A startup control method suitable for photovoltaic power station-MG system

Abstract

The present invention relates to the technical field of new energy grid connection, and discloses a start-up and control method applicable to a photovoltaic power station-MG system, including: a start-up process: the backup power supply or part of the photovoltaic array works to start the MG system; a grid connection process: detecting whether the MG system reaches the synchronous speed, if so, entering the grid connection process, otherwise returning to the start-up process; detecting whether the current meets the grid connection requirements through a quasi-synchronous grid connection device, if so, the grid connection is successful, and the MG system is successfully connected to the grid and operates; otherwise, returning to the grid connection process, the MG system still operates and waits for the opportunity to connect to the grid; a power supply and control switching process: executing a backup power supply removal strategy or a DC booster and photovoltaic inverter switching control strategy, connecting to the remaining photovoltaic arrays, and the entire station is successfully connected to the grid and operates. The present invention provides a feasible solution for photovoltaic use of new energy synchronous machines for grid connection, and also lays a certain foundation for other types of new energy use synchronous machine grid connection start-up and control methods.

Description

Technical Field

[0001] The present invention relates to the technical field of new energy grid connection, and in particular to a startup control method applicable to a photovoltaic power station-MG system. Background Art

[0002] my country has a vast territory and a large land area, and is rich in solar energy resources. Traditional photovoltaic stations transmit electricity to the power grid through electronic converters, which cannot provide inertia and damping support like thermal power plants, resulting in reduced voltage and frequency stability of the power system. Based on this, some studies have proposed technologies for connecting new energy to the grid through motor-generator systems (hereinafter referred to as MG systems) to make up for the shortcomings of insufficient inertia of power electronic converters.

[0003] For example, the invention patent CN201910774237.8 discloses a new energy grid-connected control system and method based on a new energy synchronous machine. The grid-connected system of a new energy electric field containing a direct-drive wind turbine and a photovoltaic array is composed of a power grid, a new energy synchronous machine, new energy, a voltage source inverter, a control module, and an electrical measuring instrument; the new energy synchronous machine is coaxially connected by two synchronous motors of the same capacity, and the new energy inverter drives the synchronous motor, and the synchronous motor drives the synchronous generator to directly connect to the power grid; depending on the control method, the grid-connected system and its control module structure are different. Based on the source-grid phase difference control theory, this invention proposes several control methods for new energy to be connected to the grid through a new energy synchronous machine to achieve parallel convergence of a single or multiple new energy units through a new energy synchronous machine to connect to the grid, solving many problems caused by new energy grid connection, such as stable transmission control of output power, decreased voltage and frequency stability, etc.

[0004] However, there are not many complete sets of equipment for operation and use in the field of new energy synchronous machines for photovoltaic grid connection, and this needs to be improved urgently. Summary of the invention

[0005] In view of the fact that there are not many complete sets of equipment for operation in the field of new energy synchronous machines for photovoltaic grid connection, the present invention proposes a starting and controlling method suitable for photovoltaic power station-MG system, which provides a feasible solution for photovoltaic grid connection using new energy synchronous machines, and also lays a certain foundation for starting and controlling methods for grid connection using other types of new energy synchronous machines.

[0006] The technical solution adopted by the present invention is as follows:

[0007] A startup control method applicable to a photovoltaic power station-MG system, comprising:

[0008] Startup process: The backup power supply or part of the photovoltaic array works, so that the MG system starts;

[0009] Grid connection process: Check whether the MG system reaches the synchronous speed. If so, it enters the grid connection process, otherwise it returns to the startup process; the quasi-synchronous grid connection device is used to check whether the current meets the grid connection requirements. If so, the grid connection is successful and the MG system is successfully connected to the grid and runs; otherwise, it returns to the grid connection process and the MG system still runs and waits for the opportunity to connect to the grid;

[0010] Power supply and control switching process: Execute the backup power supply removal strategy or the DC booster and PV inverter switching control strategy, connect to the remaining PV arrays, and the entire station is successfully connected to the grid.

[0011] Furthermore, when the starting power supply is a backup power supply, the starting process of the MG system includes: the backup power supply drives the speed of the MG system to gradually increase through the inverter, and when the speed of the MG system reaches the synchronous speed, the starting process of the MG system is completed.

[0012] Furthermore, when the starting power source is a partial photovoltaic array, the starting process of the MG system includes: the partial photovoltaic array generates direct current, the direct current is output as alternating current through an inverter, and the output alternating current is controlled so that the speed of the MG system gradually increases. When the speed of the MG system increases to the synchronous speed, the starting process of the MG system is completed.

[0013] Furthermore, when the starting power supply is a backup power supply or a partial photovoltaic array, the grid-connected process of the MG system includes: monitoring the amplitude difference, phase difference and frequency difference between the generator and the grid voltage through a quasi-synchronous grid-connected device. When the amplitude difference, phase difference and frequency difference meet the grid-connected conditions, the quasi-synchronous grid-connected device is closed and the grid-connected is successful.

[0014] Furthermore, when the starting power supply is the backup power supply, the power supply and control switching process of the MG system includes: after the grid is successfully connected, the backup power supply is cut off or still connected to the MG system in parallel with the photovoltaic array to supply power to the grid; the photovoltaic array is connected in parallel to the motor side of the MG system, and the MG system enters the grid-connected operation state.

[0015] Furthermore, when the starting power source is part of the photovoltaic array, the power and control switching process of the MG system includes: after successful grid connection, the part of the photovoltaic array that previously served as the starting power source is still connected to the grid and continues to supply power to the grid; the remaining photovoltaic arrays are connected in parallel to the motor side of the MG system, and the MG system enters the grid-connected operation state.

[0016] Furthermore, when the starting power source is part of the photovoltaic array, a switching control strategy for the DC booster and the photovoltaic inverter is executed, including: the photovoltaic inverter of the photovoltaic array adopts a speed closed-loop control method before being connected to the grid, and the reference value of the output voltage frequency of the photovoltaic inverter is obtained by the closed-loop control of the MG system speed; after being connected to the grid, the photovoltaic inverter adopts a constant DC voltage-based control method, and the reference value of the output voltage frequency of the photovoltaic inverter is obtained by the closed-loop control of the DC voltage.

[0017] Furthermore, the principles of coordination between different control links of the PV array after grid connection include: the DC link, i.e., the DC booster, is responsible for tracking the maximum power of the PV, the inverter link, i.e., the PV inverter, is responsible for controlling the transmission power of the MG system, and the power of the MG system connected to the grid is determined by the PV operating point.

[0018] Furthermore, when the startup power supply is the backup power supply, the backup power supply removal strategy is executed, which does not involve the switching control of the DC booster and the photovoltaic inverter. At this time, the DC booster and photovoltaic inverter of the photovoltaic array are in the normal steady-state grid-connected operation mode.

[0019] The beneficial effects of the present invention are:

[0020] 1) The startup control method for a photovoltaic power station-MG system proposed in the present invention is beneficial to the simulation modeling research of photovoltaic stations connected to the grid via MG and the application of photovoltaic stations connected to the grid via MG, and provides a reference for other types of new energy connected to the grid via MGP.

[0021] 2) The startup control method of the present invention is to switch the DC boost and inverter control strategies before and after grid connection. Both control strategies before and after switching can well realize the corresponding control functions, thereby smoothly realizing the grid connection of photovoltaic via MG system.

[0022] 3) The startup control method of the present invention provides a new solution to solve many problems caused by the connection of new energy sources to the grid, such as the lack of rotational inertia of the power grid, the decrease in voltage and frequency stability, etc. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the schematic diagram of the MG system starting power supply.

[0024] Figure 2 It is a startup flow chart based on the photovoltaic array.

[0025] Figure 3 It is a startup flow chart based on backup power supply.

[0026] Figure 4 It is a schematic diagram of the grid connection method using the photovoltaic array as the starting power source.

[0027] Figure 5This is a schematic diagram of the grid connection method in which the backup power supply is used as the starting power supply.

[0028] Figure 6 It is a schematic diagram of DC voltage control switching.

[0029] Figure 7 This is a schematic diagram of inverter control switching.

[0030] Reference numerals: 1-first photovoltaic array, 2-second photovoltaic array, 3-backup power supply, 4-MG system, 5-quasi-synchronous grid-connected device, 6-grid; 101, 201, 202, 301-switching device, 401-motor, 402-generator; Reset1-first reset signal, Reset2-second reset signal, U dc - DC voltage, U dcref - DC reference voltage, U mppt -Maximum power point tracking voltage, U pv -Inverter voltage, ω -Speed, ω ref - Reference speed. DETAILED DESCRIPTION

[0031] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, the specific implementation methods of the present invention are now described. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention, that is, the embodiments described are only part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without making creative work are within the scope of protection of the present invention.

[0032] This embodiment provides a startup control method applicable to a photovoltaic power station-MG system, including:

[0033] Startup process: The backup power supply or part of the photovoltaic array works, so that the MG system starts;

[0034] Grid connection process: Check whether the MG system reaches the synchronous speed. If so, it enters the grid connection process, otherwise it returns to the startup process; the quasi-synchronous grid connection device is used to check whether the current meets the grid connection requirements. If so, the grid connection is successful and the MG system is successfully connected to the grid and runs; otherwise, it returns to the grid connection process and the MG system still runs and waits for the opportunity to connect to the grid;

[0035] Power supply and control switching process: Execute the backup power supply removal strategy or the DC booster and PV inverter switching control strategy, connect to the remaining PV arrays, and the entire station is successfully connected to the grid.

[0036] like Figure 1The figure shows the overall structure of the system when two different starting power supplies are used, where 1 represents the first photovoltaic array, 2 represents the second photovoltaic array (as a starting power supply), 3 represents the backup power supply, 4 represents the MG system, where 401 represents the motor, 402 represents the generator, 5 represents the quasi-synchronous grid-connected device, 6 represents the power grid, and 101, 201, 202, and 301 represent switch devices.

[0037] like Figure 2 The figure shows the startup process based on the photovoltaic array (where Y is yes and N is no). When the startup power source is the second photovoltaic array 2, the process of photovoltaic grid connection via the MG system 4 is adopted. The process is: the second photovoltaic array 2 works, so that the MG system 4 starts; detects whether the MG system 4 reaches the synchronous speed, otherwise returns to the startup process, and enters the grid connection process if it is; the quasi-synchronous grid connection device 5 detects whether it meets the grid connection requirements, otherwise returns to the grid connection process, and the MG system 4 still runs and waits for the grid connection opportunity; if it is, the grid connection is successful, and the MG system 4 is successfully connected to the grid and operated, and then the DC side boost circuit and the inverter switch control strategy are connected to the first photovoltaic array 1, and the entire station is successfully connected to the grid and operated.

[0038] like Figure 3 The figure shows the startup process based on the backup power supply (where Y is yes and N is no). When the startup power supply is the backup power supply 3, the process of connecting to the grid through the photovoltaic MG system 4 is as follows: the backup power supply 3 works to start the MG system 4; it detects whether the MG system 4 reaches the synchronous speed, otherwise it returns to the startup process, and if so, it enters the grid connection process; the quasi-synchronous grid connection device 5 detects whether it meets the grid connection requirements, otherwise it returns to the grid connection process, and the MG system 4 still runs and waits for the grid connection opportunity; if so, the grid connection is successful, and the MG system 4 is successfully connected to the grid and operated, and then the backup power supply 3 is cut off and connected to the first photovoltaic array 1, and the entire station is successfully connected to the grid and operated.

[0039] like Figure 4 The figure shows the grid-connected mode of the photovoltaic array as the starting power source. When the starting power source is the second photovoltaic array 2, when starting, the switch devices 201 and 202 are closed, and the inverter of the photovoltaic array 2 is operated so that the MG system 4 is started with variable frequency. During the starting process, the DC voltage control adopts the control based on the fixed DC side voltage, so that the DC side capacitor U dc Can reach the rated value U dcref The MG system 4 is supplied with electric energy nearby, and the inverter control adopts the speed closed loop control method to make the shaft speed of the MG system 4 Can reach synchronous speed refNearby. After the startup process is completed, the quasi-synchronous grid-connected device 5 detects the closing conditions. When the amplitude, phase and frequency differences between the voltage of the generator 402 and the grid 6 meet the grid-connected conditions, the quasi-synchronous grid-connected device 5 automatically closes and the closing is successful. The inverter replacement control strategy is fixed DC voltage control, so that the DC voltage at the input end of the inverter remains constant, and the DC side control mode is changed to maximum power tracking control, so that the output power of the photovoltaic side reaches the maximum. After the control strategy is switched, the second photovoltaic array 2 continues to be connected to the grid, and then the first photovoltaic array 1 is connected to the motor 401 side of the MG system 4 through the switch device 101. All photovoltaic arrays operate in parallel, and the entire photovoltaic station is connected to the grid.

[0040] like Figure 5 The figure shows the grid-connected mode of the backup power supply as the starting power supply. When the starting power supply is the backup power supply 3, the switch device 301 is closed during startup, and the backup power supply 3 is connected to the inverter to drive the MG system 4 to start with variable frequency, so that the shaft speed of the MG system 4 reaches the synchronous speed, and the starting process ends. When the amplitude, phase and frequency difference between the voltage of the generator 402 and the grid 6 meet the grid-connected conditions, the quasi-synchronous grid-connected device automatically closes and the closing is successful. After the closing is successful, the backup power supply 3 is disconnected from the motor 401 side of the MG system 4 301, and the switch device 101 on the photovoltaic array 1 and the MG system motor 401 side is connected. The photovoltaic array 1 is successfully connected to the grid. At this time, the inverter control strategy is constant DC voltage control, so that the DC voltage at the input end of the inverter remains constant, and the DC side control mode is changed to maximum power tracking control, so that the output power of the photovoltaic side reaches the maximum. When the starting power supply is the backup power supply, the switching of the control strategy is not involved from startup to the successful grid-connected operation of the entire system.

[0041] The detailed switching process of the control strategy in this embodiment is as follows: Figure 6 and Figure 7 shown. Figure 6 The figure shows the DC voltage control switching process. The detailed switching process is as follows: before grid connection, controller 1 is turned on and controller 2 is turned off, so that the DC voltage side control strategy is fixed DC voltage control. At this time, Reset2 is set to 1, and the integrator of DC side voltage controller 2 is zero to prevent the new control loop from having a large initial value at the time of opening during the control switching process, which affects the control effect; after grid connection, controller 1 is turned off and controller 2 is turned on, so that the DC side control strategy becomes maximum power tracking control. At this time, Reset1 is set to 1 to set the integrator of DC side voltage controller 1 to zero.

[0042] like Figure 7It is the inverter control switching process. The detailed switching process is: before grid connection, controller 3 is turned on and controller 4 is turned off, so that the inverter control strategy is speed closed-loop control. At this time, Reset2 is set to 1 to make the integrator of photovoltaic voltage controller 4 zero, to prevent the new control loop from having a large initial value at the time of opening during the control switching process, affecting the control effect; after grid connection, controller 3 is turned off and controller 4 is turned on, so that the inverter control strategy is constant DC voltage control. At this time, Reset1 is set to 1 to make the integrator of photovoltaic voltage controller 4 zero.

[0043] It should be noted that, for the aforementioned method embodiments, for the sake of simplicity of description, they are expressed as a series of action combinations, but those skilled in the art should be aware that the present application is not limited by the order of the actions described, because according to the present application, certain steps can be performed in other orders or simultaneously. Secondly, those skilled in the art should also be aware that the embodiments described in the specification are all preferred embodiments, and the actions and modules involved are not necessarily required by the present application.

Claims

1. A startup control method applicable to a photovoltaic power station-MG system, characterized in that: include: Startup process: part of the photovoltaic array works to start the MG system, which is a motor-generator system; Grid connection process: Check whether the MG system reaches the synchronous speed. If so, it enters the grid connection process, otherwise it returns to the startup process; the quasi-synchronous grid connection device is used to check whether the current meets the grid connection requirements. If so, the grid connection is successful and the MG system is successfully connected to the grid and runs; otherwise, it returns to the grid connection process and the MG system still runs and waits for the opportunity to connect to the grid; Power supply and control switching process: execute the DC booster and PV inverter switching control strategy, connect to the remaining PV arrays, and the entire station is successfully connected to the grid; The power supply and control switching process includes: after the grid connection is successful, part of the photovoltaic array that was previously used as the starting power source is still connected to the grid and continues to supply power to the grid; the remaining photovoltaic arrays are connected in parallel to the motor side of the MG system, and the MG system enters the grid-connected operation state; The control strategy for switching between the DC booster and the photovoltaic inverter is implemented, including: the photovoltaic inverter of the photovoltaic array adopts a speed closed-loop control method before grid connection, and obtains a reference value of the output voltage frequency of the photovoltaic inverter by closed-loop control of the MG system speed; the photovoltaic inverter adopts a constant DC voltage control method after grid connection, and obtains a reference value of the output voltage frequency of the photovoltaic inverter by closed-loop control of the DC voltage; The principles of coordination between different control links of the photovoltaic array after grid connection include: the DC link, i.e. the DC booster, is responsible for tracking the maximum power of the photovoltaic system; the inverter link, i.e. the photovoltaic inverter, is responsible for controlling the transmission power of the MG system; and the power of the MG system connected to the grid is determined by the photovoltaic operating point.

2. A startup control method applicable to a photovoltaic power station-MG system according to claim 1, characterized in that: The startup process of the MG system includes: the part of the photovoltaic array generates direct current, the direct current is output as alternating current through the inverter, the output alternating current is controlled so that the speed of the MG system gradually increases, and when the speed of the MG system increases to the synchronous speed, the startup process of the MG system is completed.

3. A startup control method applicable to a photovoltaic power station-MG system according to claim 1, characterized in that: The grid-connected process of the MG system includes: monitoring the amplitude difference, phase difference and frequency difference between the generator and the grid voltage through the quasi-synchronous grid-connected device. When the amplitude difference, phase difference and frequency difference meet the grid-connected conditions, the quasi-synchronous grid-connected device is closed and the grid-connected is successful.

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