the controller is used to control the input voltage of the first DC-DC converter circuit and the input voltage of the second DC-DC converter circuit to increase simultaneously when the input voltage of the first DC-DC converter circuit is equal to the input voltage of the second DC-DC converter circuit during the process of controlling the input voltage of the first DC-DC converter circuit to increase, thereby improving the derating efficiency of the photovoltaic inverter.

[0015] In one possible implementation, the photovoltaic inverter includes an arc fault breaker, with multiple DC-DC converter circuits connected to the same arc fault breaker. The controller is further configured to, during the process of controlling the input voltage increase of the first DC-DC converter circuit, maintain a constant input voltage for the first DC-DC converter circuit and control the input voltage increase of the other DC-DC converter circuits when the difference between the input current of the first DC-DC converter circuit and the input current of the other DC-DC converter circuits is greater than or equal to a fourth threshold. This avoids excessively large differences in the input currents of the various DC-DC converter circuits that could interfere with the detection accuracy of the arc fault breaker.

[0016] Secondly, this application provides a derating control method for a photovoltaic inverter. The photovoltaic inverter includes an inverter circuit and multiple DC-DC converter circuits. The multiple DC-DC converter circuits are used to convert DC power from the photovoltaic modules into AC power and output it to the inverter circuit. The inverter circuit is used to convert DC power from the multiple DC-DC converter circuits into AC power. The multiple DC-DC converter circuits include a first DC-DC converter circuit and a second DC-DC converter circuit. The method includes controlling the input voltage of the first DC-DC converter circuit to increase when the photovoltaic inverter is in derating mode, thereby reducing the output power of the first DC-DC converter circuit, so that the output power of the inverter circuit decreases. The input voltage of the first DC-DC converter circuit is less than the input voltage of the second DC-DC converter circuit.

[0017] In this embodiment, when the photovoltaic inverter enters derating mode, it prioritizes controlling the photovoltaic strings with lower voltages corresponding to the maximum power point to shift the power point to the right, while keeping the output voltage of the photovoltaic strings less than or equal to the maximum input voltage among the multiple DC-DC converter circuits. This allows for greater power derating while maintaining a stable DC bus voltage and avoids overstressing of the switching transistors in the inverter circuit. The DC bus is used to electrically connect multiple DC-DC converter circuits and the inverter circuit. It is worth mentioning that, for ease of understanding, the above description is from the perspective of the photovoltaic inverter switching from the normal state, i.e., the corresponding photovoltaic string operating at the maximum power point, to the derating mode for the first time. In actual applications, there may be situations where the photovoltaic inverter still overheats after one derating. In this case, it may be necessary to increase the output voltage of the photovoltaic string that has already undergone one rightward shift of the operating point, that is, to shift the operating point of the corresponding photovoltaic string further to the right. Accordingly, the photovoltaic string with the lower current output voltage is still prioritized to shift the operating point to the right, and the output voltage of the photovoltaic string is kept less than or equal to the maximum input voltage among the current multiple DC-DC converter circuits. Under the condition of maintaining a stable DC bus voltage, the photovoltaic string with the lower output voltage can achieve more power derating, avoiding overstressing of the switching transistors in the inverter circuit.

[0018] In one possible implementation, the plurality of DC-DC converter circuits include at least one third DC-DC converter circuit, the input voltage of which is greater than that of the first DC-DC converter circuit. The method further includes prioritizing the increase of the input voltage of the first DC-DC converter circuit, wherein the input voltage of the first DC-DC converter circuit is less than or equal to the average input voltage of the plurality of DC-DC converter circuits. Since the input voltage of the first DC-DC converter circuit is in a lower range, prioritizing the increase of the input voltage of the first DC-DC converter circuit, compared to prioritizing the boost of DC-DC converter circuits with input voltages higher than the average input voltage, enables the photovoltaic inverter to achieve more power derating while maintaining the stability of the DC bus.

[0019] In one possible implementation, the method further includes prioritizing the increase of the input voltage of a first DC-DC converter circuit, wherein the first DC-DC converter circuit is the one with the lowest input voltage among a plurality of DC-DC converter circuits. Compared to boosting the input voltages of other DC-DC converter circuits, the first DC-DC converter circuit has the largest difference between its input voltage and the maximum input voltage among the plurality of DC-DC converter circuits. Prioritizing the increase of the input voltage of the first DC-DC converter circuit can achieve maximum power derating while maintaining a stable DC bus.

[0020] In one possible implementation, the method further includes controlling the input voltage of the second DC-DC converter to remain constant when the input voltage of the first DC-DC converter increases, thereby preventing the input voltage of the second DC-DC converter from exceeding the maximum input voltage among the plurality of DC-DC converters and thus preventing the bus voltage from increasing.

[0021] In one possible implementation, the method further includes controlling the input voltage of the first DC-DC converter circuit to increase from the maximum power point of the corresponding connected photovoltaic string, thereby increasing the output voltage of the photovoltaic string connected to the first DC-DC converter circuit from the voltage corresponding to the maximum power point. If the output voltage of the photovoltaic string is controlled to decrease, the output voltage of the photovoltaic string may drop to the shutdown voltage value of the photovoltaic inverter, thereby causing the photovoltaic inverter to be undervoltage at the input and triggering shutdown, affecting the normal operation of the photovoltaic inverter. Therefore, controlling the output voltage of the photovoltaic string to increase from the voltage corresponding to the maximum power point can achieve power derating while avoiding undervoltage at the input of the photovoltaic inverter. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of a photovoltaic system architecture;

[0023] Figure 2 This is a simplified schematic diagram of the photovoltaic inverter provided in this application;

[0024] Figure 3 It is a PV curve diagram of a photovoltaic string;

[0025] Figure 4 These are PV curves of different photovoltaic strings connected to the photovoltaic inverter provided in this application;

[0026] Figure 5 This is another simplified schematic diagram of the photovoltaic inverter provided in this application;

[0027] Figure 6 This is a flowchart illustrating the derating control method for the photovoltaic inverter provided in this application. Detailed Implementation

[0028] See Figure 1 , Figure 1 This is a schematic diagram of a photovoltaic system architecture. (Refer to...) Figure 1In the photovoltaic system 100, the photovoltaic module 10 directly converts solar energy into electrical energy using the photovoltaic effect. The photovoltaic module 10 typically includes multiple cells connected in series or parallel to achieve a certain output power. The photovoltaic inverter 201 converts the direct current (DC) from the photovoltaic module 10 into alternating current (AC), and sends the AC to the corresponding prefabricated substation 30 for voltage transformation. The prefabricated substation 30 can convert the low-voltage AC output from the photovoltaic inverter 201 into medium-voltage AC, and then transmit the AC to the step-up substation 40 and to the power grid 50 or other loads.

[0029] When the photovoltaic inverter 201 is operating normally, it usually operates in maximum power point tracking mode. In this mode, each DC-DC converter circuit in the photovoltaic inverter can independently control its own input voltage. By performing maximum power point tracking on the photovoltaic strings connected to each other and controlling the input voltage to the voltage corresponding to the maximum power point, the power generation efficiency of the photovoltaic system is improved.

[0030] exist Figure 1 In the photovoltaic system shown, the photovoltaic inverter needs to switch from maximum power point tracking mode to derating mode under certain circumstances. For example, when the DC bus voltage in the photovoltaic inverter is too high, or the difference between the grid voltage and the DC bus voltage is too large, or the internal temperature is too high due to a failure of the photovoltaic inverter's cooling fan, the photovoltaic inverter usually needs to operate in derating mode to reduce output power. However, in actual derating, the DC bus voltage in the photovoltaic inverter is prone to increase. This increase in bus voltage leads to an increase in the input voltage of the inverter circuit, resulting in increased stress on the switching transistors in the inverter circuit and increased heat loss of the switching transistors.

[0031] See Figure 2 , Figure 2 This is a simplified schematic diagram of the photovoltaic inverter provided in this application. The photovoltaic inverter includes an inverter circuit, multiple DC-DC conversion circuits, and a controller. Specifically, the photovoltaic inverter includes n DC-DC conversion circuits, from DC-DC conversion circuit 101 to DC-DC conversion circuit 10n, where n is an integer greater than 1. The input terminals of DC-DC conversion circuits 101 to 10n are respectively connected to photovoltaic strings 1 to n. The output terminals of the multiple DC-DC conversion circuits are connected in parallel and connected to the input terminal of the inverter circuit via a DC bus. Photovoltaic string n can be a string group, consisting of one or more photovoltaic strings connected in parallel. A bus capacitor C1 is connected between the positive DC bus BUS+ and the negative DC bus BUS-. Each DC-DC conversion circuit converts the DC power from the corresponding photovoltaic string into AC power, outputting the converted (e.g., boosted) DC power to the bus capacitor C1. The inverter circuit obtains DC power from the bus capacitor C1 and converts it into AC power, outputting AC power to the grid or load. Figure 2In the photovoltaic inverter shown, if the inverter initially operates in maximum power point tracking (MPPT) mode, the input voltages of DC-DC converters 101 to 10n are all corresponding to the maximum power point, and the output voltage of each photovoltaic string connected to a DC-DC converter is also the voltage corresponding to the maximum power point. After the photovoltaic inverter switches to derating mode, the output power of the photovoltaic inverter is typically reduced by controlling the operating point of the photovoltaic strings to shift to the right, i.e., increasing the input voltage of the DC-DC converters. It is worth noting that the magnitude of the DC bus voltage in the photovoltaic inverter is directly related only to the input voltage of the DC-DC converter with the highest input voltage among the multiple DC-DC converters. Specifically, if the input voltage of the DC-DC converter with the highest input voltage increases, the DC bus voltage in the photovoltaic inverter also increases. If the input voltages of other DC-DC converters change but do not exceed the aforementioned highest input voltage, the DC bus voltage remains stable.

[0032] For example, among the current DC-DC converters 101 to 10n, the input voltage of DC-DC converter 10n is the highest (e.g., 490V). If the input voltage of a certain DC-DC converter exceeds the input voltage of DC-DC converter 10n, the bus voltage in the photovoltaic inverter, i.e., the voltage across the bus capacitor C1, will increase. If the input voltages of DC-DC converters 101 and 102 are 400V and 450V respectively, i.e., the input voltage of DC-DC converter 101 is less than the input voltage of DC-DC converter 102, in order to reduce the fluctuation amplitude of the bus voltage while achieving derating of the photovoltaic inverter, the input voltage of DC-DC converter 101 is first increased. The photovoltaic string 1 connected to DC-DC converter 101 deviates from the maximum power point, and the output power of photovoltaic string 1 decreases to drate the photovoltaic inverter. Here, since the input voltage of DC-DC converter 101 differs from that of DC-DC converter 10n by 90V (490V-400V), while the input voltage of DC-DC converter 102 differs from that of DC-DC converter 10n by only 40V (490V-450V), the difference between the input voltage of DC-DC converter 101 and the maximum input voltage among the multiple DC-DC converters is larger. Therefore, prioritizing the increase of the input voltage of DC-DC converter 101 can enable the photovoltaic inverter to achieve more power derating while maintaining the stability of the DC bus.

[0033] In some feasible implementations, after the photovoltaic inverter switches to derating mode, the controller in the photovoltaic inverter controls the input voltage of the first DC-DC conversion circuit to increase from the maximum power point of the corresponding connected photovoltaic string, i.e., the case of the photovoltaic inverter's first derating. See also Figure 3 , Figure 3 This is a PV curve diagram of a photovoltaic string. For example... Figure 3As shown, under the same illumination and temperature conditions, when the output voltage V of the photovoltaic string changes, its output power P also changes. When the photovoltaic string operates at its maximum power point, the output voltage of the photovoltaic string is the maximum power point voltage Vmppt, and the output power of the photovoltaic string reaches the maximum power Pmppt. When the photovoltaic string connected to the first DC-DC converter circuit operates at its maximum power point, the operating point of the photovoltaic string is shifted to the left or right from the maximum power point to deviate from it. If the operating point of the photovoltaic string is shifted to the left from the maximum power point, that is, the output voltage of the photovoltaic string is reduced, the output voltage of the photovoltaic string may drop to the shutdown voltage value of the photovoltaic inverter, which will lead to undervoltage at the input of the photovoltaic inverter and trigger shutdown, affecting the normal operation of the photovoltaic inverter. Therefore, this application achieves derating of the photovoltaic inverter by controlling the operating point of the photovoltaic string to the right from the maximum power point, that is, by controlling the output voltage of the photovoltaic string to increase (up to the open circuit voltage Voc), while avoiding shutdown caused by undervoltage at the input of the photovoltaic inverter.

[0034] Optionally, in practical applications, there may be situations where the photovoltaic inverter still overheats after one derating. In this case, it is necessary to increase the output voltage of the photovoltaic string that has already undergone one rightward shift of its operating point, that is, to further shift the operating point of the corresponding photovoltaic string to the right. For example, as described above... Figure 2 Taking the photovoltaic inverter shown as an example, during the initial derating of the photovoltaic inverter, the photovoltaic string 1 connected to the DC-DC converter circuit 101 is shifted to the right from its maximum power point. For example, the output voltage of photovoltaic string 1 is controlled from 400V to 440V to achieve power derating. If the photovoltaic inverter still overheats after the initial derating, since the current output voltage of photovoltaic string 1 is still lower than the output voltage of photovoltaic string 2 (450V), the operating point of photovoltaic string 1 is further shifted to the right. This allows for more power derating while maintaining a stable DC bus voltage, further meeting the derating requirements of the photovoltaic inverter.

[0035] Optionally, in practical applications, during the process of the inverter controlling the input voltage of the first DC-DC converter circuit to rise, if the output power of the photovoltaic inverter is less than a set threshold, it is determined that the current output power of the photovoltaic inverter has reached the derating requirement, and then the photovoltaic inverter is controlled to exit the derating mode to avoid the output power of the photovoltaic inverter being too low.

[0036] Optionally, during the process of controlling the input voltage of the first DC-DC converter to increase, when the input voltage of the first DC-DC converter reaches the maximum input voltage among multiple DC-DC converters, and the output power of the photovoltaic inverter is greater than or equal to a set threshold, the input voltage of the first DC-DC converter is kept constant while the input voltage of the second DC-DC converter is increased. Similarly, as described above... Figure 2Taking the photovoltaic inverter shown as an example, after the photovoltaic inverter switches to derating mode, the input voltage of the DC-DC converter 101 is first increased. If the input voltage of the DC-DC converter 101 increases to 490V, the output power of the photovoltaic inverter is greater than or equal to a set threshold, that is, the input voltage of the DC-DC converter 101 reaches the maximum input voltage among multiple DC-DC converters, while the current output power of the photovoltaic inverter does not meet the derating requirement. Therefore, the controller keeps the input voltage of the DC-DC converter 101 unchanged to prevent the bus voltage from increasing, while simultaneously increasing the input voltage of the DC-DC converter 102 to reduce the output power of the DC-DC converter 102, thus enabling the photovoltaic inverter to achieve more power derating.

[0037] Optionally, when the controller controls the input voltage of the first DC-DC converter to increase, and the input voltage of the first DC-DC converter increases to the same level as the input voltage of the second DC-DC converter, the controller simultaneously controls the input voltages of both the first and second DC-DC converters to increase. Similarly, as described above... Figure 2 Taking the photovoltaic inverter shown as an example, after the photovoltaic inverter switches to derating mode, the input voltage of DC-DC converter 101 is first increased. After the input voltage of DC-DC converter 101 increases to 450V, the input voltages of DC-DC converter 101 and DC-DC converter 102 are the same. Then, the input voltages of DC-DC converter 101 and DC-DC converter 102 are increased simultaneously. The photovoltaic strings connected to DC-DC converter 101 and DC-DC converter 102 are all deviated from the maximum power point, and the output power of DC-DC converter 101 and DC-DC converter 102 is reduced, thereby improving the derating efficiency of the photovoltaic inverter.

[0038] In some feasible implementations, when the controller increases the input voltage of the first DC-DC converter circuit, the input voltage of the second DC-DC converter circuit remains unchanged. Similarly, as described above... Figure 2 Taking the photovoltaic inverter shown as an example, after the photovoltaic inverter switches to derating mode, in order to achieve more power derating while maintaining a stable DC bus, the input voltage of the DC-DC converter 101 with a smaller input voltage is first increased. This causes the photovoltaic string 1 connected to the DC-DC converter 101 to deviate from its maximum power point, thereby reducing the output power of the DC-DC converter 101. Since the magnitude of the DC bus voltage is directly related to the maximum input voltage among the multiple DC-DC converters, the input voltage of the DC-DC converter 101 is kept less than or equal to the maximum input voltage among the multiple DC-DC converters during the process of increasing the input voltage of the DC-DC converter 101. Since the input voltage of the DC-DC converter 102 is larger, the input voltage of the DC-DC converter 102 is kept constant to prevent the input voltage of the DC-DC converter 102 from exceeding the maximum input voltage among the multiple DC-DC converters, thereby preventing an increase in the bus voltage.

[0039] In some feasible implementations, the photovoltaic inverter includes at least one third DC-DC converter circuit among its multiple DC-DC converter circuits. The input voltage of this third DC-DC converter circuit is greater than the input voltage of the first DC-DC converter circuit. After the photovoltaic inverter switches to derating mode, the controller in the photovoltaic inverter controls the input voltage of the first DC-DC converter circuit to increase, and the input voltage of the first DC-DC converter circuit is less than or equal to the average input voltage of the multiple DC-DC converter circuits. Specifically, for example... Figure 2 Taking the photovoltaic inverter shown as an example, after the photovoltaic inverter switches to derating mode, it acquires the input voltages of DC-DC converters 101 to 10n, and obtains the average input voltage of multiple DC-DC converters based on their respective input voltages. Then, the controller designates the DC-DC converter with an input voltage lower than the average input voltage as the first DC-DC converter and controls the input voltage of this first DC-DC converter to increase. Because the input voltage of the first DC-DC converter is lower than the average input voltage, prioritizing the increase of the input voltage of the first DC-DC converter, compared to prioritizing the boost of DC-DC converters with input voltages higher than the average input voltage, allows the photovoltaic inverter to achieve more power derating while maintaining a stable DC bus.

[0040] In some feasible implementations, after the photovoltaic inverter switches to derating mode, the controller in the photovoltaic inverter is used to control the input voltage of the first DC-DC converter to increase, and this first DC-DC converter is the DC-DC converter with the lowest input voltage among multiple DC-DC converters. Specifically, taking... Figure 2 Taking the photovoltaic inverter shown as an example, after the photovoltaic inverter switches to derating mode, the input voltages of DC-DC converters 101 to 10n are obtained, and the DC-DC converter with the lowest input voltage among multiple DC-DC converters is determined. For example, if n is 3, meaning the photovoltaic inverter includes three DC-DC converters: DC-DC converter 101, DC-DC converter 102, and DC-DC converter 103, when the photovoltaic inverter is operating in maximum power point tracking mode, if the input voltages of DC-DC converters 101, 102, and 103 are 400V, 450V, and 490V respectively, then the voltages corresponding to the maximum power points of photovoltaic strings 1, 2, and 3 are 400V, 450V, and 490V respectively. Please refer to [further details omitted]. Figure 4 , Figure 4 These are PV curves of different photovoltaic strings connected to the photovoltaic inverter provided in this application. Figure 4This includes the PV curves for photovoltaic strings 1, 2, and 3. When the photovoltaic inverter operates in maximum power point tracking (MPPT) mode, photovoltaic strings 1, 2, and 3 are at their respective maximum power points, with voltages of 400V, 450V, and 490V. The voltage across the bus capacitor C1 in the current photovoltaic inverter is the bus voltage (e.g., 1500V). The magnitude of the DC bus voltage in the photovoltaic inverter is directly related only to the input voltage of the DC-DC converter with the highest input voltage among the multiple DC-DC converter circuits. Since DC-DC converter 103 has the highest input voltage, if the input voltage of a certain DC-DC converter exceeds the input voltage of DC-DC converter 103, the bus voltage will increase. Since the input voltage of DC-DC converter 101 differs from that of DC-DC converter 103 by 90V (490V-400V); the input voltage of DC-DC converter 102 differs by only 40V (490V-450V); and the input voltage of DC-DC converter 103 differs by 0V, and the difference between the input voltages of DC-DC converter 101 and DC-DC converter 103 is the largest, the input voltage of the first DC-DC converter is preferentially increased, which can achieve the maximum power derating while maintaining the stability of the DC bus.

[0041] Optionally, when the controller increases the input voltage of the first DC-DC converter, it simultaneously increases the input voltage of multiple DC-DC converters, including the first DC-DC converter. Specifically, taking the first DC-DC converter as the one with the lowest input voltage among the multiple DC-DC converters, after the photovoltaic inverter switches to derating mode, the input voltages of DC-DC converters 101 to 10n are obtained, and the input voltages of the DC-DC converters with the lowest input voltages among the multiple DC-DC converters are sequentially increased from DC-DC converters 101 to 10n. Then, the controller prioritizes simultaneously increasing the input voltages of DC-DC converters 101 and 102 (whose input voltage is only slightly higher than that of DC-DC converter 101), while keeping the input voltages of DC-DC converters 103 to 10n unchanged. The photovoltaic strings connected to DC-DC converters 101 and 102 deviate from their maximum power points, thereby simultaneously reducing the output power of DC-DC converters 101 and 102 and improving the derating efficiency of the photovoltaic inverter.

[0042] In some feasible implementations, when the photovoltaic inverter is in derating mode, the inverter temperature is higher than or equal to a first threshold, or the DC bus voltage is higher than or equal to a second threshold, or the difference between the grid voltage and the DC bus voltage is higher than or equal to a third threshold. Specifically, when the photovoltaic inverter is operating in maximum power point tracking mode, the controller acquires the inverter temperature. If the temperature is higher than or equal to the first threshold, it determines that the inverter is overheating and controls it to enter derating mode to prevent the inverter temperature from continuously rising and affecting equipment safety. Alternatively, the controller acquires the voltage across the DC bus. If the voltage across the DC bus is higher than or equal to the second threshold, it determines that the DC bus voltage is overvoltage and controls the inverter to enter derating mode to prevent excessive inverter losses. Alternatively, the controller acquires the voltage across the DC bus and the grid voltage. If the difference between the grid voltage and the DC bus voltage is higher than or equal to the third threshold, it determines that the grid voltage is overvoltage and controls the inverter to enter derating mode to prevent excessive inverter losses.

[0043] In some feasible implementations, the photovoltaic inverter includes an Arc-Fault Circuit Interrupter (AFCI). AFCI detects DC arc faults in the photovoltaic string and inverter connection circuit by identifying arc fault characteristic signals. It disconnects the circuit before the arc fault develops into a fire or a short circuit, ensuring the safe operation of the photovoltaic inverter. See also Figure 5 , Figure 5 This is another simplified schematic diagram of the photovoltaic inverter provided in this application. For example... Figure 5As shown, multiple DC-DC converters in the photovoltaic inverter are connected to the same AFC I. During the process of controlling the input voltage increase of the first DC-DC converter, the controller is also used to, when the difference between the input current of the first DC-DC converter and the input current of other DC-DC converters is greater than or equal to a fourth threshold, control the input voltage of the first DC-DC converter to remain constant, and control the input voltage of the other DC-DC converters to increase. Specifically, taking the first DC-DC converter as the one with the smallest input voltage among the multiple DC-DC converters as an example, after the photovoltaic inverter switches to derating mode, the controller obtains the input voltages of DC-DC converters 101 to 10n, and determines that the DC-DC converter with the smallest input voltage among the multiple DC-DC converters is DC-DC converter 101, and the input voltages of DC-DC converters 102 to 10n increase sequentially. Then, the controller prioritizes controlling the increase of the input voltage of the DC-DC converter 101 with the smallest input voltage, causing the photovoltaic string 1 connected to DC-DC converter 101 to deviate from its maximum power point, reducing the output power of DC-DC converter 101, and thus reducing the output power of the inverter circuits in the photovoltaic inverter. During the process of increasing the input voltage of DC-DC converter 101, if the difference between the input current of DC-DC converter 101 and the input current of any one of the DC-DC converters 102 to 10n reaches a value greater than or equal to a fourth threshold, then the current difference between the input currents of each DC-DC converter is too large. An excessively large current difference can easily affect the detection accuracy of AFC I, leading to misjudgment of arc faults. When the difference between the input current of DC-DC converter 101 and the input current of any other DC-DC converter reaches a value greater than or equal to the fourth threshold (e.g., set between 0.2A and 10A), the controller keeps the input voltage of DC-DC converter 101 constant and increases the input voltage of other DC-DC converters. For example, it controls the input voltage of DC-DC converter 102 (whose input voltage is only greater than that of DC-DC converter 101) to increase, reducing the output power of DC-DC converter 102, thus continuing to derating while ensuring the AFC I detection function.

[0044] See Figure 6 , Figure 6 This is a flowchart illustrating the derating control method for photovoltaic inverters provided in this application. The derating control method for photovoltaic inverters provided in this application is applicable to the above-mentioned... Figure 2 and Figure 5 The control process of the photovoltaic inverter shown is as follows: Figure 6 As shown, the derating control method for photovoltaic inverters provided in this application includes the following steps:

[0045] S101 controls the photovoltaic inverter to be in derating mode.

[0046] S102 prioritizes increasing the input voltage of the first DC-DC converter circuit, where the input voltage of the first DC-DC converter circuit is less than the input voltage of the second DC-DC converter circuit.

[0047] In some feasible implementations, after the photovoltaic inverter switches to derating mode, the input voltages of at least two DC-DC converters in the photovoltaic inverter are obtained. For example, the input voltages of the first DC-DC converter and the second DC-DC converter are obtained as 400V and 450V respectively, meaning the input voltage of the first DC-DC converter is less than that of the second DC-DC converter. If the input voltage of the DC-DC converter with the highest input voltage among the multiple DC-DC converters is 490V, since the magnitude of the DC bus voltage in the photovoltaic inverter is only directly related to the input voltage of the DC-DC converter with the highest input voltage, if the input voltage of a certain DC-DC converter is controlled to exceed the maximum input voltage, the bus voltage in the photovoltaic inverter will increase. In order to reduce the fluctuation amplitude of the bus voltage while achieving photovoltaic inverter derating, the input voltage of the first DC-DC converter is first controlled to increase, and the photovoltaic string connected to the first DC-DC converter is deviated from the maximum power point to drate the photovoltaic inverter. Here, since the input voltage of the first DC-DC converter differs from the maximum input voltage by 90V (490V-400V), while the second DC-DC converter differs from the maximum input voltage by only 40V (490V-450V), the difference between the input voltage of the first DC-DC converter and the maximum input voltage among the multiple DC-DC converters is larger. Therefore, prioritizing the increase of the input voltage of the first DC-DC converter can enable the photovoltaic inverter to achieve more power derating while maintaining the stability of the DC bus.

[0048] Optionally, after the photovoltaic inverter switches to derating mode, the input voltage of the first DC-DC converter is increased from the maximum power point of the corresponding connected photovoltaic string. If the operating point of the photovoltaic string is shifted to the left from the maximum power point, i.e., the output voltage of the photovoltaic string is reduced, the output voltage of the photovoltaic string may drop to the shutdown voltage value of the photovoltaic inverter, thereby causing the photovoltaic inverter to shut down due to input undervoltage, affecting the normal operation of the photovoltaic inverter. Therefore, this application achieves photovoltaic inverter derating by shifting the operating point of the photovoltaic string to the right from the maximum power point, i.e., by increasing the output voltage of the photovoltaic string, while avoiding shutdown caused by input undervoltage.

[0049] Optionally, in practical applications, there may be situations where the photovoltaic inverter still overheats after one derating. In this case, it is necessary to further increase the output voltage of the photovoltaic string that has already undergone one rightward shift of its operating point, i.e., to further shift the operating point of the corresponding photovoltaic string to the right. For example, in the initial derating of the photovoltaic inverter, the photovoltaic string connected to the first DC-DC converter is controlled to shift right from its maximum power point, such as controlling the output voltage of the photovoltaic string from 400V to 440V to achieve power derating. If the photovoltaic inverter still overheats after the initial derating, since the output voltage of the photovoltaic string connected to the first DC-DC converter is still lower than the output voltage (450V) connected to the second DC-DC converter, it is still preferable to further shift the operating point of the photovoltaic string connected to the first DC-DC converter to the right. This allows for more power derating while maintaining a stable DC bus voltage, further meeting the derating requirements of the photovoltaic inverter.

[0050] Optionally, when the controller increases the input voltage of the first DC-DC converter circuit until it reaches the same level as the input voltage of the second DC-DC converter circuit, the controller simultaneously increases the input voltages of both the first and second DC-DC converter circuits. This reduces the output power of both circuits simultaneously, improving the derating efficiency of the photovoltaic inverter.

[0051] Furthermore, during the process of controlling the input voltage of the first DC-DC converter circuit to increase, since the input voltage of the second DC-DC converter circuit is larger, the input voltage of the second DC-DC converter circuit is kept constant to avoid the input voltage of the second DC-DC converter circuit exceeding the maximum input voltage among the multiple DC-DC converter circuits, which would lead to an increase in the bus voltage.

[0052] In some feasible implementations, after the photovoltaic inverter switches to derating mode, the input voltage of each DC-DC converter in the photovoltaic inverter is acquired, and the average input voltage of multiple DC-DC converters is obtained based on the input voltage of each DC-DC converter. Then, the DC-DC converter with an input voltage lower than the average input voltage is designated as the first DC-DC converter, and its input voltage is increased. Because the input voltage of the first DC-DC converter is lower than the average input voltage, prioritizing the increase of the input voltage of the first DC-DC converter, compared to prioritizing the boost of DC-DC converters with input voltages higher than the average input voltage, allows the photovoltaic inverter to achieve more power derating while maintaining DC bus stability.

[0053] In some feasible implementations, after the photovoltaic inverter switches to derating mode, the input voltage of each DC-DC converter in the photovoltaic inverter is acquired. Based on the input voltage of each DC-DC converter, the DC-DC converter with the lowest input voltage is selected as the first DC-DC converter, and its input voltage is controlled to increase. Compared to boosting the input voltage of other DC-DC converters, the difference between the input voltage of the first DC-DC converter and the maximum input voltage among the multiple DC-DC converters is the largest. Prioritizing the increase of the input voltage of the first DC-DC converter achieves maximum power derating while maintaining DC bus stability.

[0054] In some feasible implementations, the photovoltaic inverter includes an AFC I (Automatic Current Conversion I), and multiple DC-DC converter circuits in the photovoltaic inverter are connected to the same AFC I. During the process of controlling the input voltage increase of the first DC-DC converter circuit, when the difference between the input current of the first DC-DC converter circuit and the input current of other DC-DC converter circuits is greater than or equal to a fourth threshold, the input voltage of the first DC-DC converter circuit is kept constant, while the input voltages of the other DC-DC converter circuits are increased. Specifically, during the process of controlling the input voltage increase of the first DC-DC converter circuit, if the difference between the input current of the first DC-DC converter circuit and the input current of any other DC-DC converter circuit reaches a value greater than or equal to the fourth threshold, the current difference between the input currents of each DC-DC converter circuit is too large. An excessively large current difference can easily affect the detection accuracy of the AFC I, leading to misjudgment of arc faults. By controlling the input voltage of the first DC-DC converter circuit to remain constant and the input voltages of the other DC-DC converter circuits to increase when the difference between the input current of the first DC-DC converter circuit and the input current of any other DC-DC converter circuit reaches a value greater than or equal to the fourth threshold, derating is continued while ensuring the AFC I detection function.

Claims

1. A photovoltaic inverter, characterized in that, It includes an inverter circuit, multiple DC-DC conversion circuits, and a controller, among which: The plurality of DC-DC converter circuits are used to convert the DC power from the photovoltaic module into DC power and output it to the inverter circuit. The plurality of DC-DC converter circuits include a first DC-DC converter circuit and a second DC-DC converter circuit. The inverter circuit is used to convert the direct current from the plurality of DC-DC converter circuits into alternating current; When the photovoltaic inverter is in derating mode, the controller is used to control the input voltage of the first DC-DC converter to increase in order to reduce the output power of the first DC-DC converter, thereby reducing the output power of the inverter circuit, wherein the input voltage of the first DC-DC converter is less than the input voltage of the second DC-DC converter.

2. The photovoltaic inverter according to claim 1, characterized in that, The plurality of DC-DC converter circuits include at least one third DC-DC converter circuit, wherein the input voltage of the third DC-DC converter circuit is greater than the input voltage of the first DC-DC converter circuit, and the input voltage of the first DC-DC converter circuit is less than or equal to the average input voltage of the plurality of DC-DC converter circuits.

3. The photovoltaic inverter according to claim 2, characterized in that, The first DC-DC converter circuit is the DC-DC converter circuit with the smallest input voltage among the plurality of DC-DC converter circuits.

4. The photovoltaic inverter according to any one of claims 1-3, characterized in that, The photovoltaic inverter includes a DC bus, and the output terminals of the plurality of DC-DC conversion circuits are connected in parallel and then connected to the input terminal of the inverter circuit through the DC bus; The photovoltaic inverter is in derating mode when the temperature of the photovoltaic inverter is higher than or equal to a first threshold, or the DC bus voltage is higher than or equal to a second threshold, or the difference between the grid voltage and the DC bus voltage is higher than or equal to a third threshold.

5. The photovoltaic inverter according to any one of claims 1-4, characterized in that, When the controller increases the input voltage of the first DC-DC converter, the input voltage of the second DC-DC converter remains unchanged.

6. The photovoltaic inverter according to any one of claims 1-5, characterized in that, The controller is used to control the input voltage of the first DC-DC converter circuit to increase from the maximum power point of the corresponding connected photovoltaic string.

7. The photovoltaic inverter according to any one of claims 1-6, characterized in that, The controller is used to control the photovoltaic inverter to exit the derating mode when the output power of the photovoltaic inverter is less than a set threshold during the process of controlling the input voltage of the first DC-DC converter to increase.

8. The photovoltaic inverter according to any one of claims 1-6, characterized in that, The controller is configured to, during the process of controlling the input voltage of the first DC-DC converter to increase, when the input voltage of the first DC-DC converter reaches the maximum input voltage among the plurality of DC-DC converters, and the output power of the photovoltaic inverter is greater than or equal to a set threshold, control the input voltage of the first DC-DC converter to remain unchanged and control the input voltage of the second DC-DC converter to increase.

9. The photovoltaic inverter according to any one of claims 1-6, characterized in that, The controller is configured to, during the process of controlling the input voltage of the first DC-DC converter to increase, when the input voltage of the first DC-DC converter is equal to the input voltage of the second DC-DC converter, control the input voltage of both the first DC-DC converter and the second DC-DC converter to increase simultaneously.

10. The photovoltaic inverter according to any one of claims 1-9, characterized in that, The photovoltaic inverter includes an arc fault disconnector, and the multiple DC-DC conversion circuits are all connected to the same arc fault disconnector. The controller is further configured to, during the process of controlling the input voltage of the first DC-DC converter to increase, when the difference between the input current of the first DC-DC converter and the input current of other DC-DC converters is greater than or equal to a fourth threshold, control the input voltage of the first DC-DC converter to remain unchanged and control the input voltage of the other DC-DC converters to increase.

11. A derating control method for a photovoltaic inverter, characterized in that, The photovoltaic inverter includes an inverter circuit and multiple DC-DC conversion circuits. The multiple DC-DC conversion circuits convert DC power from the photovoltaic modules and output it to the inverter circuit. The inverter circuit converts the DC power from the multiple DC-DC conversion circuits into AC power. The multiple DC-DC conversion circuits include a first DC-DC conversion circuit and a second DC-DC conversion circuit. The method includes: When the photovoltaic inverter is in derating mode, the input voltage of the first DC-DC converter is increased to reduce the output power of the first DC-DC converter, thereby reducing the output power of the inverter circuit. The input voltage of the first DC-DC converter is less than the input voltage of the second DC-DC converter.

12. The method according to claim 11, characterized in that, The plurality of DC-DC converter circuits includes at least one third DC-DC converter circuit, wherein the input voltage of the third DC-DC converter circuit is greater than the input voltage of the first DC-DC converter circuit, and the method further includes: The input voltage of the first DC-DC converter is controlled to increase, and the input voltage of the first DC-DC converter is less than or equal to the average input voltage of the plurality of DC-DC converters.

13. The method according to claim 12, characterized in that, The method further includes: The input voltage of the first DC-DC converter is increased, and the first DC-DC converter is the DC-DC converter with the smallest input voltage among the plurality of DC-DC converters.

14. The method according to any one of claims 11-13, characterized in that, The method further includes: When the input voltage of the first DC-DC converter circuit is increased, the input voltage of the second DC-DC converter circuit remains unchanged.

15. The method according to any one of claims 11-14, characterized in that, The method further includes: The input voltage of the first DC-DC converter is increased from the maximum power point of the corresponding connected photovoltaic string.

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