Photovoltaic system

Abstract

The utility model is suitable for the technical field of power electronics, and provides a photovoltaic system, which comprises a photovoltaic string unit, an inverter, a grid-connected switch and a frame circuit breaker, the photovoltaic string unit is connected with the input end of the inverter; the output end of the inverter is connected with the input end of the grid-connected switch; the output end of the grid-connected switch is connected with the input end of the frame circuit breaker; the output end of the frame circuit breaker is used for being connected with a three-phase power grid. The grid-connected switch comprises a plurality of power switch tubes; the frame circuit breaker is used for closing under the condition that the inverter responds to a grid-connected instruction; and the plurality of power switch tubes are used for conducting under the condition that the electric signal output by the inverter is consistent with the electric signal of the three-phase power grid, so as to complete the closing of the grid-connected switch. The photovoltaic system provided by the utility model realizes rapid grid connection.

Description

Technical Field

[0001] This utility model belongs to the field of power electronics technology, and in particular relates to a photovoltaic system. Background Technology

[0002] In related technologies, photovoltaic systems use centralized inverters and employ the following two schemes to achieve grid connection.

[0003] like Figure 1 As shown, the first scheme is that the photovoltaic system is equipped with an AC contactor and a frame circuit breaker. The frame circuit breaker is only used as a disconnecting device. When the inverter starts up slowly until it meets the grid connection conditions, a command to close the AC contactor is issued. After the AC contactor is engaged, the grid connection is successful.

[0004] like Figure 2 As shown, the second option is a photovoltaic system with a frame circuit breaker with a high count life. The frame circuit breaker not only serves as a disconnecting device but also functions as a grid-connected switch. When the inverter starts up slowly until it meets the grid-connection conditions, a command to close the frame circuit breaker is issued. After the frame circuit breaker is energized, grid connection is successful.

[0005] The second option is less expensive than the first, saving the cost of one AC contactor. However, both options suffer from a longer grid connection switch closing time. There is an increasing demand in the industry for inverters to quickly start up and connect to the grid after receiving a grid dispatch command, with the shortest possible connection time. The speed of grid connection directly affects the stability and reliability of the power system; excessively long connection times can lead to power system instability and even grid failures. Utility Model Content

[0006] This utility model provides a photovoltaic system designed to address the problem that prolonged grid connection time may lead to power system instability or even grid failure.

[0007] This utility model provides a photovoltaic system, including: a photovoltaic string unit, an inverter, a grid-connected switch, and a frame circuit breaker;

[0008] The photovoltaic string unit is connected to the input terminal of the inverter; the output terminal of the inverter is connected to the input terminal of the grid-connected switch; the output terminal of the grid-connected switch is connected to the input terminal of the frame circuit breaker; the output terminal of the frame circuit breaker is used to connect to the three-phase power grid.

[0009] The grid-connected switch includes several power switching transistors;

[0010] The frame circuit breaker is used to close when the inverter responds to the grid connection command;

[0011] The plurality of power switching transistors are used to conduct when the electrical signal output by the inverter is consistent with the electrical signal of the three-phase power grid, so as to complete the closing of the grid-connected switch.

[0012] In some embodiments, each phase output of the inverter is connected to at least one power switch.

[0013] In some embodiments, when there are multiple power switching transistors, the at least one power switching transistor is connected in series.

[0014] In some embodiments, when there are multiple power switches, the at least one power switch is connected in parallel.

[0015] In some embodiments, when the number of at least one power switch transistors is odd, the at least one power switch transistor is divided into multiple groups of power switch transistors, each group of power switch transistors includes one or more power switch transistors, and the multiple groups of power switch transistors are connected in parallel.

[0016] In some embodiments, when there is an even number of power switches, the at least one power switch is divided into multiple groups of power switches, each group having the same number of power switches and the power switches in each group being connected in series, and the multiple groups of power switches being connected in parallel.

[0017] In some embodiments, the multiple sets of power switches are an even array, with each pair of power switches connected in reverse parallel.

[0018] In some embodiments, the power switch is an IGBT.

[0019] In some embodiments, the time when the plurality of power switches completes conduction is earlier than the time when the frame circuit breaker completes closure.

[0020] The photovoltaic system provided by this utility model combines the fast closing time of the power switches, the high breaking capacity of the frame circuit breaker, and the ability of the frame circuit breaker to physically isolate the grid by using several power switching transistors as grid-connected switches in series with the frame circuit breaker. When the inverter responds to the grid-connection command, the frame circuit breaker closes, and when the inverter meets the grid-connection conditions, the grid-connection switch closes, forming a fast grid-connection system. This system can realize the fast grid-connection function of the inverter and meet the needs of fast inverter dispatch. Attached Figure Description

[0021] Figure 1 This is one of the structural schematic diagrams of a photovoltaic system provided by existing technology;

[0022] Figure 2This is the second schematic diagram of a photovoltaic system provided by existing technology;

[0023] Figure 3 This is a grid connection timing diagram for photovoltaic systems provided by existing technology;

[0024] Figure 4 This is one of the structural schematic diagrams of the photovoltaic system provided in this embodiment of the utility model;

[0025] Figure 5 This is one of the structural schematic diagrams of the grid-connected switch of the photovoltaic system provided in this embodiment of the utility model;

[0026] Figure 6 This is the second schematic diagram of the grid-connected switch of the photovoltaic system provided in this embodiment of the utility model;

[0027] Figure 7 This is a schematic diagram of the grid connection process of the photovoltaic system provided in this embodiment of the utility model;

[0028] Figure 8 This is the second schematic diagram of the photovoltaic system provided in this embodiment of the present invention;

[0029] Figure 9 This is a grid connection timing diagram of a photovoltaic system provided in an embodiment of this utility model. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of this utility model clearer, the present utility model will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the present utility model and are not intended to limit the present utility model.

[0031] In this invention, the reference to "embodiment" or "implementation" means that a specific feature, component, or characteristic described in connection with an embodiment or implementation may be included in at least one embodiment of this invention. The appearance of this phrase in various places in the specification does not necessarily refer to the same embodiment, nor is it a mutually exclusive, independent, or alternative embodiment. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0032] In the description of this utility model, it should be understood that the terms "length," "width," "upper," "lower," "left," "right," "horizontal," "top," and "bottom," etc., indicating orientation or positional relationships based on the orientation or positional relationships shown in the accompanying drawings, are only for the convenience of describing this utility model and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of this utility model. Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first" and "second" may explicitly or implicitly include one or more of the stated features. In the description of this utility model, "multiple" means two or more, unless otherwise explicitly specified.

[0033] In the description of this utility model, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "joining" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection, an electrical connection, or a connection that allows for communication; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this utility model according to the specific circumstances.

[0034] In related technologies, given the severe cost challenges facing the photovoltaic industry, most suppliers are reducing costs by increasing the power rating of inverters or raising the system voltage level. Consequently, the power of inverters in photovoltaic systems is increasing, and system voltage levels are rising from the original 1000V to 1500V and are moving towards systems above 2000V. Currently, high-voltage photovoltaic systems mostly use centralized inverters, further reducing costs by increasing the power rating of centralized inverters or raising the system voltage level.

[0035] Figure 1 and Figure 2 The AC contactors or frame circuit breakers used in the current system are all mechanical switches. The closing time of mechanical switches is discrete, generally around 30ms to 60ms. Adding the AC soft-start time of the inverter, which is at least 20ms, the total grid connection time exceeds 70ms. However, the current requirement is that the inverter must complete the grid connection process within 50ms of the grid dispatch command.

[0036] For the grid connection process of the solutions in the relevant technologies, the grid connection timing diagram is as follows: Figure 3As shown. At time t0, the inverter receives the power station's start-up command, i.e., the grid connection command. The inverter performs AC soft start, and the inverter's output electrical signal tracks the grid's electrical signal from zero. At time t1, the inverter's output electrical signal is consistent with the grid's electrical signal, meeting the grid connection conditions, and a closing command is issued to the frame circuit breaker. The frame circuit breaker is a mechanical switch, and it takes approximately 50ms for it to complete the closing action after receiving the closing command at time t1. That is, at time t2, the inverter outputs grid-connected current, and the inverter completes the grid connection operation. From the time the power station issues the command to the time the inverter completes the grid connection operation and integrates the grid-connected current into the grid, it takes approximately 70ms from time t0 to time t2, which is a relatively long grid connection time.

[0037] The following is combined with Figures 4-9 This invention describes the photovoltaic system provided by this utility model.

[0038] Figure 4 This is a schematic diagram of the photovoltaic system provided in an embodiment of the present invention. (Refer to...) Figure 4 The photovoltaic system provided by this utility model includes:

[0039] Photovoltaic string unit 10, inverter 20, grid-connected switch 30 and frame circuit breaker 40;

[0040] The photovoltaic string unit 10 is connected to the input terminal of the inverter 20; the output terminal of the inverter 20 is connected to the input terminal of the grid-connected switch 30; the output terminal of the grid-connected switch 30 is connected to the input terminal of the frame circuit breaker 40; the output terminal of the frame circuit breaker 40 is used to connect to the three-phase power grid 50.

[0041] The grid-connected switch 30 includes several power switching transistors;

[0042] The frame circuit breaker 40 is used to close in response to the grid connection command of the inverter 20;

[0043] Several power switching transistors are used to turn on when the electrical signal output by the inverter 20 is consistent with the electrical signal of the three-phase power grid 50, so as to complete the closing of the grid-connected switch 30.

[0044] In practice, the photovoltaic string unit 10 is the smallest unit that connects multiple photovoltaic modules in series to form the required DC output voltage. The number of photovoltaic modules in the photovoltaic string unit 10 depends on the system design requirements and is not specifically limited here.

[0045] Inverter 20 is mainly used to convert the DC power output from photovoltaic string unit 10 into AC power so that it can be connected to the three-phase power grid 50 or used by local loads.

[0046] By disconnecting or connecting the circuit, the grid-connected switch 30 can regulate the three-phase power grid 50, thereby ensuring the safe operation of the three-phase power grid 50. The grid-connected switch 30 uses several power switching transistors with high withstand voltage, low conduction loss, and fast switching speed, which helps to achieve rapid grid connection.

[0047] Power switching transistors include, for example, insulated gate bipolar transistors (IGBTs), metal-oxide-semiconductor field-effect transistors (MOSFETs), thyristors, or power transistors.

[0048] The frame circuit breaker 40 is mainly used as a disconnecting device in photovoltaic systems. It has high breaking capacity and high reliability, and can quickly disconnect the circuit in the event of faults such as short circuits or overloads to protect the safety of equipment and systems.

[0049] In actual operation, the inverter 20 in the photovoltaic system receives the grid connection command from the power station and simultaneously issues a closing command to the frame circuit breaker 40, then performs AC soft start. When the electrical signal output by the inverter 20 is consistent with the electrical signal of the three-phase power grid 50, it issues a closing command to the grid connection switch 30. The power switching transistor used in the grid connection switch 30 has a very fast conduction time; therefore, the grid connection switch 30 can complete the closing process within microseconds.

[0050] It should be noted that ensuring the electrical signal output by inverter 20 is consistent with the electrical signal of the three-phase power grid 50 is a fundamental requirement for the grid-connected operation of inverter 20. This consistency involves multiple parameters, including voltage, frequency, and phase, which collectively determine whether inverter 20 can be smoothly and efficiently connected to the grid.

[0051] I. Voltage Consistency

[0052] The voltage output by inverter 20 needs to be consistent with the voltage of the three-phase power grid 50 in both amplitude and waveform. Specifically, the voltage amplitude output by inverter 20 should be equal to or slightly higher than the voltage of the three-phase power grid 50 to ensure that it does not cause any impact on the three-phase power grid 50 during grid connection. Simultaneously, the voltage waveform output by inverter 20 should be close to a sine wave to reduce harmonic interference to the three-phase power grid 50. In practical applications, inverter 20 is typically equipped with a voltage regulation function, which can automatically adjust the output voltage according to changes in the voltage of the three-phase power grid 50 to maintain voltage consistency.

[0053] II. Frequency Consistency

[0054] The AC output frequency of inverter 20 needs to be consistent with the frequency of the three-phase power grid 50. In most areas, the frequency of the three-phase power grid 50 is 50Hz or 60Hz, therefore the AC output frequency of inverter 20 should also match this. Frequency consistency is crucial for the synchronous operation and stability of the power system. If the output frequency of inverter 20 is inconsistent with the frequency of the three-phase power grid 50, it will cause frequency fluctuations in the three-phase power grid 50, thereby affecting the stability and power quality of the three-phase power grid 50.

[0055] III. Phase Consistency

[0056] The phase of the AC output from inverter 20 needs to be synchronized with the voltage phase of the three-phase grid 50. Phase synchronization is crucial for the grid-connected operation of inverter 20. If the phase of the inverter 20's output is not synchronized with the voltage phase of the three-phase grid 50, it will lead to current surges and a decrease in power factor during grid connection. Therefore, inverter 20 typically employs phase-locked loop (PLL) technology to track the phase of the three-phase grid 50, ensuring that the phase of the AC output from inverter 20 is synchronized with the grid voltage phase.

[0057] IV. Harmonic Suppression

[0058] Inverter 20 may generate harmonics during the conversion of DC to AC, which can cause interference and losses to the three-phase power grid 50. Therefore, inverter 20 needs to meet certain harmonic limitation standards to reduce the impact of harmonics on the three-phase power grid 50. In practical applications, inverter 20 is usually equipped with filters or modulation strategies to suppress the generation of harmonics.

[0059] In summary, for the electrical signal output by inverter 20 to be consistent with the electrical signal of the three-phase power grid 50, the consistency requirements of multiple parameters such as voltage, frequency, and phase must be met. Simultaneously, inverter 20 also needs to employ a suitable grid-connected control strategy to ensure stable and efficient grid-connected operation.

[0060] It is understandable that grid connection is considered successful when both the frame circuit breaker 40 and the grid-connected switch 30 are closed and the inverter 20 outputs grid-connected current.

[0061] The photovoltaic system provided by this utility model combines the fast closing time of the power switches, the high breaking capacity of the frame circuit breaker, and the ability of the frame circuit breaker to physically isolate the grid by using several power switching transistors as grid-connected switches in series with the frame circuit breaker. When the inverter responds to the grid-connection command, the frame circuit breaker closes, and when the inverter meets the grid-connection conditions, the grid-connection switch closes, forming a fast grid-connection system. This system can realize the fast grid-connection function of the inverter and meet the needs of fast inverter dispatch.

[0062] In some embodiments, each phase output of inverter 20 is connected to at least one power switch.

[0063] In actual implementation, the grid-connected switch 30 has three-phase input terminals and three-phase output terminals. Each phase input terminal is electrically connected to the inverter 20, and each phase output terminal is electrically connected to the frame circuit breaker 40. The grid-connected switch 30 includes a plurality of power switching transistors, with at least one power switching transistor corresponding to each phase. The input terminal of each corresponding power switching transistor is electrically connected to the inverter 20, and the output terminal of each corresponding power switching transistor is electrically connected to the frame circuit breaker 40. The plurality of power switching transistors is greater than or equal to three. The number of power switching transistors per phase can be set according to actual needs and is not specifically limited here. In this embodiment of the invention, increasing the number of power switching transistors can improve the reliability and stability of the photovoltaic system and reduce the possibility of failure.

[0064] A power switch is a semiconductor device that controls current. Its operation is achieved by controlling voltage changes to turn the power switch on and off. When the control voltage is high, the power switch is in the on state, allowing current to flow; when the control voltage is low, the power switch is in the off state, preventing current from flowing.

[0065] In some embodiments, when there are multiple power switching transistors, the at least one power switching transistor is connected in series.

[0066] In actual implementation, when there is at least one power switch for each corresponding case, the power switch is directly connected in series between the inverter 20 and the frame circuit breaker 40.

[0067] In the case where there are two or more corresponding power switches, at least one power switch is connected in series. The power switches connected in series can form a whole and be connected in series between the inverter 20 and the frame circuit breaker 40.

[0068] In some embodiments, when there are multiple power switches, the at least one power switch is connected in parallel.

[0069] In practice, when there are two power switches for each corresponding power switch, the two power switches are connected in parallel. This parallel connection can be either forward parallel or reverse parallel.

[0070] Forward parallel connection refers to directly connecting the corresponding pins of two power switches together and then connecting them to the circuit. Ensure that the same pins of each power switch are connected together to achieve a forward parallel connection.

[0071] Reverse series connection refers to connecting the current input terminal of one power switch to the current output terminal of another power switch, and connecting the current output terminal of one power switch to the current input terminal of another power switch, thus achieving a reverse parallel connection.

[0072] For example, the three electrodes of a field-effect transistor (FET) are the source, gate, and drain. The source of the FET is the current input terminal, and the drain is the current output terminal. Similarly, the three electrodes of a bipolar junction transistor (BJT) are the emitter, base, and collector. The emitter of the BJT is the current input terminal, and the collector is the current output terminal. The three electrodes of an insulated-gate bipolar transistor (IGBT) are the emitter, gate, and collector. The emitter of the IGBT is the current input terminal, and the collector is the current output terminal.

[0073] When there are two or more power switches in each corresponding phase, the power switches are connected in parallel. The number of power switches in each phase can be the same or different, and no specific limitation is made here.

[0074] In some embodiments, when there is an odd number of power switches, the power switches are divided into multiple groups of power switches, each group of power switches includes one or more power switches, and the multiple groups of power switches are connected in parallel.

[0075] In actual implementation, at least one power switch transistor can be 3, 5, 7...2N+1, where N is a positive integer. At least one power switch transistor is divided into two or more groups, each group of which includes one or more power switch transistors. The number of power switch transistors in at least two groups is different, and the power switch transistors in each group are connected in parallel.

[0076] For example: if there are at least 3 power switches, then at least 3 power switches can be divided into 3 groups, with each group corresponding to 1 power switch, and the 3 power switches are connected in parallel; if there are at least 3 power switches, then at least 3 power switches can be divided into 2 groups, with the first group corresponding to 1 power switch and the second group corresponding to 2 power switches, and the two groups of power switches are connected in parallel.

[0077] If there are at least 5 power switches, then at least 5 power switches can be divided into 2 groups: the first group can correspond to 2 power switches, and the second group can correspond to 3 power switches; or at least 5 power switches can be divided into 3 groups: the first group can correspond to 2 power switches, the second group can correspond to 2 power switches, and the third group can correspond to 2 power switches; or at least 5 power switches can be divided into 4 groups: the first group can correspond to 2 power switches, the second group can correspond to 1 power switch, the third group can correspond to 1 power switch, and the fourth group can correspond to 1 power switch. Regardless of which grouping method is chosen, the power switches in each group are connected in parallel.

[0078] In some embodiments, when there is an even number of power switches, the power switches are divided into multiple groups of power switches, each group having the same number of power switches and the power switches in each group being connected in series, and the multiple groups of power switches being connected in parallel.

[0079] In practice, the number of power switches can be 4, 6, 8...2N, where N is a positive integer. These power switches are divided into multiple groups, each group containing at least one power switch, and the power switches in each group are connected in parallel.

[0080] For example: if there are 4 power switches, then at least one power switch can be divided into 2 groups, with each group corresponding to 2 power switches, and the 2 groups of power switches are connected in parallel; if there are 6 power switches, then at least one power switch can be divided into 3 groups, with each group corresponding to 2 power switches, and the 3 groups of power switches are connected in parallel; or at least one power switch can be divided into 2 groups, with each group corresponding to 3 power switches, and the 2 groups of power switches are connected in parallel; regardless of which grouping method is chosen, the power switches in each group are connected in parallel.

[0081] In some embodiments, the multiple power switching transistors are an even number of groups, and every two groups of power switching transistors are connected in anti-parallel.

[0082] In actual implementation, at least one power switch can be divided into 2 groups, 4 groups, 6 groups...2N groups of power switches, where N is a positive integer, and each group contains the same number of power switches, and the power switches in each group are connected in series.

[0083] In this embodiment, the power switching transistors are connected in reverse parallel. In each pair of power switching transistors, the current input terminal of the first power switching transistor is connected to the current output terminal of the second power switching transistor, and the current output terminal of the first power switching transistor is connected to the current input terminal of the second power switching transistor.

[0084] Taking IGBTs as an example, let's explain the power switching transistor connected in one phase.

[0085] like Figure 5 As shown, each corresponding group of power switches consists of two transistors. These two power switches are divided into two groups and are connected in reverse parallel. That is, the emitter of the first IGBT is connected to the collector of the second IGBT, and the collector of the first IGBT is connected to the emitter of the second IGBT, thus achieving the reverse parallel connection.

[0086] like Figure 6 As shown, each corresponding group of power switches consists of four transistors. These four power switches are divided into two groups, each group containing two forward-connected power switches connected in series. The two groups of power switches are connected in anti-parallel. The emitter of the first group of IGBTs is connected to the collector of the second group of IGBTs, and the collector of the first group of IGBTs is connected to the emitter of the second group of IGBTs, thus achieving an anti-parallel connection.

[0087] In some embodiments, the power switching transistor is an IGBT.

[0088] In practice, the closing time of IGBTs is very fast, and grid connection can be completed in microseconds.

[0089] In some embodiments, the time when several power switches complete conduction is earlier than the time when the frame circuit breaker completes closure.

[0090] In practice, the timing of the frame circuit breaker's closing action coincides with the timing of the inverter's start tracking of the power grid.

[0091] It is understandable that since the frame circuit breaker is a mechanical switch, the time it takes to complete the closing action is always longer than the time it takes for the power switch tubes to complete the conduction action. Therefore, the time when several power switch tubes complete the conduction action is earlier than the time when the frame circuit breaker completes the closing action.

[0092] Figure 7 This is a schematic flowchart of the grid connection method for a photovoltaic system provided in an embodiment of this utility model. (Refer to...) Figure 7 This utility model provides a grid connection method for a photovoltaic system, applicable to the photovoltaic system in any of the above embodiments, the method comprising:

[0093] Step 110: Receive grid connection command;

[0094] Step 120: Respond to the grid connection command, issue the first closing command to the frame circuit breaker, and determine the electrical signal output by the inverter;

[0095] Step 130: When the electrical signal output by the inverter is consistent with the electrical signal of the three-phase power grid, a second closing command is issued to the grid-connected switch. The second closing command is used to indicate that several power switching transistors are turned on.

[0096] Step 140: If both the frame circuit breaker and the grid-connected switch are closed and the inverter outputs grid-connected current, then grid connection is confirmed to be successful.

[0097] In actual implementation, the inverter in the photovoltaic system receives a grid connection command. The grid connection command refers to the power station sending a start-up command to the inverter of the photovoltaic system, which is used to instruct the inverter to start operation.

[0098] The inverter receives the grid connection command from the power station and starts up slowly. At the same time as receiving the grid connection command, the inverter issues the first closing command to the frame circuit breaker, the frame circuit breaker begins to close, and the inverter performs AC soft start.

[0099] The inverter output voltage gradually increases, and the relevant inverter algorithm initiates the AC inverter voltage to track the grid from zero until the inverter output signal matches the three-phase grid signal, meeting the grid connection requirements. Then, a second closing command is issued to the grid connection switch, turning on several power switches. These power switches are voltage-type devices and can complete the closing process within a few microseconds. Once both the frame circuit breaker and the grid connection switch have closed, and the inverter outputs grid-connected current, grid connection is confirmed to be successful, thus completing the grid connection process.

[0100] The grid connection method for photovoltaic systems provided by this utility model combines the fast closing time of the power switches, the high breaking capacity of the frame circuit breaker, and the ability of the frame circuit breaker to physically isolate the grid by using several power switching transistors as grid connection switches in series with the frame circuit breaker, thus forming a fast grid connection system. This system can realize the fast grid connection function of the inverter and meet the needs of fast inverter dispatch.

[0101] The embodiments of this utility model will be described below in conjunction with actual application scenarios.

[0102] like Figure 8 As shown, the photovoltaic system includes photovoltaic string units, a DC load switch, an inverter, an inverter inductor, a grid-connected switch, a frame circuit breaker, and a transformer, all connected in sequence. An AC filter capacitor is connected in parallel to the output terminal of the inverter inductor, and the output terminal of the transformer is connected to a 35kV high-voltage three-phase power grid.

[0103] The photovoltaic system uses a frame circuit breaker as the breaking device, which has high breaking capacity and the ability to physically isolate the power grid; it uses several IGBT modules as grid-connected switches. IGBT modules are voltage-type devices that can complete the closing process within a few microseconds and have the ability to close quickly.

[0104] like Figure 9 As shown, the grid connection process of a photovoltaic system can be illustrated by a grid connection timing diagram.

[0105] At time t0, the inverter in the photovoltaic system receives the power station's start-up command and simultaneously issues a closing command to the frame circuit breaker. The frame circuit breaker is a mechanical switch, and it takes approximately 50ms to complete the closing action after receiving the closing command at time t0. The inverter starts up at time t0, and its output electrical signal tracks the grid from zero, with the voltage gradually increasing, requiring at least 20ms. By time t1, the inverter's output electrical signal is consistent with the grid's electrical signal, meeting the grid connection requirements, and several closing commands are issued to the IGBT modules.

[0106] The IGBT module is a voltage-type device that can complete the closing process within a few microseconds. By time t2, the IGBT module has turned on, and the grid-connected switch has closed. At this time, the electrical signal output by the inverter is consistent with the electrical signal of the power grid. This continues until time t3, when the frame circuit breaker also closes, and the inverter completes its grid-connected operation, outputting grid-connected current, thus successfully connecting the photovoltaic system to the grid. In other words, the time from when the power station issues the grid-connection command to when the inverter completes its grid-connection operation and integrates the grid-connected current into the grid, from time t0 to time t3, is approximately 50ms. Since the IGBT turn-on time only takes a few microseconds, most of the 50ms time is the closing action time of the frame circuit breaker. The inverter's grid-connection time is much smaller than that of traditional solutions.

[0107] The grid connection method for photovoltaic systems provided by this utility model adopts a series connection of frame circuit breakers and IGBT modules, combining the fast closing time of IGBT modules, the high breaking capacity of frame circuit breakers, and the ability to physically isolate the power grid to form a fast grid connection system that can meet the current market requirements for fast grid connection of inverters.

[0108] It is understood that those skilled in the art can combine various implementation methods in the above embodiments under the guidance of the above examples to obtain technical solutions with multiple implementation methods.

[0109] The above description is only a preferred embodiment of the present utility model and is not intended to limit the present utility model. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present utility model should be included within the protection scope of the present utility model.

Claims

1. A photovoltaic system, characterized by, The utility model relates to a photovoltaic string unit, an inverter, a grid-connected switch and a frame circuit breaker. The photovoltaic string unit is connected to the input of the inverter; the output of the inverter is connected to the input of the grid-connected switch; the output of the grid-connected switch is connected to the input of the frame circuit breaker; and the output of the frame circuit breaker is used to be connected to a three-phase power grid. The grid-connected switch comprises a plurality of power switch tubes. The frame circuit breaker is used to be closed when the inverter responds to a grid-connected instruction. The plurality of power switch tubes are used to be turned on when the electrical signal output by the inverter and the electrical signal of the three-phase power grid reach consistency, so as to complete the closing of the grid-connected switch. Each phase output of the inverter is connected to at least one power switch tube, wherein: When the at least one power switch tube is multiple, the at least one power switch tube is connected in series; when the at least one power switch tube corresponding to each phase is one, the power switch tube is directly connected in series between the inverter and the frame circuit breaker; and when the at least one power switch tube corresponding to each phase is two or more, at least one power switch tube is connected in full series and forms an integral whole, which is connected in series between the inverter and the frame circuit breaker; or When the at least one power switch tube is multiple, the at least one power switch tube is connected in parallel. When the at least one power switch tube is connected in parallel, when the at least one power switch tube is odd, the at least one power switch tube is divided into a plurality of groups of power switch tubes, each group of power switch tubes comprising one or more power switch tubes, and the plurality of groups of power switch tubes are connected in parallel.

2. The photovoltaic system of claim 1, wherein, When the at least one power switch tube is connected in parallel, when the at least one power switch tube is even, the at least one power switch tube is divided into a plurality of groups of power switch tubes, each group of power switch tubes having the same number of power switch tubes and the power switch tubes in each group of power switch tubes being connected in series, and the plurality of groups of power switch tubes being connected in parallel.

3. The photovoltaic system of claim 1, wherein, The plurality of groups of power switch tubes are even, and every two groups of power switch tubes are connected in reverse parallel.

4. The photovoltaic system of claim 3, wherein, The power switch tube is an IGBT.

5. The photovoltaic system according to any of claims 1-4, characterized in that, The time at which the plurality of power switch tubes complete turning on is earlier than the time at which the frame circuit breaker completes closing.

6. The photovoltaic system according to any of claims 1-4, characterized in that, ​

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