Grid connection starting strategy of full-power variable-speed pumped storage unit under low-water-head working condition

Abstract

The invention discloses a starting grid-connected strategy of a full-power variable-speed pumped storage unit under a low-water-head working condition. The starting grid-connected strategy comprises the following steps: establishing a water diversion system model; establishing a full-characteristic model of the pump turbine; establishing control models of a speed regulator, a synchronous generator, an excitation system and a converter; establishing a pre-synchronization control model; when the pumped storage unit reaches the no-load rotating speed, the excitation system starts to work, and excitation voltage building is conducted on the synchronous generator; and after the pumped storage unit reaches the grid connection condition, the power grid and the converter switch are switched off. According to the method, during no-load forwarding power supply, VSG control and pre-synchronization control are used for jointly adjusting output of the full-power variable-speed pumped storage unit, synchronization of the full-power variable-speed unit is accelerated, and during no-load forwarding power supply, working points of an excitation system and a converter of the full-power variable-speed unit and a control system of the full-power variable-speed unit are provided; the time from starting to power generation is shortened, and technical support is provided for starting and grid connection of the full-power variable-speed pumped storage unit.

Description

Technical Field

[0001] The present invention relates to the technical field of pumped storage, and in particular to a starting and grid-connecting strategy for a full-power variable-speed pumped storage unit under low water head conditions. Background Art

[0002] Full-power variable-speed pumped-storage units have the advantages of fast power adjustment, fast response speed, and independent decoupling of active and reactive power. They can provide peak load regulation, frequency regulation, reactive voltage support, demand-side response and other services for power grid operation, and are one of the technical directions for the development of pumped-storage technology at this stage. The traditional reversible unit operation mode is a constant speed operation mode, which requires the adjustment of the speed regulation system to complete the generator stator voltage frequency to reach the grid frequency. Due to the reverse "S" characteristic area of ​​the reversible unit, it is difficult for the traditional reversible unit to achieve synchronization between the generator stator voltage and the grid voltage under low head and no-load conditions. At present, the startup strategy of conventional pumped-storage units is still used for the power generation startup conditions of full-power variable-speed pumped-storage units, and variable-speed operation is not considered, so the advantages of variable speed cannot be reflected, and the startup to grid synchronization process is considered in few studies.

[0003] CN115614210A discloses a variable speed pumped storage unit low head power generation start method, including the following steps: operating the controller, starting the converter, and charging the bus voltage. The above method can be connected to the grid when the mechanical speed of the unit reaches the lower limit of the grid-connected speed, which is beneficial to saving the start-up time of the unit, alleviating the wear of the guide vane sleeve, and increasing the service life of the guide vane; improving the operating stability of the variable pumped storage unit, but the power generation start method proposed in this invention patent is suitable for double-fed variable speed pumped storage units, and does not actually solve the above-mentioned problems for full-power variable speed pumped storage units. Summary of the invention

[0004] The purpose of the present invention is to solve the shortcomings existing in the prior art and to propose a starting and grid-connecting strategy for a full-power variable-speed pumped storage unit under low water head conditions.

[0005] In order to achieve the above technical objectives, the present invention is implemented by the following technical solutions: a startup and grid-connected strategy for a full-power variable-speed pumped storage unit under low head conditions, the method involves mathematical modeling and a startup and grid-connected strategy, the mathematical modeling includes a water diversion system, a pump turbine, a speed governor, a synchronous generator, an excitation system, a converter, a pre-synchronization control, and a power grid, and the startup and grid-connected strategy is a method for a full-power variable-speed unit under low head conditions to complete the grid-connected demand based on the above modeling, and the content includes:

[0006] Establish a water diversion system model based on the characteristic line method;

[0007] Establish a full characteristic model of the pump turbine;

[0008] Establish control models for speed regulator, synchronous generator, excitation system, and converter;

[0009] Establish a pre-synchronization control model;

[0010] When the full-power variable-speed pumped storage unit is operating under low-head conditions, the no-load speed of the variable-speed unit is non-rated speed during the process from startup to no-load state;

[0011] When the no-load speed is reached, the excitation system starts to work and excites and builds up voltage for the synchronous generator;

[0012] The converter control is started while the excitation voltage is built up, and the pre-synchronization control and the converter control are used together to adjust the converter output;

[0013] After the full-power variable-speed pumped-storage unit reaches the grid-connected conditions, the circuit breaker switches between the grid and the converter are closed, allowing the variable-speed unit to be grid-connected and transmit power.

[0014] As a further description of the above technical solution, for the large fluctuation transition process of the startup condition, a pressure pipeline mathematical model based on the characteristic line method should be established. The unsteady flow mathematical model of the one-dimensional pressurized water diversion system inside the pipeline is as follows:

[0015]

[0016] As a further description of the above technical solution, when the characteristic line method is used to solve the above equation, the characteristic equation is introduced Then the equation can be transformed into:

[0017]

[0018] As a further description of the above technical solution, a characteristic grid is constructed according to the finite difference method, the above equation is converted into a difference equation, and then the flow rate and water head of the pipeline at any time are solved.

[0019] Where: H is the pressure pipe head; Q is the pipe flow; x is the pipe length; t is the discrete time step; A is the pipe cross-sectional area; g is the gravitational acceleration; f is the friction coefficient; D is the pipe diameter; a is the water hammer wave velocity.

[0020] As a further description of the above technical solution, the full characteristic model of the pump-turbine is a model obtained by processing the full characteristic curve of the reversible pump-turbine by improving the Suter transformation method.

[0021] As a further description of the above technical solution, the speed regulator model is a parallel PI control model. If the input value is the speed, the speed regulator model is:

[0022]

[0023] Among them, K p K is the proportional coefficient of the speed regulator; i is the speed regulator integral coefficient; T y is the main servomotor time constant; y is the guide vane opening; ω is the speed deviation;

[0024] As a further description of the above technical solution, if there is power input, the speed regulator model in the power mode is:

[0025]

[0026] Where P is the power deviation, e p It is the inverse of the adjustment coefficient;

[0027] As a further description of the above technical solution, the synchronous power generation model is the voltage equation, flux equation and rotation equation in the dq coordinate system, which are respectively:

[0028]

[0029] Among them, u d 、u q is the voltage component in the dq coordinate system; ψ d , q is the flux component in the dq coordinate system; id, iq are the current components in the dq coordinate system; r s is the stator resistance; ω is the angular velocity; u f is the excitation voltage; ψ f is the excitation flux; r f is the resistance of the excitation winding; i f is the excitation current; Ld and Lq are the inductance components in the dq coordinate system; m af is the mutual inductance between the stator and the field winding; L f is the inductance of the excitation winding; T m is the output torque of the reversible pump turbine; T e is the electromagnetic torque; T a is the inertia time constant of the unit.

[0030] As a further description of the above technical solution, the excitation system adopts the mathematical model of the thyristor excitation regulator in the IEEE standard model considering the saturation characteristics of the excitation machine. The mathematical model of the series correction unit is:

[0031]

[0032] Among them, T b and T c is the time constant of the lead-lag link;

[0033] The mathematical model of the power unit is:

[0034]

[0035] Among them, K A and T A are the coefficients and time constants of the amplifier;

[0036] The mathematical model of the exciter is:

[0037]

[0038] Among them, K e , T e They are the self-excitation coefficient and time constant of the exciter respectively;

[0039] The mathematical model of the rotor soft feedback unit is:

[0040]

[0041] Among them, K F is the rotor soft feedback unit amplification factor; T F is the time constant of the rotor soft feedback unit.

[0042] As a further description of the above technical solution, the full-power converter is divided into a machine-side converter and a grid-side converter, and each converter corresponds to its own control model.

[0043] As a further description of the above technical solution, the outer loop control model of the machine-side converter is a DC voltage loop, which is intended to follow the voltage of the DC bus, and its mathematical model is:

[0044]

[0045] in, is the reference value of the machine side current loop; K p , K i are the proportional gain and integral gain of the DC voltage outer loop; V dc is the reference value and measured value of DC voltage; Q dc It is the reference value and measured value of the reactive power on the generator side;

[0046] As a further description of the above technical solution, the inner loop control model of the machine-side converter is a current loop, which adopts a voltage feedforward decoupling control strategy and PI regulation, and its mathematical model is:

[0047]

[0048] Among them, u d 、uq is the dq component of the AC side voltage of the machine-side converter; K pi , K ii is the proportional gain and integral gain of the current inner loop; L is the machine-side filter inductance; e d 、e q is the dq component of the synchronous generator terminal voltage;

[0049] As a further description of the above technical solution, the control model of the grid-side converter is a virtual synchronous generator (VSG) and a double closed-loop control model. The VSG control model has the external characteristics of a synchronous generator, and combined with pre-synchronization control, it can improve the success rate of grid connection of full-power variable-speed pumped-storage units.

[0050] As a further description of the above technical solution, the VSG control model is divided into active power-frequency droop control and reactive power-voltage droop control;

[0051] The mathematical model of active power-frequency droop control is:

[0052]

[0053] Among them, P ref is the given power value; P is the measured power; k p is the active power-frequency droop coefficient; J is the virtual moment of inertia; ω0 is the rated angular velocity; Δω1 is the output value of the pre-synchronization control; D is the virtual damping coefficient;

[0054] The mathematical model of reactive power-voltage control droop is:

[0055]

[0056] Among them, K p1 , K i1 is the proportional gain and integral gain; Q ref is the reactive power reference value; U ref is the voltage reference value; ΔU1 is the output value of the pre-synchronization control; k q is the reactive-voltage control droop coefficient; E0 is the no-load potential;

[0057] As a further description of the above technical solution, the outer loop control of the double closed-loop control model is voltage control, and its mathematical model is:

[0058]

[0059] in, is the reference value of the inner current loop; K pu , K iu is the proportional gain and integral gain of the voltage outer loop; u dref 、u qrefis the reference value of the voltage outer loop; C f is the grid-side filter capacitor; e od 、e oq is the dq component of the grid-side output voltage of the converter;

[0060] As a further description of the above technical solution, the inner loop control of the double closed-loop control model is current control, and its mathematical model is:

[0061]

[0062] Among them, v d 、v q K is the input reference value of the PWM modulation strategy; pi , K ii is the proportional gain and integral gain of the current inner loop; L f It is the grid-side filter inductor;

[0063] As a further description of the above technical solution, the pre-synchronization control model is divided into voltage pre-synchronization control and frequency phase pre-synchronization control; the mathematical model of voltage pre-synchronization control is:

[0064]

[0065] Among them, K pv , K iv is the proportional gain and integral gain of voltage presynchronization control; U ref is the grid voltage; U is the output voltage of the converter;

[0066] As a further description of the above technical solution, the mathematical model of frequency phase presynchronization control is:

[0067]

[0068] Among them, K pω , K iω is the proportional gain and integral gain of frequency phase presynchronization control; θ ref is the phase of the power grid; θ is the phase of the converter;

[0069] As a further description of the above technical solution, when the full-power variable-speed pumped-storage unit works in a low-head condition, during the process from starting to no-load state of the variable-speed unit, in order to prevent the full-power variable-speed pumped-storage unit from entering the "S" characteristic zone when starting, 80%-95% of the rated speed is selected as the full-power pumped-storage unit starting no-load speed according to actual conditions;

[0070] As a further description of the above technical solution, when the full-power variable-speed pumped storage unit reaches the set no-load speed, the excitation system is started to build up the voltage for the synchronous generator excitation, and the pre-synchronization control and the converter controller are simultaneously started to jointly adjust the voltage amplitude, phase and frequency output by the converter;

[0071] As a further description of the above technical solution, after the full-power variable-speed pumped-storage unit reaches the grid-connected conditions, the circuit breaker switch between the grid and the converter is closed, so that the full-power variable-speed unit is connected to the grid. After the full-power variable-speed unit receives the load instruction, it enters the load state. The load power instruction is used as the reference input of the PID controller of the unit speed control system. The output control signal of the PID controller is used to adjust the main relay stroke and control the guide vane opening; the reference input of the full-power converter grid-side controller is the output power of the pump turbine, and the full-power converter grid-side controller is used to control the output power of the full-power variable-speed pumped-storage unit.

[0072] Compared with the prior art, the effects of the technology of the present invention are as follows:

[0073] 1. The invention proposes a startup and grid-connection strategy for a full-power variable-speed pumped-storage unit under low-head conditions. The strategy is based on the technology of full-power variable-speed pumped-storage units. Under low-head conditions, the no-load speed of the full-power variable-speed unit is set to a non-rated speed, which avoids the unit from entering the reverse "S" characteristic zone when starting to no-load, thereby improving the stability of the full-power variable-speed unit under low-head conditions.

[0074] 2. When transmitting electricity at no load, the present invention jointly adjusts the output of the full-power variable-speed pumped-storage unit by VSG control and pre-synchronization control, thereby accelerating the synchronization of the full-power variable-speed unit. In addition, the present invention proposes that when transmitting electricity at no load, the working point of the excitation system and converter and its control system of the full-power variable-speed unit is adjusted, thereby shortening the time from startup to power generation, thereby providing technical support for the startup and grid connection of the full-power variable-speed pumped-storage unit. BRIEF DESCRIPTION OF THE DRAWINGS

[0075] In the attached picture:

[0076] Figure 1 The diagram is a water-machine-electric coupling model consisting of a water diversion system, a water pump-turbine, a speed governor, a synchronous generator, an excitation system, a full power converter, a control system, and a power grid;

[0077] Figure 2 Schematic diagram of the unit startup strategy of "one-stage guide vane direct opening + PI control"

[0078] Figure 3 This is the PI controller and actuator model block diagram of the full-power variable-speed pumped storage unit speed control system;

[0079] Figure 4 It is the pre-synchronization control block diagram of the grid-side converter of the full-power variable-speed pumped storage generator;

[0080] Figure 5 This is the VSG control block diagram of the grid-side converter of the full-power variable speed pumped storage generator;

[0081] Figure 6 This is the hydraulic and electrical characteristics diagram of the full-power variable-speed pumped storage unit when it is started and connected to the grid at low water head. DETAILED DESCRIPTION

[0082] In order to make the purpose, technical scheme and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and specific examples, but it is not intended to limit the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work are within the scope of protection of the present invention.

[0083] The present invention relates to a startup and grid-connection strategy for a full-power variable-speed pumped-storage unit under low-head working conditions, wherein a simulation system comprises eight parts, namely, a water diversion system, a water pump-turbine, a speed regulator, a synchronous generator, an excitation system, a full-power converter and its control system, a pre-synchronization control system, and a power grid of the full-power variable-speed pumped-storage system.

[0084] The modeling idea of ​​the water diversion system is based on the characteristic line method. The unsteady flow mathematical model of the one-dimensional pressurized water diversion system inside the pipeline is solved by the characteristic line method to obtain the flow rate and water head of the pipeline at any time.

[0085] The pump-turbine model is obtained by processing the full characteristic curve of the pump-turbine using the improved Suter transformation method; the synchronous generator and excitation system adopt the IEEE standard model;

[0086] The full-power converter includes a machine-side converter and a grid-side converter. The machine-side converter is controlled by a double closed loop consisting of a DC voltage outer loop and a current inner loop. The grid-side converter is controlled by VSG control, pre-synchronization control, and voltage-current double closed loops. The VSG control is to obtain the voltage amplitude and phase angle output by the grid-side converter.

[0087] The pre-synchronization control compares the voltage amplitude, phase angle, and frequency of the converter with those of the grid, and introduces the obtained deviation into the VSG control to accelerate the synchronization of the grid-side converter and the grid.

[0088] The starting and grid-connected strategy of the full-power variable-speed pumped storage unit under low-head working conditions is that during the process from starting to no-load state of the full-power variable-speed unit, the no-load speed of the variable-speed unit is non-rated speed, and its speed range is 80%-95% of the rated speed; when the no-load speed is reached, the excitation system works to excite and build voltage for the synchronous generator; while excitation and voltage are built, the converter control is started, and the pre-synchronization control and converter control are used to jointly adjust the converter output; after the full-power variable-speed pumped storage unit reaches the grid-connected conditions, the circuit breaker switch between the grid and the converter is closed, so that the variable-speed unit is connected to the grid and transmits power. This starting and grid-connected method can finely reflect the water-machine-electric coupling characteristics of the full-power variable-speed pumped storage unit when it is started and connected to the grid at low water head.

[0089] The technical solution of this embodiment is as follows:

[0090] 1. Establishing a water diversion system model based on the characteristic line method

[0091] For the large fluctuation transition process of the startup condition, a mathematical model of the pressure pipeline based on the characteristic line method should be established. The mathematical model of the unsteady flow of the one-dimensional pressurized water diversion system inside the pipeline is as follows:

[0092]

[0093] When solving the above equation by the characteristic line method, the characteristic equation is introduced Then the equation can be transformed into:

[0094]

[0095] Then, the characteristic grid is constructed according to the finite difference method, the above equation is converted into a difference equation, and then the flow rate and water head of the pipeline at any time are solved.

[0096] Where: H is the pressure pipe head; Q is the pipe flow; x is the pipe length; t is the discrete time step; A is the pipe cross-sectional area; g is the gravitational acceleration; f is the friction coefficient; D is the pipe diameter; a is the water hammer wave velocity.

[0097] 2. Establishing the full characteristic model of the pump turbine

[0098] The full characteristic model of the pump-turbine is a model obtained by processing the full characteristic curve of the pump-turbine by improving the Suter transformation method.

[0099] The formula for improving the Suter transformation method is as follows:

[0100]

[0101] Among them, WH(x,y) and WM(x,y) are variables describing the full characteristic curve after the improved Suter transformation, x is the independent variable; h, y, n, q, and m are the relative value of head, relative value of guide vane opening, relative value of speed, relative value of flow, and relative value of torque, respectively; k1>|M 11max | / M 11r ; k2 = 0.5 ~ 1.2; C y =0.1~0.3; C h =0.4~0.6.

[0102] 3. Establishing the governor model

[0103] The speed governor adopts a parallel PI regulator. After the signal is output, the main relay is adjusted through the actuator to control the opening of the guide vane.

[0104] If the input value of the speed regulator model is the speed, its model is:

[0105]

[0106] Among them, K p K is the proportional coefficient of the speed regulator; i is the speed regulator integral coefficient; T y is the main servomotor time constant; y is the guide vane opening; ω is the speed deviation;

[0107] If there is power input, the speed regulator model in power mode is:

[0108]

[0109] Where P is the power deviation, e p It is the inverse of the adjustment coefficient;

[0110] 4. Establishing Synchronous Generator Model

[0111] The synchronous power generation model is the voltage equation, flux equation and rotation equation in the dq coordinate system, which are:

[0112]

[0113] Among them, u d 、u q is the voltage component in the dq coordinate system; ψ d , q is the flux component in the dq coordinate system; id, iq are the current components in the dq coordinate system; r s is the stator resistance; ω is the angular velocity; u f is the excitation voltage; ψ f is the excitation flux; r f is the resistance of the excitation winding; i fis the excitation current; Ld and Lq are the inductance components in the dq coordinate system; m af is the mutual inductance between the stator and the field winding; L f is the inductance of the excitation winding; T m is the output torque of the reversible pump turbine; T e is the electromagnetic torque; T a is the inertia time constant of the unit.

[0114] 5. Establishing the excitation system model

[0115] The excitation system adopts the mathematical model of thyristor excitation regulator in the IEEE standard model that takes into account the saturation characteristics of the excitation machine. Among them, the mathematical model of the series correction unit is:

[0116]

[0117] Among them, T b and T c is the time constant of the lead-lag link;

[0118] The mathematical model of the power unit is:

[0119]

[0120] Among them, K A and T A are the coefficients and time constants of the amplifier;

[0121] The mathematical model of the exciter is:

[0122]

[0123] Among them, K e 、T e They are the self-excitation coefficient and time constant of the exciter respectively;

[0124] The mathematical model of the rotor soft feedback unit is:

[0125]

[0126] Among them, K F is the rotor soft feedback unit amplification factor; T F is the time constant of the rotor soft feedback unit;

[0127] 6. Establishing the full power converter and its control model

[0128] The full-power converter is divided into a machine-side converter and a grid-side converter, and each converter corresponds to its own control model.

[0129] The outer loop control model of the machine-side converter is a DC voltage loop, which is designed to follow the voltage of the DC bus. Its mathematical model is:

[0130]

[0131] in, is the reference value of the machine side current loop; K p , K i are the proportional gain and integral gain of the DC voltage outer loop; V dc is the reference value and measured value of DC voltage; Q dc It is the reference value and measured value of the reactive power on the generator side;

[0132] The inner loop control model of the machine-side converter is the current loop, which adopts the voltage feedforward decoupling control strategy and PI regulation. Its mathematical model is:

[0133]

[0134] Among them, u d 、u q is the dq component of the AC side voltage of the machine-side converter; K pi , K ii is the proportional gain and integral gain of the current inner loop; L is the machine-side filter inductance; e d 、e q is the dq component of the synchronous generator terminal voltage;

[0135] The control model of the grid-side converter is a virtual synchronous generator (VSG) and a double closed-loop control model. The VSG control model has the external characteristics of a synchronous generator. Combined with pre-synchronization control, it can improve the success rate of grid connection of full-power variable-speed pumped storage units.

[0136] The VSG control model is divided into active power-frequency droop control and reactive power-voltage droop control;

[0137] The mathematical model of active power-frequency droop control is:

[0138]

[0139] Among them, P ref is the given power value; P is the measured power; k p is the active power-frequency droop coefficient; J is the virtual moment of inertia; ω0 is the rated angular velocity; Δω1 is the output value of the pre-synchronization control; D is the virtual damping coefficient;

[0140] The mathematical model of reactive power-voltage control droop is:

[0141]

[0142] Among them, K p1 , Ki1 is the proportional gain and integral gain; Q ref is the reactive power reference value; U ref is the voltage reference value; ΔU1 is the output value of the pre-synchronization control; k q is the reactive-voltage control droop coefficient; E0 is the no-load potential;

[0143] The outer loop control of the double closed-loop control model is voltage control, and its mathematical model is:

[0144]

[0145] in, is the reference value of the inner current loop; K pu , K iu is the proportional gain and integral gain of the voltage outer loop; u dref 、u qref is the reference value of the voltage outer loop; C f is the grid-side filter capacitor; e od 、e oq is the dq component of the grid-side output voltage of the converter;

[0146] The inner loop control of the double closed-loop control model is current control, and its mathematical model is:

[0147]

[0148] Among them, v d 、v q K is the input reference value of the PWM modulation strategy; pi , K ii is the proportional gain and integral gain of the current inner loop; L f It is the grid-side filter inductor;

[0149] 7. Establishing a pre-synchronization control model

[0150] The presynchronization control model is divided into voltage presynchronization control and frequency phase presynchronization control; the mathematical model of voltage presynchronization control is:

[0151]

[0152] Among them, K pv , K iv is the proportional gain and integral gain of voltage presynchronization control; U ref is the grid voltage; U is the output voltage of the converter;

[0153] The mathematical model of frequency phase presynchronization control is:

[0154]

[0155] Among them, Kpω , K iω is the proportional gain and integral gain of frequency phase presynchronization control; θ ref is the phase of the power grid; θ is the phase of the converter;

[0156] In specific implementation, a full-power variable-speed pumped storage unit startup and grid-connection strategy under low head conditions, the system includes eight parts: water diversion system, pump turbine, speed governor, synchronous generator, excitation system, full-power converter, pre-synchronization control and power grid, such as Figure 1 shown.

[0157] The startup and grid-connection strategy of the full-power variable-speed pumped storage unit under low-head working conditions is a strategy proposed in combination with the above basic model, which is applicable to the full-power variable-speed unit under low-head working conditions from startup-no-load-excitation-generation and grid-connection. Specifically, the method of this embodiment is described in detail:

[0158] S1: By setting the initial values ​​of the above model, the model is run in the unstarted state;

[0159] S2: When the full-power variable-speed pumped storage unit operates under low head conditions, the guide vanes are Figure 2 The unit startup strategy of "one-stage guide vane direct opening + PI control" is shown in the figure. At this time, the speed governor model is as follows Figure 3 When the full-power variable-speed unit is in the process of starting to no-load, the speed governor uses the speed control mode; when the full-power variable-speed unit is in the process of generating electricity and connecting to the grid, the speed governor uses the power control mode, turns off the control signal switch, and enters the power control loop;

[0160] S3: In order to prevent the full-power variable-speed pumped-storage unit from entering the "S" characteristic zone when starting under low-head conditions, 80%-95% of the rated speed is selected as the no-load speed of the full-power pumped-storage unit when starting according to actual conditions. In this embodiment, the no-load speed of the full-power variable-speed pumped-storage unit is 85% of the rated speed;

[0161] S4: When the full-power variable-speed pumped storage unit reaches the set no-load speed, the excitation system is started to excite and build up the voltage of the synchronous generator, and the pre-synchronization control is started at the same time ( Figure 4 ) and converter control (VSG control such as Figure 5 ) jointly adjust the voltage amplitude, frequency and phase of the converter output;

[0162] S5: After the full-power variable-speed pumped-storage unit reaches the grid-connected conditions, the circuit breaker switch between the grid and the converter is turned off, and the full-power variable-speed unit is connected to the grid.

[0163] S6: After the full-power variable-speed unit receives the load instruction, it enters the load state. The load power instruction serves as the reference input of the PID controller of the unit speed control system. The output control signal of the PID controller is used to adjust the main relay stroke and control the guide vane opening. The reference input of the full-power converter grid-side controller is the output power of the pump-turbine. The full-power converter grid-side controller is used to control the output power of the full-power variable-speed pumped-storage unit.

[0164] The specific simulation results are as follows Figure 6 As shown, it can be seen that the non-rated speed starting strategy can prevent the unit from entering the anti-"S" characteristic area and realize the reliable grid-connected operation of the full-power variable-speed unit; the variable-speed unit controls the voltage amplitude, frequency and phase of the full-power converter output through VSG and pre-synchronization, thereby reducing the synchronization time of the full-power variable-speed unit.

[0165] The above is the technical idea for implementing the present invention, but it does not limit the implementation mode and protection scope of the present invention. For those skilled in the art, it should be aware that all solutions obtained by equivalent substitutions and obvious changes made using the contents of the specification of the present invention should be included in the protection scope of the present invention.

Claims

1. A startup and grid-connection strategy for a full-power variable-speed pumped-storage unit under low-head conditions, wherein the pumped-storage unit is composed of a characteristic line model of a water diversion system, a full-characteristic model of a pump-turbine, a synchronous generator model, an excitation system model, a full-power converter and its control system model, a pre-synchronization control model, and a power grid, and is characterized in that: include: Establish a water diversion system model; Establish a full characteristic model of the pump turbine; Establish control models for speed regulator, synchronous generator, excitation system and converter; Establish a pre-synchronization control model; Wherein, when the pumped storage unit is in a low head working condition, during the process from startup to no-load state, the no-load speed of the pumped storage unit is a non-rated speed. When the pumped storage unit reaches the no-load speed, the excitation system starts to work to excite and build up pressure for the synchronous generator; While the excitation voltage is being built up, the control of the converter is started, and the pre-synchronization control is used together with the control of the converter to adjust the voltage amplitude, phase and frequency of the output of the converter; After the pumped-storage unit reaches the grid-connected condition, the circuit breaker switch between the grid and the converter is turned off to achieve grid-connected connection of the pumped-storage unit and power transmission.

2. The starting and grid-connecting strategy of a full-power variable-speed pumped storage unit under low head conditions according to claim 1 is characterized in that: A mathematical model of the pressure pipeline based on the characteristic line method is established, in which the mathematical model of the unsteady flow of the one-dimensional pressurized water diversion system inside the pipeline is as follows: When solving the above equation using the characteristic line method, the characteristic equation is introduced Then the equation can be transformed into: Then, the characteristic grid is constructed according to the finite difference method, the above equation is converted into a difference equation, and then the flow rate and water head of the pipeline at any time are solved; Among them, H is the pressure pipe head; Q is the pipe flow rate; x is the pipe length; t is the discrete time step; A is the pipe cross-sectional area; g is the gravitational acceleration; f is the friction coefficient; D is the pipe diameter; and a is the water hammer wave velocity.

3. The starting and grid-connecting strategy of a full-power variable-speed pumped storage unit under low water head conditions according to claim 1 is characterized in that: The speed regulator model is a parallel PI control model. If the input value is the speed, the speed regulator model is: Among them, K p K is the proportional coefficient of the speed regulator; i is the speed regulator integral coefficient; T y is the main servomotor time constant; y is the guide vane opening; ω is the speed deviation; If there is power input, the speed regulator model in power mode is: Where P is the power deviation, e p It is the inverse of the adjustment coefficient; The voltage equation of the synchronous power generation model in the dq coordinate system is: The magnetic flux equation of the synchronous power generation model in the dq coordinate system is: The rotation equation of the synchronous power generation model in the dq coordinate system is: Among them, u d 、u q is the voltage component in the dq coordinate system; ψ d , q is the flux component in the dq coordinate system; id, iq are the current components in the dq coordinate system; r s is the stator resistance; ω is the angular velocity; u f is the excitation voltage; ψ f is the excitation flux; r f is the excitation winding resistance; i f is the excitation current; Ld and Lq are the inductance components in the dq coordinate system; m af is the mutual inductance between the stator and the field winding; L f is the inductance of the excitation winding; T m is the output torque of the reversible pump turbine; T e is the electromagnetic torque; T a is the inertia time constant of the unit; The excitation system adopts the mathematical model of the thyristor excitation regulator in the IEEE standard model, wherein the mathematical model of the series correction unit is: Among them, T b and T c is the time constant of the lead-lag link; The mathematical model of the power unit is: Among them, K A and T A are the coefficients and time constants of the amplifier; The mathematical model of the exciter is: Among them, K e , T e They are the self-excitation coefficient and time constant of the exciter respectively; The mathematical model of the rotor soft feedback unit is: Among them, K F is the rotor soft feedback unit amplification factor; T F is the time constant of the rotor soft feedback unit.

4. The starting and grid-connecting strategy of a full-power variable-speed pumped storage unit under low water head conditions according to claim 1 is characterized in that: The converter includes a machine-side converter and a grid-side converter, and each converter corresponds to its own control model; The outer loop control model of the machine-side converter is a DC voltage loop, which is used to follow the voltage of the DC bus. Its mathematical model is: in, is the reference value of the machine side current loop; K p , K i are the proportional gain and integral gain of the DC voltage outer loop; V dc is the reference value and measured value of DC voltage; Q dc It is the reference value and measured value of the reactive power on the machine side; The inner loop control model of the machine-side converter is the current loop, which adopts the voltage feedforward decoupling control strategy and PI regulation. Its mathematical model is: Among them, u d 、u q is the dq component of the AC side voltage of the machine-side converter; K pi , K ii is the proportional gain and integral gain of the current inner loop; L is the machine-side filter inductance; e d 、e q is the dq component of the synchronous generator terminal voltage.

5. The starting and grid-connecting strategy of a full-power variable-speed pumped storage unit under low-head conditions according to claim 4 is characterized in that: The control model of the grid-side converter is a virtual synchronous generator (VSG) and a double closed-loop control model. The VSG control model has the external characteristics of a synchronous generator. Combined with pre-synchronization control, it can improve the success rate of grid connection of full-power variable-speed pumped storage units.

6. The starting and grid-connecting strategy of a full-power variable-speed pumped storage unit under low-head conditions according to claim 5 is characterized in that: The VSG control model is divided into active power-frequency droop control and reactive power-voltage droop control; The active power-frequency droop control mathematical model is: Among them, P ref is the given power value; P is the measured power; k p is the active power-frequency droop coefficient; J is the virtual moment of inertia; ω0 is the rated angular velocity; Δω1 is the output value of the pre-synchronization control; D is the virtual damping coefficient; The reactive power-voltage control droop mathematical model is: Among them, K p1 , K i1 is the proportional gain and integral gain; Q ref is the reactive power reference value; U ref is the voltage reference value; ΔU1 is the output value of the pre-synchronization control; k q is the reactive-voltage control droop coefficient; E0 is the no-load potential.

7. The starting and grid-connecting strategy of a full-power variable-speed pumped storage unit under low-head conditions according to claim 5 is characterized in that: The outer loop control of the double closed-loop control model is voltage control, and its mathematical model is: in, is the reference value of the inner current loop; K pu , K iu is the proportional gain and integral gain of the voltage outer loop; u dref 、u qref is the reference value of the voltage outer loop; C f is the grid-side filter capacitor; e od 、e oq is the dq component of the grid-side output voltage of the converter; The inner loop control of the double closed-loop control model is current control, and its mathematical model is: Among them, v d 、v q K is the input reference value of the PWM modulation strategy; pi , K ii is the proportional gain and integral gain of the current inner loop; L f It is the grid-side filter inductor.

8. The starting and grid-connecting strategy of a full-power variable-speed pumped storage unit under low-head conditions according to claim 1 is characterized in that: The pre-synchronization control model is divided into voltage pre-synchronization control and frequency phase pre-synchronization control; the mathematical model of voltage pre-synchronization control is: Among them, K pv , K iv is the proportional gain and integral gain of voltage presynchronization control; U ref is the grid voltage; U is the output voltage of the converter; The mathematical model of frequency phase presynchronization control is: Among them, K pω , K iω is the proportional gain and integral gain of frequency phase presynchronization control; θ ref is the phase of the grid; θ is the phase of the converter.

9. The starting and grid-connecting strategy of a full-power variable-speed pumped storage unit under low water head conditions according to claim 1, characterized in that: When the pumped storage unit operates in a low head condition, during the process from the start-up to the no-load state of the variable speed unit, the starting no-load speed of the pumped storage unit is 80%-95% of the rated speed.

10. The starting and grid-connecting strategy of a full-power variable-speed pumped storage unit under low-head conditions according to claim 1, characterized in that: After the pumped storage unit meets the grid connection conditions, the circuit breaker switch between the grid and the converter is turned off, so that the full-power variable-speed unit is connected to the grid; After receiving the load instruction, the pumped storage unit enters the load state, and the load power instruction is used as the reference input of the PID controller of the unit speed control system. The output control signal of the PID controller is used to adjust the main relay stroke and control the guide vane opening; The reference input of the converter grid-side controller is the output power of the pump-turbine. The full-power converter grid-side controller is used to control the output power of the full-power variable-speed pumped-storage unit.

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