Starting method and system for dragging variable-speed pumped storage unit based on static frequency converter

Abstract

According to the starting method and system for dragging the variable-speed pumped storage unit based on the static frequency converter, the variable-speed pumped storage unit is connected to a power grid through a grid-connected switch and connected to one side of the static frequency converter through a wire outlet switch, and the other side of the static frequency converter is connected to the power grid; the variable-speed pumped storage unit is connected to one side of the machine-side converter, the other side of the machine-side converter is connected with one side of the grid-side converter through a direct-current bus, and the other side of the grid-side converter is connected to a power grid; under the starting constraint condition, the alternating-current excitation frequency introduced into the rotor side of the variable-speed pumped storage unit is determined; the static frequency converter is started to send instructions to the control system of the machine-side converter and the control system of the grid-side converter, and alternating-current exciting current is introduced into the rotor side of the variable-speed pumped storage unit; after the outgoing line switch is closed, the static frequency converter drags the variable-speed pumped storage unit at a constant speed to reach a target rotating speed; and when the target rotating speed is reached, the outlet switch is disconnected and the grid-connected switch is closed. Rotor position calculation accuracy in the static frequency converter low-frequency starting stage is improved, and unit starting success rate is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of motor control, and relates to a starting method and system for introducing low-frequency AC excitation on the rotor side when a static frequency converter drives a variable-speed pumped storage unit. Background Art

[0002] With the massive access of intermittent new energy sources such as wind power and photovoltaic power, the safe and stable operation of the power grid faces severe challenges. In solving the instability of new energy power generation, the role of energy storage technology in peak shaving and valley filling is becoming increasingly important. Among the many energy storage technologies today, pumped storage technology is one of the most economical, efficient and largest energy storage technologies. Pumped storage units are mainly divided into fixed speed and variable speed units. At present, most pumped storage power stations put into operation in China use conventional fixed speed units, which are synchronous motors. The fixed speed unit is often started by a static frequency converter (SFC) under pumping conditions. The fixed speed unit can only operate at the rated speed, and has shortcomings in operating efficiency, adjustment range and flexibility, while the variable speed unit has obvious advantages in these aspects. The variable speed unit is an asynchronous induction double-fed motor. By introducing AC excitation of different frequencies on the rotor side, the operating state of the unit can be adjusted arbitrarily according to needs, and has good operating characteristics.

[0003] In the prior art, there are two main methods for starting a variable speed unit: one is to use the converter on the rotor side for self-starting; the other is to start by dragging with a static frequency converter. The self-starting of the variable speed unit has very high requirements on the capacity and high voltage resistance of the converter on the rotor side. Such a full-power converter is very expensive, which will increase the construction cost of the pumped storage power station. There is also a non-full-power converter for the variable speed unit. The non-full-power variable speed unit generally cannot start by itself and needs to be dragged by a static frequency converter. When using a static frequency converter to drag a variable speed unit, the general method is to pass direct current on the rotor side of the variable speed unit to form a fixed rotor magnetic field, so that the asynchronous induction motor is equivalent to a winding type synchronous motor, so that the variable speed unit can be started by starting a fixed speed synchronous motor. From the principle of the static frequency converter, it can be seen that in the low-frequency rotation stage of the unit (below 0.5Hz), it calculates the rotor magnetic flux position based on the rotor motion equation, and the resistance torque in the rotor motion equation cannot be obtained particularly accurately, so there is a principle defect of large rotor position calculation and measurement error. The usual practice is: after measuring the static position of the rotor, a large impact current is passed through the stator side to generate a sufficiently large electromagnetic torque to accelerate the rotor to rotate quickly, so that it passes through the low-frequency rotation stage directly to the medium-high frequency rotation stage in a short time, so as to use the terminal voltage induced by the high-frequency rotation of the rotor to accurately calculate the position of the rotor or magnetic flux to control the acceleration of the unit. This method will cause a large impact on the motor. The pumped storage unit will switch back and forth between power generation and pumping conditions during operation. Frequent starting in this way is very unfriendly to the unit winding. Moreover, the pumped storage unit has a large capacity and a large moment of inertia, so the impact torque required will also be very large. In addition, the resistance torque of the unit cannot be accurately calculated. With the change of seasonal temperature and the degree of maintenance, its resistance torque also changes. This will increase the probability of failure to start the unit in the low-frequency stage, which is very unfavorable to the rapid regulation of the power grid load and safe and stable operation. Summary of the invention

[0004] In order to solve the deficiencies in the prior art, the present invention provides a starting method and system for a variable-speed pumped storage unit based on a static frequency converter, which effectively solves the shortcomings of large rotor position calculation error and large starting torque impact during the low-frequency starting stage of the static frequency converter, and effectively improves the success rate of unit starting.

[0005] The present invention adopts the following technical solution.

[0006] The present invention proposes a starting method based on a static frequency converter driving a variable speed pumped storage unit, comprising:

[0007] The variable speed pumped storage unit is connected to the grid through the grid-connected switch; the variable speed pumped storage unit is also connected to one side of the static frequency converter through the outgoing line switch, and the other side of the static frequency converter is connected to the grid; the variable speed pumped storage unit is connected to one side of the machine-side converter, and the other side of the machine-side converter is connected to one side of the grid-side converter through the DC bus, and the other side of the grid-side converter is connected to the grid;

[0008] The capacity of the variable speed pumped storage unit, the rated power under pumping conditions, the inertia time constant, the operating efficiency and the starting time are obtained. Based on the energy flow principle of the doubly fed induction motor, the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit is determined under the starting constraint conditions; wherein the starting constraint conditions include: the slip power transmitted from the stator side of the variable speed pumped storage unit to the machine-side converter is not greater than the capacity of the machine-side converter;

[0009] The static inverter is started according to its target speed, and the static inverter sends instructions to the control system of the generator-side converter and to the control system of the grid-side converter. The AC current of the power grid passes through the grid-side converter and the generator-side converter to obtain the AC excitation current introduced into the rotor side of the variable speed pumped storage unit.

[0010] After the outgoing line switch is closed, the static frequency converter obtains the difference between the target speed and the AC excitation frequency, and according to the terminal voltage of the variable speed pumped storage unit, it uniformly drives the variable speed pumped storage unit to reach the target speed;

[0011] When the variable speed pumped storage unit reaches the target speed, the outgoing line switch is disconnected, and after the grid-connected switch is closed, the variable speed pumped storage unit completes synchronous grid connection.

[0012] Preferably, the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit satisfies the following relationship:

[0013]

[0014] Where, sP m is the slip power from the stator side to the rotor side of the variable speed pumped storage unit, s is the slip rate, P m is the power input to the stator side of the variable speed pumped storage unit, P e is the rated power of the variable speed pumped storage unit under pumping conditions, f r is the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit, η is the operating efficiency of the variable speed pumped storage unit corresponding to the rated speed under pumping conditions, H is the inertia time constant of the variable speed pumped storage unit, and t is the starting time for the rotor of the variable speed pumped storage unit to accelerate to the target speed at a constant acceleration.

[0015] Preferably, the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit determined under the starting constraint conditions is greater than or equal to 0.5 Hz and less than or equal to 5 Hz.

[0016] Preferably, the target speed of the static frequency converter is the target speed of the variable speed pumped storage unit, which is set according to the specific required parameters of the actual project.

[0017] Preferably, the machine-side converter adopts voltage-frequency control to control the output frequency of the machine-side converter to be the AC excitation frequency and the output voltage to be the AC excitation voltage under the AC excitation frequency; the grid-side converter adopts constant DC bus voltage control.

[0018] Preferably, the static frequency converter is used to send instructions to the control system of the machine-side converter to control the machine-side converter to invert the AC excitation frequency and the AC excitation voltage at the AC excitation frequency, thereby controlling the low-frequency AC excitation current supplied to the rotor side of the variable speed pumped storage unit.

[0019] Preferably, the static frequency converter is also used to send instructions to the control system of the grid-side converter to control the grid-side converter to charge the DC bus and maintain the bus voltage at a constant value.

[0020] Preferably, the difference between the target speed and the AC excitation frequency is the actual speed of the rotor of the variable speed pumped storage unit.

[0021] Preferably, when the outgoing line switch is disconnected, the static frequency converter stops after sending a control mode switching instruction to the machine-side converter.

[0022] The present invention also proposes a starting system based on a static frequency converter driving a variable speed pumped storage unit, comprising:

[0023] Grid-connected switch, outgoing line switch, static frequency converter, machine-side converter, grid-side converter and control module; wherein the variable speed pumped storage unit is connected to the grid through the grid-connected switch; the variable speed pumped storage unit is also connected to one side of the static frequency converter through the outgoing line switch, and the other side of the static frequency converter is connected to the grid; the variable speed pumped storage unit is connected to one side of the machine-side converter, and the other side of the machine-side converter is connected to one side of the grid-side converter through the DC bus, and the other side of the grid-side converter is connected to the grid;

[0024] The control module is used to obtain the capacity of the variable speed pumped storage unit, the rated power under the pumping condition, the inertia time constant, the operating efficiency and the starting time, and based on the energy flow principle of the doubly fed induction motor, determine the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit under the starting constraint condition; wherein, the starting constraint condition includes: the slip power transmitted from the stator side of the variable speed pumped storage unit to the machine side converter is not greater than the capacity of the machine side converter; the static inverter is started according to the target speed of the static inverter, the static inverter sends instructions to the control system of the machine side converter and to the control system of the grid side converter, and the AC current of the power grid passes through the grid side converter and the machine side converter to obtain the AC excitation current introduced into the rotor side of the variable speed pumped storage unit; after the outgoing line switch is closed, the static inverter obtains the difference between the target speed and the AC excitation frequency, and according to the terminal voltage of the variable speed pumped storage unit, uniformly drags the variable speed pumped storage unit to reach the target speed; when the variable speed pumped storage unit reaches the target speed, the outgoing line switch is disconnected, and after the grid connection switch is closed, the variable speed pumped storage unit completes synchronous grid connection.

[0025] The beneficial effect of the present invention lies in that, compared with the prior art, it at least includes the following aspects: the method proposed by the present invention, when the variable speed pumped storage unit is converted from static to water pump working condition, unlocks the static frequency converter to send a start command to the rotor AC excitation system; after receiving the start command, the AC excitation controller starts the machine-side converter in voltage-frequency control mode, and passes a low-frequency AC excitation current with a frequency of 0.5Hz-5Hz into the rotor winding, and the rotor excitation current will induce a machine-end voltage of the corresponding frequency on the stator side; then the static frequency converter closes the output switch and uniformly drags the unit to the target speed according to the collected machine-end voltage information; low-frequency AC excitation is passed on the rotor side of the variable speed pumped storage unit, and the static frequency converter is started to gradually drag the unit to the target speed, thereby solving the shortcomings of large rotor position calculation error and large starting torque impact in the low-frequency starting stage of the static frequency converter, and effectively improving the success rate of unit starting. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a flow chart of a starting method of a variable speed pumped storage unit driven by a static frequency converter proposed by the present invention;

[0027] Figure 2 It is a system architecture diagram of a typical static inverter driving a variable speed pumped storage unit in an embodiment of the present invention. DETAILED DESCRIPTION

[0028] In order to make the purpose, technical scheme and advantages of the present invention clearer, the technical scheme of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. The embodiments described in this application are only embodiments of a part of the present invention, rather than all embodiments. Based on the spirit of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work belong to the protection scope of the present invention.

[0029] The present invention proposes a starting method and system based on a static frequency converter driving a variable speed pumped storage unit. Low frequency AC excitation is introduced into the rotor side of the variable speed pumped storage unit, and the static frequency converter gradually drives the variable speed pumped storage unit to reach the target speed. Figure 1 As shown, including:

[0030] Step 1, the variable speed pumped storage unit is connected to the grid through the grid-connected switch; the variable speed pumped storage unit is also connected to one side of the static frequency converter through the outgoing line switch, and the other side of the static frequency converter is connected to the grid; the variable speed pumped storage unit is connected to one side of the machine-side converter, and the other side of the machine-side converter is connected to one side of the grid-side converter through the DC bus, and the other side of the grid-side converter is connected to the grid.

[0031] The variable speed pumped storage unit system implemented based on step 1 is as follows Figure 2 As shown. The variable speed pumped storage unit is a doubly fed induction motor; the doubly fed induction motor drives the pump turbine, and the doubly fed induction motor is connected to the grid through the GCB grid-connected switch; the doubly fed induction motor is also connected to one side of the static frequency converter SFC through the SFC outgoing switch, and the other side of the static frequency converter SFC is connected to the grid; the variable speed pumped storage unit is connected to one side of the machine-side converter, and the other side of the machine-side converter is connected to one side of the grid-side converter through the DC bus, and the other side of the grid-side converter is connected to the grid.

[0032] Step 2, obtain the capacity of the variable speed pumped storage unit, the rated power under pumping conditions, the inertia time constant, the operating efficiency and the starting time, and determine the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit under the starting constraint conditions based on the energy flow principle of the doubly fed induction motor; wherein the starting constraint conditions include: the slip power transmitted from the stator side of the variable speed pumped storage unit to the machine-side converter is not greater than the capacity of the machine-side converter.

[0033] The variable speed pumped storage unit is an asynchronous double-fed induction motor. The power P m , including: the slip power sP from the stator side to the rotor side of the variable speed pumped storage unit m , Output power of variable speed pumped storage unit rotor side (1-s)P m Therefore, the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit satisfies the following relationship:

[0034]

[0035] Where, sP m is the slip power from the stator side to the rotor side of the variable speed pumped storage unit, s is the slip rate, P m is the power input to the stator side of the variable speed pumped storage unit, P e is the rated power of the variable speed pumped storage unit under pumping conditions, f r is the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit, η is the operating efficiency of the variable speed pumped storage unit corresponding to the rated speed under pumping conditions, H is the inertia time constant of the variable speed pumped storage unit, and t is the starting time for the rotor of the variable speed pumped storage unit to accelerate to the target speed at a constant acceleration.

[0036] During the startup process, the slip power from the stator side of the variable speed pumped storage unit to the machine-side converter is not greater than the capacity of the machine-side converter as a constraint condition;

[0037] In the embodiment, the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit determined under the starting constraint conditions is greater than or equal to 0.5 Hz and less than or equal to 5 Hz.

[0038] Step 3, starting the static inverter according to its target speed, the static inverter sends instructions to the control system of the machine-side converter and to the control system of the grid-side converter, and the AC current of the grid passes through the grid-side converter and the machine-side converter to obtain the AC excitation current introduced into the rotor side of the variable speed pumped storage unit.

[0039] Among them, the machine-side converter adopts voltage-frequency control to control the output frequency of the machine-side converter to be the AC excitation frequency and the output voltage to be the AC excitation voltage under the AC excitation frequency; the grid-side converter adopts constant DC bus voltage control.

[0040] The target speed of the static inverter is the target speed of the variable speed pumped storage unit, which is set according to the specific required parameters of the actual project.

[0041] The static frequency converter is used to send instructions to the control system of the machine-side converter, controlling the machine-side converter to invert according to the AC excitation frequency and the AC excitation voltage at the AC excitation frequency, thereby controlling the low-frequency AC excitation current supplied to the rotor side of the variable-speed pumped storage unit; the static frequency converter is also used to send instructions to the control system of the grid-side converter, controlling the grid-side converter to charge the DC bus and maintain the bus voltage at a constant value.

[0042] In the prior art, a DC excitation current is introduced into the rotor side of the variable speed pumped storage unit, which may cause the problem of inaccurate rotor position calculation in the low-frequency rotation stage of the variable speed pumped storage unit; in the present invention, after the machine-side converter adopts voltage-frequency control, the output frequency of the machine-side converter is achieved to be the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit determined in step 1, and the output voltage of the machine-side converter is the AC excitation voltage at the AC excitation frequency, so that low-frequency AC excitation is introduced into the rotor side to improve the accuracy of the rotor position calculation in the low-frequency rotation stage, thereby improving the startup success rate.

[0043] When the unit is stationary, a low-frequency AC excitation current is passed through the rotor side. The rotor low-frequency AC excitation current will generate a rotating magnetic field of the corresponding frequency on the rotor. This magnetic field cuts the stator winding through the air gap, thereby inducing a three-phase electromotive force of the corresponding frequency on the stator side. When the induced electromotive force is large enough, the static inverter can accurately calculate the flux position based on the machine-end voltage information, and when this induced electromotive force is large enough, it can also meet the voltage requirements for the natural commutation of the thyristor. In this way, the static inverter will no longer go through the control of the static and low-frequency stages, and directly use the induced machine-end voltage information to calculate the precise flux position to drag the unit to start. The principle of calculating the flux in the medium and high frequency of the static inverter is very reliable, and there is no need to modify the control logic of the static inverter. This can greatly increase the probability of successful startup of the unit and is more friendly to the safe and stable operation of the power grid.

[0044] Step 4: After the outgoing line switch is closed, the static frequency converter obtains the difference between the target speed and the AC excitation frequency, and according to the terminal voltage of the variable speed pumped storage unit, uniformly drags the variable speed pumped storage unit to reach the target speed.

[0045] In the embodiment, the difference between the target speed and the AC excitation frequency is the actual speed of the rotor of the variable speed pumped storage unit.

[0046] Step 5: When the variable speed pumped storage unit reaches the target speed, the outgoing line switch is disconnected, and after the grid connection switch is closed, the variable speed pumped storage unit completes synchronous grid connection.

[0047] After the outgoing line switch is disconnected, the static inverter sends a control mode switching command to the machine-side converter and then stops.

[0048] The present disclosure may be a system, a method and / or a computer program product. The computer program product may include a computer-readable storage medium carrying computer-readable program instructions for causing a processor to implement various aspects of the present disclosure.

[0049] A computer-readable storage medium may be a tangible device that can hold and store instructions used by an instruction execution device. A computer-readable storage medium may be, for example, but not limited to, an electrical storage device, a magnetic storage device, an optical storage device, an electromagnetic storage device, a semiconductor storage device, or any suitable combination of the foregoing. More specific examples of computer-readable storage media (a non-exhaustive list) include: a portable computer disk, a hard disk, a random access memory (RAM), a read-only memory (ROM), an erasable programmable read-only memory (EPROM or flash memory), a static random access memory (SRAM), a portable compact disk read-only memory (CD-ROM), a digital versatile disk (DVD), a memory stick, a floppy disk, a mechanical encoding device, such as a punch card or a raised structure in a groove on which instructions are stored, and any suitable combination of the foregoing. As used herein, a computer-readable storage medium is not to be interpreted as a transient signal per se, such as a radio wave or other freely propagating electromagnetic wave, an electromagnetic wave propagating through a waveguide or other transmission medium (e.g., a light pulse through a fiber optic cable), or an electrical signal transmitted through a wire.

[0050] The computer-readable program instructions described herein can be downloaded from a computer-readable storage medium to each computing / processing device, or downloaded to an external computer or external storage device via a network, such as the Internet, a local area network, a wide area network, and / or a wireless network. The network can include copper transmission cables, optical fiber transmissions, wireless transmissions, routers, firewalls, switches, gateway computers, and / or edge servers. The network adapter card or network interface in each computing / processing device receives the computer-readable program instructions from the network and forwards the computer-readable program instructions for storage in the computer-readable storage medium in each computing / processing device.

[0051] The computer program instructions for performing the operation of the present disclosure may be assembly instructions, instruction set architecture (ISA) instructions, machine instructions, machine-related instructions, microcode, firmware instructions, state setting data, or source code or object code written in any combination of one or more programming languages, including object-oriented programming languages, such as Smalltalk, C++, etc., and conventional procedural programming languages, such as "C" language or similar programming languages. Computer-readable program instructions may be executed completely on a user's computer, partially on a user's computer, as an independent software package, partially on a user's computer, partially on a remote computer, or completely on a remote computer or server. In the case of a remote computer, the remote computer may be connected to the user's computer via any type of network, including a local area network (LAN) or a wide area network (WAN), or may be connected to an external computer (e.g., using an Internet service provider to connect via the Internet). In some embodiments, an electronic circuit, such as a programmable logic circuit, a field programmable gate array (FPGA), or a programmable logic array (PLA), may be customized by utilizing the state information of the computer-readable program instructions, and the electronic circuit may execute the computer-readable program instructions, thereby realizing various aspects of the present disclosure.

[0052] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, ordinary technicians in the relevant field should understand that the specific implementation methods of the present invention can still be modified or replaced by equivalents, and any modifications or equivalent replacements that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A starting method based on a variable speed pumped storage unit driven by a static frequency converter, characterized in that: include: The variable speed pumped storage unit is connected to the grid through the grid-connected switch; the variable speed pumped storage unit is also connected to one side of the static frequency converter through the outgoing line switch, and the other side of the static frequency converter is connected to the grid; the variable speed pumped storage unit is connected to one side of the machine-side converter, and the other side of the machine-side converter is connected to one side of the grid-side converter through the DC bus, and the other side of the grid-side converter is connected to the grid; The capacity of the variable speed pumped storage unit, the rated power under pumping conditions, the inertia time constant, the operating efficiency and the starting time are obtained. Based on the energy flow principle of the doubly fed induction motor, the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit is determined under the starting constraint conditions; wherein the starting constraint conditions include: the slip power transmitted from the stator side of the variable speed pumped storage unit to the machine-side converter is not greater than the capacity of the machine-side converter; The static inverter is started according to its target speed, and the static inverter sends instructions to the control system of the generator-side converter and to the control system of the grid-side converter. The AC current of the power grid passes through the grid-side converter and the generator-side converter to obtain the AC excitation current introduced into the rotor side of the variable speed pumped storage unit. After the outgoing line switch is closed, the static frequency converter obtains the difference between the target speed and the AC excitation frequency, and according to the terminal voltage of the variable speed pumped storage unit, it uniformly drives the variable speed pumped storage unit to reach the target speed; When the variable speed pumped storage unit reaches the target speed, the outgoing line switch is disconnected, and after the grid-connected switch is closed, the variable speed pumped storage unit completes synchronous grid connection.

2. The starting method based on a variable speed pumped storage unit driven by a static frequency converter according to claim 1 is characterized in that: The AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit satisfies the following relationship: Where, sP m is the slip power from the stator side to the rotor side of the variable speed pumped storage unit, s is the slip rate, P m is the power input to the stator side of the variable speed pumped storage unit, P e is the rated power of the variable speed pumped storage unit under pumping conditions, f r is the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit, η is the operating efficiency of the variable speed pumped storage unit corresponding to the rated speed under pumping conditions, H is the inertia time constant of the variable speed pumped storage unit, and t is the starting time for the rotor of the variable speed pumped storage unit to accelerate to the target speed at a constant acceleration.

3. The starting method based on static frequency converter driving variable speed pumped storage unit according to claim 2 is characterized in that: The AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit determined under the starting constraint conditions is greater than or equal to 0.5 Hz and less than or equal to 5 Hz.

4. The starting method based on a variable speed pumped storage unit driven by a static frequency converter according to claim 1 is characterized in that: The target speed of the static inverter is the target speed of the variable speed pumped storage unit, which is set according to the specific required parameters of the actual project.

5. The starting method based on static frequency converter driving variable speed pumped storage unit according to claim 1 is characterized in that: The machine-side converter adopts voltage-frequency control to control the output frequency of the machine-side converter to be the AC excitation frequency and the output voltage to be the AC excitation voltage under the AC excitation frequency; the grid-side converter adopts constant DC bus voltage control.

6. The starting method based on static frequency converter driving variable speed pumped storage unit according to claim 5 is characterized in that: The static frequency converter is used to send instructions to the control system of the machine-side converter, controlling the machine-side converter to invert the AC excitation frequency and the AC excitation voltage at the AC excitation frequency, thereby controlling the low-frequency AC excitation current supplied to the rotor side of the variable speed pumped storage unit.

7. The starting method based on a variable speed pumped storage unit driven by a static frequency converter according to claim 6 is characterized in that: The static frequency converter is also used to send instructions to the control system of the grid-side converter to control the grid-side converter to charge the DC bus and maintain the bus voltage at a constant value.

8. The starting method based on static frequency converter driving variable speed pumped storage unit according to claim 1 is characterized in that: The difference between the target speed and the AC excitation frequency is the actual speed of the rotor of the variable speed pumped storage unit.

9. The starting method based on static frequency converter driving variable speed pumped storage unit according to claim 1, characterized in that: When the outgoing line switch is disconnected, the static inverter sends a control mode switching command to the machine-side converter and then stops.

10. A starting system based on a variable speed pumped storage unit driven by a static frequency converter, characterized in that: include: Grid-connected switch, outgoing line switch, static frequency converter, machine-side converter, grid-side converter and control module; wherein the variable speed pumped storage unit is connected to the grid through the grid-connected switch; the variable speed pumped storage unit is also connected to one side of the static frequency converter through the outgoing line switch, and the other side of the static frequency converter is connected to the grid; the variable speed pumped storage unit is connected to one side of the machine-side converter, and the other side of the machine-side converter is connected to one side of the grid-side converter through the DC bus, and the other side of the grid-side converter is connected to the grid; The control module is used to obtain the capacity of the variable speed pumped storage unit, the rated power under the pumping condition, the inertia time constant, the operating efficiency and the starting time, and based on the energy flow principle of the doubly fed induction motor, determine the AC excitation frequency introduced into the rotor side of the variable speed pumped storage unit under the starting constraint condition; wherein, the starting constraint condition includes: the slip power transmitted from the stator side of the variable speed pumped storage unit to the machine side converter is not greater than the capacity of the machine side converter; the static inverter is started according to the target speed of the static inverter, the static inverter sends instructions to the control system of the machine side converter and to the control system of the grid side converter, and the AC current of the power grid passes through the grid side converter and the machine side converter to obtain the AC excitation current introduced into the rotor side of the variable speed pumped storage unit; after the outgoing line switch is closed, the static inverter obtains the difference between the target speed and the AC excitation frequency, and according to the terminal voltage of the variable speed pumped storage unit, uniformly drags the variable speed pumped storage unit to reach the target speed; when the variable speed pumped storage unit reaches the target speed, the outgoing line switch is disconnected, and after the grid connection switch is closed, the variable speed pumped storage unit completes synchronous grid connection.

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