A photovoltaic flexible grid-connected system

Abstract

The utility model discloses a photovoltaic flexible grid-connected system, including photovoltaic system and with its connection photovoltaic flexible grid-connected device, photovoltaic flexible grid-connected device includes wire and the flexible grid-connected system high voltage side isolator, phase change, fuse, flexible grid-connected system low voltage side isolator, resistance switch box, flexible grid-connected low voltage side switch, control unit and display screen that set up on wire, one end of wire is connected with high voltage side power switch and phase change isolator in photovoltaic system respectively, its other end is connected with low voltage side 400V bus incoming line switch in photovoltaic system, and flexible grid-connected system high voltage side isolator is close to high voltage side power switch one side, and flexible grid-connected low voltage side switch is close to low voltage side 400V bus incoming line switch one side. The utility model discloses can be good to realize automatic flexible station in -use electric equipment power switch, avoid the loss of photovoltaic system standby at night, save the power cost of photovoltaic power station.

Description

Technical Field

[0001] This utility model belongs to the technical field of relay protection and automatic safety devices, and relates to a photovoltaic flexible grid-connected system. Background Technology

[0002] After a photovoltaic (PV) power generation system is connected to the grid, the grid connection switch is generally not disconnected, and the transformers in the PV power station are always energized. At night, the PV inverters are off, and the transformers operate under no-load conditions. Therefore, a large number of medium-voltage transformers consume the system's active and reactive power, generating unnecessary electricity costs. For example, a 20MW PV power station can incur over 100,000 yuan in active power costs annually, increasing the power plant's operating costs. Currently, the method used is to manually open the collector switch at night and manually close it again before generating electricity during the day. This practice has the following problems: 1) Frequent switching can easily cause switch failures; 2) Frequent closing will generate inrush current in the medium-voltage transformer, which will accelerate transformer aging and shorten its lifespan under the influence of electrodynamic forces; 3) It increases the workload of maintenance personnel; 4) The timing of switch opening and closing is not precise enough, resulting in a loss of some power generation.

[0003] To address these issues, a flexible grid-connected photovoltaic system was designed. Utility Model Content

[0004] The purpose of this utility model is to overcome the shortcomings of the existing technology and provide a simple and reasonable photovoltaic flexible grid-connected system. Through the designed grid-connected system, the medium-voltage transformer in the station is automatically shut down during the inverter shutdown period and automatically starts operating before the inverter starts generating voltage. This is used to reduce the standby loss of the photovoltaic power station, reduce the demand for active power of the system, reduce the active power burden of the system, and is conducive to energy saving and consumption reduction of the entire photovoltaic system.

[0005] This utility model is achieved through the following technical solution: a flexible photovoltaic grid-connected system, including a photovoltaic system and a flexible photovoltaic grid-connected device connected to the photovoltaic system. The flexible photovoltaic grid-connected device includes a conductor and a flexible grid-connected system high-voltage side disconnecting switch, a phase changer, a fuse, a flexible grid-connected system low-voltage side disconnecting switch, a resistor switching box, a flexible grid-connected low-voltage side switch, a control unit, and a display screen, which are sequentially arranged on the conductor. One end of the conductor is connected to the high-voltage side power switch and the phase changer disconnecting switch in the photovoltaic system, and the other end of the conductor is connected to the low-voltage side 400V bus incoming switch in the photovoltaic system. The flexible grid-connected system high-voltage side disconnecting switch is closer to the high-voltage side power switch, and the flexible grid-connected low-voltage side switch is closer to the low-voltage side 400V bus incoming switch. The phase change™ wiring group is DY11, model SC10-100 / 10.5TM, rated capacity 100kVA, short-circuit impedance 4%, used to convert 10kV voltage to 380V voltage; The rated fusing current of the fuse is 1.1 times the rated current of the low-voltage side of the phase change circuit, and it is used to protect the circuit. The resistor switching box is equipped with resistors R1-R10 connected in series. Each of the resistors R1-R10 is connected in parallel with a switch K1-K10. The resistance values ​​decrease sequentially from 150Ω to 5Ω to achieve voltage division. The flexible grid-connected low-voltage side switch is model TJ-1.14 / 125, which is used to automatically open after a delay after the flexible grid-connected output switch is closed. A flexible grid-connected output switch is installed on the conductor between the high-voltage side power switch and the phase-change isolating switch. A new synchronizing detection relay, model DT-1, rated voltage 100V, is also installed on the flexible grid-connected output switch to detect whether the voltage difference, frequency difference, and phase difference between the upper and lower ports of the flexible grid-connected output switch meet the set value requirements. The new synchronizing detection relay is connected to the secondary circuit of the original bus PT and the secondary circuit of the new phase-change high-voltage side PT through conductors. The flexible grid-connected output switch is model VS1-12 / 1250A-31.5, which is used to achieve grid connection when the three requirements of the same period are met, and to automatically disconnect the high-voltage side power switch when the photovoltaic system is in standby mode. The high-voltage side disconnecting switch, phase changer, fuse, low-voltage side disconnecting switch, resistor switching box, low-voltage side switch, and display screen of the flexible grid-connected system are all connected to the control unit. The control unit controls the device to start and stop, closes the resistor switching box to switch switches K1 to K10, detects the secondary voltage of the bus PT and the high-voltage side voltage of the medium-voltage transformer, and determines whether the voltage amplitude difference, phase difference, and frequency difference meet the setting requirements. A new phase-change high-voltage side is installed on the wire between the high-voltage side power switch and the phase-change isolating switch, which is used to collect the primary voltage at the lower port of the flexible grid-connected output switch and send it to the synchronization detection relay.

[0006] Preferably, the control unit (KZ) is equipped with interfaces D0:1-12, DI:1-4, analog input interfaces AI:1-8 and HMI_485, a 24V power output interface, and a 24V DC power input interface. The digital output interfaces DO:1-12 are connected to the closing positions of switches K1-K10, the flexible grid-connected output switch (QFH), the flexible grid-connected low-voltage side switch, and the high-voltage side power switch, respectively, to control the closing commands of K1 to K10, the flexible grid-connected output switch, the flexible grid-connected low-voltage side switch, and the high-voltage side power switch. A latitude and longitude time control switch is installed on the DI:1 interface line to automatically control the closing of the flexible grid-connected output switch according to daylight / darkness. The circuit breaker is open and closed, and the latitude and longitude time control switch is powered by +24V power. The DI:2-4 interfaces are connected to the closed positions of the high-voltage side disconnect switch, the low-voltage side switch, and the high-voltage side power switch of the flexible grid-connected system, respectively. The analog input interface AI:1-4 interfaces are connected to the secondary circuit of the original bus PT, and the analog input interface AI:5-8 interfaces are connected to the secondary circuit of the newly added phase change high-voltage side PT. The HMI_485 interface, the 24V power output interface, and the 24V DC power input interface are connected to the display screen and the power module, respectively. The display screen is used to display the status of each input and output contact, the sampled value, and the parameter settings. The power module is used to convert 220V AC power to 24V DC power for use by the control unit.

[0007] Preferably, the switching switches K1-K10 of the resistor switching box are model TJ-1.14 / 125, with a rated voltage of 1.14kV and a rated current of 125A.

[0008] Preferably, the fuse has a fusing current of 130A.

[0009] Preferably, the load-side voltage of the synchronous detection relay is taken from the secondary voltage of the newly added phase-change high-voltage side PT, and the system-side voltage is taken from the secondary voltage of the original photovoltaic 10kV bus PT.

[0010] Preferably, the synchronous detection relay is model DT-1 with a rated voltage of 100V.

[0011] The beneficial effects of this utility model are as follows: The photovoltaic flexible grid-connected system designed in this utility model uses a medium-voltage flexible start-stop system to automatically shut down the medium-voltage transformer in the station during the inverter shutdown period and automatically start the medium-voltage transformer in the station before the inverter starts generating voltage. This is used to reduce the standby loss of the photovoltaic power station, reduce the demand for active power of the system, reduce the active power burden of the system, and help the photovoltaic system save energy and reduce consumption. Attached Figure Description

[0012] Figure 1 This is the original primary system diagram of the substation busbar in a photovoltaic system; Figure 2 shows the primary system diagram of the medium-pressure system of this utility model; Figure 3. Schematic diagram of the internal structure of the resistor switching box in this utility model; Figure 4. Schematic diagram of the control unit in this utility model. Detailed Implementation

[0013] To enable those skilled in the art to more clearly understand the purpose, technical solution and advantages of this utility model, the present utility model will be further described below in conjunction with the accompanying drawings and embodiments.

[0014] In the description of this utility model, it should be understood that the orientation or positional relationship indicated by terms such as "upper", "lower", "left", "right", "inner", "outer", "horizontal", and "vertical" are based on the orientation or positional relationship shown in the accompanying drawings and are only for the convenience of describing this utility model, and are not intended to indicate or imply that the device or component referred to must have a specific orientation, and therefore should not be construed as a limitation of this utility model.

[0015] The present invention will now be described in detail with reference to the accompanying drawings: Figure 2-3 As shown, a flexible grid-connected photovoltaic system includes a photovoltaic system and a flexible grid-connected photovoltaic device connected to the photovoltaic system. The flexible grid-connected photovoltaic device includes a conductor and a flexible grid-connected system high-voltage side disconnecting switch QSH, a phase changer TM, a fuse FU, a flexible grid-connected system low-voltage side disconnecting switch QSL, a resistor switching box R, a flexible grid-connected low-voltage side switch QFL, a control unit KZ, and a display screen XS, which are sequentially arranged on the conductor. One end of the conductor is connected to the high-voltage side power switch QF and the phase change disconnecting switch in the photovoltaic system, respectively. The other end of the conductor is connected to the low-voltage side 400V bus incoming switch in the photovoltaic system. The flexible grid-connected system high-voltage side disconnecting switch QSH is closer to the high-voltage side power switch QF, and the flexible grid-connected low-voltage side switch QFL is closer to the low-voltage side 400V bus incoming switch. The phase change™ wiring group is DY11, model SC10-100 / 10.5TM, rated capacity 100kVA, short-circuit impedance 4%, used to convert 10kV voltage to 380V voltage; The rated fusing current of the fuse FU is 1.1 times the rated current of the low-voltage side of the phase change TM, and it is used to protect the circuit. The resistor switching box R is internally configured with resistors R1-R10 (10-segment high-power aluminum shell resistors) connected in series. Each of the resistors R1-R10 is connected in parallel with a switch K1-K10. The resistance values ​​decrease sequentially from 150Ω to 5Ω to achieve voltage division. The flexible grid-connected low-voltage side switch QFL is model TJ-1.14 / 125, and is used to automatically open after a delay after the flexible grid-connected output switch QFH is closed. A flexible grid-connected output switch QFH is installed on the conductor between the high-voltage side power switch QF and the phase-change isolating switch. A new synchronization detection relay KY, model DT-1, rated voltage 100V, is also installed on the flexible grid-connected output switch QFH to detect whether the voltage difference, frequency difference, and phase difference between the upper and lower ports of the flexible grid-connected output switch QFH meet the set value requirements. The new synchronization detection relay KY is connected to the secondary circuit of the original bus PT and the secondary circuit of the new phase-change high-voltage side PT through conductors. The flexible grid-connected output switch QFH is model VS1-12 / 1250A-31.5, which is used to achieve grid connection when the three requirements of the same period are met, and to automatically disconnect the high-voltage side power switch QF when the photovoltaic system is in standby mode; The high-voltage side disconnector QSH, phase changer TM, fuse FU, low-voltage side disconnector QSL, resistor switching box R, low-voltage side switch QFL, and display screen XS of the flexible grid-connected system are all connected to the control unit KZ. The control unit KZ controls the device to start and stop, closes the resistor switching box R to switch switches K1 to K10, detects the secondary voltage of the bus PT and the high-voltage side voltage of the medium-voltage transformer, and determines whether the voltage amplitude difference, phase difference, and frequency difference meet the setting requirements. A new phase-change high-voltage side PT is installed on the wire between the high-voltage side power switch QF and the phase-change isolating switch to collect the primary voltage at the lower port of the flexible grid-connected output switch QFH and send it to the synchronization detection relay KY.

[0016] like Figure 4As shown, the control unit KZ is equipped with interfaces D0:1-12, DI:1-4, analog input interfaces AI:1-8 and HMI_485, a 24V power output interface, and a 24V DC power input interface. The digital output interfaces DO:1-12 are connected to the closing positions of switches K1-K10, the flexible grid-connected output switch QFH, the flexible grid-connected low-voltage side switch QFL, and the high-voltage side power switch QF, respectively, to control the closing commands of K1 to K10, the flexible grid-connected output switch QFH, the flexible grid-connected low-voltage side switch QFL, and the high-voltage side power switch QF. A latitude and longitude time control switch SK is installed on the DI:1 interface line to automatically control the closing and opening of the flexible grid-connected output switch QFH according to daylight / darkness. The latitude and longitude time control switch SK is powered by a +24V power supply. The DI:2-4 interfaces are connected to the closed positions of the high-voltage side isolating switch QSH, the low-voltage side switch QFL, and the high-voltage side power switch QF of the flexible grid-connected system, respectively. The analog input interface AI:1-4 interfaces are connected to the secondary circuit of the original bus PT, and the analog input interface AI:5-8 interfaces are connected to the secondary circuit of the newly added phase-change high-voltage side PT. The HMI_485 interface, the 24V power output interface, and the 24V DC power input interface are connected to the display screen XS and the power module DY, respectively. The display screen XS is used to display the status of each input and output contact, the sampled value, and the parameter settings. The power module DY is used to convert 220V AC power to 24V DC power for use by the control unit.

[0017] The switching switches K1-K10 in the resistor switching box R are model TJ-1.14 / 125, with a rated voltage of 1.14kV and a rated current of 125A. The fuse FU has a fusing current of 130A. The load-side voltage of the synchronizing relay KY is taken from the secondary voltage of the newly added phase change high-voltage side PT, and the system-side voltage is taken from the secondary voltage of the original photovoltaic 10kV bus PT. The synchronizing relay KY is model DT-1, with a rated voltage of 100V.

[0018] A method for using a flexible grid-connected photovoltaic system, the method comprising the following steps: 1) Grid connection of the transformer substation in the photovoltaic system; S1) The high-voltage side disconnector QSH of the flexible grid-connected system is in the closed position; S2) The low-voltage side disconnector QSL of the flexible grid-connected system is in the closed position; S3) The resistor switching box R switching switches K1 to K10 are in the open position; S4) Flexible grid-connected output switch QFH is in the open position; S5) Flexible grid-connected low-voltage side switch QFL is in the open position; S6) Reset the output contacts of the latitude and longitude time control switch SK; S7) Control unit KZ issues a closing command for flexible grid-connected low-voltage side switch QFL; S8) The transformer substation in the photovoltaic system is energized; S9) The switching switches K1 to K10 of the resistor switching box are gradually closed to flexibly increase the voltage of the high voltage box in the photovoltaic system until the high voltage side voltage reaches the rated value. S10) Control unit KZ detects the secondary voltage of the detection bus PT and the high voltage of the medium voltage transformer, and determines whether the voltage amplitude difference, phase difference and frequency difference on both sides meet the setting requirements. S11) Once the conditions are confirmed to be met, a closing command is issued for the flexible grid connection output switch QFH, and the medium-voltage transformer restores the system power supply. S12) Disconnect the flexible grid-connected low-voltage side switch QFL; S13) Disconnect the switching switches K1 to K10 of the resistor switching box; S14) The photovoltaic system resumes power generation; 2) The transformer in the photovoltaic system is switched off-grid; S1) The latitude and longitude time control switch SK output contact action; S2) Control unit KZ issues a tripping command to the flexible grid-connected output switch QFH; S3) Power outage of transformer substation in photovoltaic system The design features of this utility model are as follows: The phase changer TM in this invention is connected at its upper end to the lower end of the high-voltage side power switch QF in the flexible grid-connected system, and at its lower end to the upper end of the fuse FU. It is used to convert the 10kV voltage to the required 380V voltage. Its wiring group needs to be consistent with the original photovoltaic system's substation. The original photovoltaic system's substation phase changer TM1 model is: SCB10-1000 / 10.5, wiring group DY11. The selected phase changer TM model is: SC10-100 / 10.5TM, wiring group DY11, rated capacity 100kVA, short-circuit impedance 4%.

[0019] The flexible grid-connected output switch QFH in this invention is used to connect the original photovoltaic medium-voltage transformer from its off-grid state back to the grid. Before closing the switch, it must simultaneously meet the three synchronization requirements to prevent closing under asynchronous conditions, which could pose a safety hazard to the equipment. When the photovoltaic system is in standby mode, switch QFH automatically disconnects, enabling the photovoltaic medium-voltage transformer to automatically disconnect from the grid and reducing standby losses in the photovoltaic system. The flexible grid-connected output switch QFH model is VS1-12 / 1250A-31.5.

[0020] In this invention, the flexible grid-connected low-voltage side switch QFL is connected at its upper end to the lower end of the resistor switching box R, and at its lower end to the 400V busbar on the low-voltage side of the phase changer TM1. It is used to supply power from the low-voltage side of the phase changer TM1 to the original photovoltaic phase changer TM1 low-voltage busbar. After the original photovoltaic system's low-voltage busbar is energized, the photovoltaic system's medium-voltage phase changer TM1 will operate energized. After the flexible grid-connected output switch QFH is closed, the flexible grid-connected system low-voltage side switch QFL automatically opens after a delay. The model of the flexible grid-connected system low-voltage side switch QFL is TJ-1.14 / 125.

[0021] This invention adds a phase-change high-voltage side PT, which is connected to the lower end of the flexible grid-connected output switch QFH. It is used to collect the primary voltage at the lower end of the flexible grid-connected output switch QFH, and its secondary circuit sends the data to the newly added synchronization detection relay KY.

[0022] In this invention, the upper end of the resistor switching box R is connected to the lower end of the low-voltage side isolating switch QS in the flexible grid-connected system, and the lower end of the resistor switching box R is connected to the upper end of the flexible grid-connected low-voltage side switch QFL. It is used to flexibly transmit the voltage from the low-voltage side of the phase-change TM to the original photovoltaic phase-change TM1 low-voltage bus. The resistor switching box R is internally equipped with 10 high-power resistors to achieve voltage division. After the flexible grid-connected low-voltage side switch QFL is closed, the switching switch gradually closes from K1 to K10, and the resistance value of the resistor switching box gradually decreases from high to low, reducing the power-on impact of the photovoltaic phase-change TM1 and reducing the influence of transformer inrush current. Select switch K model: TJ-1.14 / 125, rated voltage 1.14kV, rated current 125A; select high-power resistor R as aluminum shell resistor, power 10kW, resistance values ​​from R1 to R10 are 150Ω, 100Ω, 75Ω, 60Ω, 50Ω, 40Ω, 30Ω, 20Ω, 10Ω, 5Ω respectively.

[0023] In this invention, the fuse FU is connected at its upper end to the low-voltage side of the phase changer™ and at its lower end to the upper end of the disconnecting switch QS on the low-voltage side of the flexible grid-connected system. It is used to prevent overcurrent on the low-voltage side of the phase changer™, thus providing protection. Its rated fusing current is 1.1 times the rated current of the low-voltage side of the phase changer™. Taking the substation model SC10-100 / 10.5 as an example, the fusing current of fuse FU is 130A.

[0024] The high-voltage side disconnecting switch QSH of the flexible grid-connected system in this utility model is connected at the upper end to the lower end of the original photovoltaic system collector wire 1 switch QF and at the lower end to the upper end of the phase changer TM, and is used for maintenance isolation of the phase changer TM.

[0025] The upper end of the low-voltage side disconnecting switch QS in the flexible grid-connected system is connected to the lower end of the fuse FU, and the lower end is connected to the upper end of the resistor switching box R, for maintenance isolation of the phase change TM.

[0026] The newly added synchronization detection relay KY in this utility model is used to determine whether the primary voltages at the upper and lower ports of the grid-connected output switch QFH of the flexible grid-connected system meet the three synchronization requirements, namely, voltage difference, frequency difference, and phase difference meeting the set value requirements. The relay KY system measures the voltage from the secondary voltage of the original 10kV photovoltaic bus PT, and the load-side voltage is taken from the secondary voltage of the newly added phase-change high-voltage side PT. When the synchronization requirements are met, the closing circuit of the grid-connected output switch QFH of the flexible grid-connected system is opened. The synchronization detection relay KY model is DT-1, with a rated voltage of 100V.

[0027] The latitude and longitude time control switch SK of this utility model, when it is dark, inputs to the control unit through the configured switch contacts. When the DI1 command is detected as true, the grid connection output switch QFH of the flexible grid-connected system is disconnected, and the substation in the photovoltaic system is de-energized. When it is light, the latitude and longitude time control switch SK contacts return, the DI1 command is false, the photovoltaic flexible grid connection program is started, the grid connection output switch QFH of the flexible grid-connected system is closed, and the substation in the photovoltaic system resumes operation.

[0028] The control unit of this utility model is used to control the start and stop of the photovoltaic flexible grid-connected device. When the photovoltaic system flexible grid-connected program is started, the flexible grid-connected low-voltage side switch QFL is closed, the photovoltaic system medium-voltage transformer is powered on, and the resistor switching box switches K1 to K10 are closed in sequence to flexibly adjust the high-voltage side voltage of the photovoltaic system medium-voltage transformer until the rated voltage is reached. The control unit KZ detects the secondary voltage of the bus PT and the high-voltage measurement voltage of the medium-voltage transformer, and judges whether the voltage amplitude difference, phase difference and frequency difference on both sides meet the setting requirements. When it is confirmed that the conditions are met, the flexible grid-connected system grid-connected output switch QFH is closed, the medium-voltage transformer restores the system power supply, the flexible grid-connected low-voltage side switch QFL is opened, the resistor switching box switches K1 to K10 are opened, and the photovoltaic system resumes the power generation state.

[0029] The display screen of this invention is connected to the control unit to display the status and sampling of each input / output contact, and is also used for parameter tuning and viewing.

[0030] The specific embodiments described herein are merely illustrative of the principles and effects of this utility model and are not intended to limit the scope of this utility model. Any person skilled in the art can modify or alter the above embodiments without departing from the spirit and scope of this utility model. Therefore, all equivalent modifications or alterations made by those skilled in the art without departing from the spirit and technical concept disclosed in this utility model should still be covered by the claims of this utility model.

Claims

1. A flexible photovoltaic grid-connected system, comprising a photovoltaic system and a flexible photovoltaic grid-connected device connected to the photovoltaic system, characterized in that: The flexible grid-connected photovoltaic device includes a conductor and, sequentially arranged on the conductor, a high-voltage side disconnect switch (QSH), a phase changer (TM), a fuse (FU), a low-voltage side disconnect switch (QSL), a resistor switching box (R), a low-voltage side switch (QFL), a control unit (KZ), and a display screen (XS). One end of the conductor is connected to the high-voltage side power switch (QF) and the phase change disconnect switch in the photovoltaic system, respectively, and the other end of the conductor is connected to the low-voltage side 400V bus incoming switch in the photovoltaic system. The high-voltage side disconnect switch (QSH) is closer to the high-voltage side power switch (QF), and the low-voltage side switch (QFL) is closer to the low-voltage side 400V bus incoming switch. The phase change™ wiring group is DY11, model SC10-100 / 10.5TM, rated capacity 100kVA, short-circuit impedance 4%, used to convert 10kV voltage to 380V voltage; The rated fusing current of the fuse (FU) is 1.1 times the rated current of the low-voltage side of the phase change (TM) circuit, and it is used to protect the circuit. The resistor switching box (R) is equipped with resistors R1-R10 connected in series. Each of the resistors R1-R10 is connected in parallel with a switch K1-K10. The resistance values ​​decrease sequentially from 150Ω to 5Ω to achieve voltage division. The flexible grid-connected low-voltage side switch (QFL) is model TJ-1.14 / 125, and is used to automatically trip after a delay after the flexible grid-connected output switch (QFH) is closed. A flexible grid-connected output switch (QFH) is installed on the conductor between the high-voltage side power switch (QF) and the phase-change isolating switch. A new synchronization detection relay (KY), model DT-1, rated voltage 100V, is also installed on the flexible grid-connected output switch (QFH) to detect whether the voltage difference, frequency difference, and phase difference between the upper and lower ports of the flexible grid-connected output switch (QFH) meet the set value requirements. The new synchronization detection relay (KY) is connected to the secondary circuit of the original bus PT and the secondary circuit of the new phase-change high-voltage side PT through conductors. The flexible grid-connected output switch (QFH) is model VS1-12 / 1250A-31.5, which is used to achieve grid connection when the three requirements of the same period are met, and to automatically disconnect the high-voltage side power switch (QF) when the photovoltaic system is in standby mode. The high-voltage side disconnect switch (QSH), phase changer (TM), fuse (FU), low-voltage side disconnect switch (QSL), resistor switching box (R), low-voltage side switch (QFL), and display screen (XS) of the flexible grid-connected system are all connected to the control unit (KZ). The control unit (KZ) controls the device to start and stop, closes the resistor switching box (R) to switch switches K1 to K10, detects the secondary voltage of the bus PT and the high-voltage side voltage of the medium-voltage transformer, and determines whether the voltage amplitude difference, phase difference, and frequency difference meet the setting requirements. A new phase-change high-voltage side (PT) is installed on the wire between the high-voltage side power switch (QF) and the phase-change isolating switch to collect the primary voltage at the lower port of the flexible grid-connected output switch (QFH) and send it to the synchronization detection relay (KY).

2. The photovoltaic flexible grid-connected system according to claim 1, characterized in that: The control unit (KZ) is equipped with interfaces D0:1-12, DI:1-4, analog input interfaces AI:1-8 and HMI_485, a 24V power output interface, and a 24V DC power input interface. The digital output interfaces DO:1-12 are connected to the closing positions of switches K1-K10, the flexible grid-connected output switch (QFH), the flexible grid-connected low-voltage side switch (QFL), and the high-voltage side power switch (QF), respectively, to control the closing commands of K1 to K10, the flexible grid-connected output switch (QFH), the flexible grid-connected low-voltage side switch (QFL), and the high-voltage side power switch (QF). The DI:1 interface line is equipped with a latitude and longitude time control switch (SK), which automatically controls the closing and opening of the flexible grid-connected output switch (QFH) according to daylight and darkness. The latitude and longitude time control switch (SK) is powered by a +24V power supply. The DI:2-4 interfaces are connected to the closing positions of the high-voltage side disconnect switch (QSH), the low-voltage side switch (QFL), and the high-voltage side power switch (QF) of the flexible grid-connected system, respectively. The analog input interface AI:1-4 interfaces are connected to the secondary circuit of the original bus PT, and the analog input interface AI:5-8 interfaces are connected to the secondary circuit of the newly added phase-change high-voltage side PT. The HMI_485 interface, the 24V power output interface, and the 24V DC power input interface are connected to the display screen (XS) and the power module (DY), respectively. The display screen (XS) is used to display the status of each input and output contact, the sampled value, and the parameter settings. The power module (DY) is used to convert 220V AC power to 24V DC power for use by the control unit.

3. The photovoltaic flexible grid-connected system according to claim 1, characterized in that: The switching switches K1-K10 of the resistor switching box (R) are model TJ-1.14 / 125, with a rated voltage of 1.14kV and a rated current of 125A.

4. The photovoltaic flexible grid-connected system according to claim 1, characterized in that: The fuse (FU) has a fusing current of 130A.

5. The photovoltaic flexible grid-connected system according to claim 1, characterized in that: The load-side voltage of the synchronous detection relay (KY) is taken from the secondary voltage of the newly added phase change high-voltage side PT, and the system-side voltage is taken from the secondary voltage of the original photovoltaic 10kV bus PT.

6. The photovoltaic flexible grid-connected system according to claim 1, characterized in that: The synchronous detection relay (KY) is model DT-1 with a rated voltage of 100V.

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