Off-grid photovoltaic power generation system based on flywheel clean battery energy storage

Abstract

The invention discloses an off-grid photovoltaic power generation system based on flywheel clean battery energy storage. A cleaning model and a charging / discharging equivalent model circuit under a d-q shaft system are established; a control strategy in the flywheel battery charging / discharging process is analyzed and determined, a photovoltaic system and a flywheel battery are combined, a combined system is divided into four working models according to the supply-demand relation between the output power of the photovoltaic system and the power required by a load, and a control method of a combined controller is formulated. And analyzing and optimizing the operation state of the system under each working model and the energy storage and release effects of the flywheel battery. The combined system can stably operate under each working model and among the working models; the charging / discharging efficiency and the discharging depth are relatively high, and a quick response can be made to the difference value change of the output and the load power of the photovoltaic system; and energy is stored when the output of the photovoltaic system is residual, and energy is released to make up for the demand when the output of the photovoltaic system is insufficient, so that the electric energy quality is effectively improved, and the resource waste is reduced.

Description

Technical Field

[0001] The present application relates to a flywheel energy storage photovoltaic power generation system, and in particular to an off-grid photovoltaic power generation system based on flywheel clean battery energy storage, belonging to the technical field of clean energy storage photovoltaic power generation. Background Art

[0002] With the exhaustion of fossil fuels and the intensification of environmental problems, the proportion of clean and renewable energy such as solar energy, wind energy, and ocean energy in the power generation system is increasing year by year. Solar energy resources are abundant and have great development potential. It is the most promising new energy among the five energy sources including wind energy, hydropower, geothermal energy, and ocean energy. It has the characteristics of full cleanliness, safety, and extensiveness. At present, we are vigorously promoting the application of distributed photovoltaics and "photovoltaic +", realizing the diversification of photovoltaic power generation, and innovating the application model of distributed photovoltaics.

[0003] The off-grid photovoltaic power generation system is separated from the public power grid. Its output is seriously affected by environmental factors such as temperature and radiation intensity. It is highly random, volatile and uncontrollable. When the system cannot absorb the energy emitted by photovoltaic cells, the phenomenon of "abandoning light" will occur, resulting in a large amount of electricity waste. Based on this, it is necessary to introduce energy storage devices in the off-grid photovoltaic power generation system to track the planned output and smooth the output to provide more stable and reliable electricity, enhance the flexibility and reliability of the system, optimize the system energy management path, and increase the freedom of energy trading.

[0004] At present, the energy storage technologies used more in photovoltaic power generation systems include chemical energy storage, electromagnetic energy storage and mechanical energy storage. The application of electrochemical energy storage in power systems is growing rapidly, but chemical batteries have low energy storage density, shallow discharge depth, and are limited by the temperature range, which also pollutes the environment to a certain extent. Compared with chemical batteries, superconducting batteries have greater energy storage density and discharge depth, and less pollution to the environment, but the superconducting state requires the coil to be in an extremely low temperature environment to achieve, and maintaining this extremely low temperature environment requires most of the energy consumption, and the maintenance device is large in size, which is difficult to achieve home use, and is not the best choice for off-grid photovoltaic power generation systems. Flywheel batteries not only have high energy storage density and deep discharge depth, but also are pollution-free to the environment, have a similar service life to photovoltaic batteries, and are convenient for the overall operation and management of the system. In the case of the same power, the relative size is the smallest, and the cost is low during its life cycle. It has good application prospects in the short-term energy storage of off-grid photovoltaic power generation systems. Exploring the working characteristics of flywheel batteries and their compensation effect on photovoltaic systems is of great significance and role in the efficient and stable operation of off-grid photovoltaic power generation systems and the development of smart grids.

[0005] The problems that need to be solved by the existing clean energy storage photovoltaic power generation technology and the key technical difficulties of this application include:

[0006] (1) The off-grid photovoltaic power generation system is separated from the public power grid. Its output is seriously affected by environmental factors such as temperature and radiation intensity. It is highly random, volatile and uncontrollable. When the system cannot absorb the energy emitted by the photovoltaic cells, light abandonment will occur, resulting in a large amount of electric energy waste. The existing technology has not introduced efficient and reliable energy storage devices in the off-grid photovoltaic power generation system, and there is a lack of off-grid photovoltaic power generation systems based on flywheel clean battery energy storage. There is a lack of flywheel battery energy storage optimization and cleaning model, no optimization model for photovoltaic cells, no analysis of its IV and PV characteristics corresponding to different light intensities and temperatures, and a lack of flywheel battery power electronic conversion and motor linkage system. It is impossible to effectively track the planned output and smooth the output, and it is impossible to provide more stable and reliable electricity, enhance the flexibility and reliability of the system, optimize the system energy management path, and increase the freedom of energy trading.

[0007] (2) The energy storage technologies used more frequently in photovoltaic power generation systems include chemical energy storage, electromagnetic energy storage and mechanical energy storage. Chemical batteries have low energy storage density, shallow discharge depth, and are limited by the temperature range, which also pollutes the environment to a certain extent. Compared with chemical batteries, superconducting batteries have greater energy storage density and discharge depth, but the superconducting state requires the coil to be in an extremely low temperature environment to achieve, and maintaining this extremely low temperature environment consumes most of the energy, and the maintenance device is large in size, making it difficult to achieve household use. For off-grid photovoltaic power generation systems, they are not the best choice. Flywheel batteries not only have high energy storage density and deep discharge depth, but also are pollution-free and have a similar service life to photovoltaic batteries, which is convenient for the overall operation and management of the system. In addition, under the same power, they have the smallest relative size and low cost during their life cycle. They have good application prospects in the short-term energy storage of off-grid photovoltaic power generation systems. However, the existing technology does not study the working characteristics of flywheel batteries enough, especially there is no in-depth analysis to establish a clean energy storage model for flywheel batteries, no flywheel battery charge / discharge optimization control strategy has been formulated, and the role of photovoltaic system compensation in the efficient and stable operation of off-grid photovoltaic power generation systems and the development of smart grids has not been fully explored.

[0008] (3) The prior art lacks analysis on establishing a clean energy storage model for flywheel batteries, and has not established a clean model for flywheel batteries composed of a permanent magnet synchronous motor (PMSM) and a PWM converter. It lacks analysis and modeling of the motion equation, voltage equation, electromagnetic torque equation and magnetic flux equation of the permanent magnet synchronous motor, has not established a four-quadrant operation model and a switching function model for the PWM converter, and has not built a charge / discharge equivalent model circuit for the flywheel battery in the dq axis. The prior art has not formulated a flywheel battery charge / discharge optimization control strategy, has not analyzed its charge and discharge models, and cannot optimize the charge / discharge characteristics of the flywheel battery. The speed rises / falls unsteadily, and the given voltage cannot be output steadily during discharge. The prior art has no The method of combining flywheel batteries as energy storage devices with photovoltaic systems lacks the setting of the operating state of the combined system into four working models based on the relationship between the output power of the photovoltaic system and the power required by the load. There is a lack of analysis of the various working models of the combined system, and no control strategy for the combined controller has been formulated. The flywheel battery cannot quickly switch the working model according to the difference between the output power of the photovoltaic system and the load power, cannot store electrical energy when the photovoltaic system output is sufficient, and cannot compensate for it when the photovoltaic system output is insufficient. It cannot provide stable electrical energy for the load, resulting in low energy utilization, and cannot effectively utilize the excess electrical energy of the photovoltaic system. The problem of abandoned light is serious, which is not conducive to the long-term operation of the photovoltaic system. Summary of the invention

[0009] This application divides the flywheel battery into two parts: a permanent magnet synchronous motor and a PWM rectifier, and establishes a clean model and a charge / discharge equivalent model circuit under a dq axis system respectively; analyzes and establishes the control mode and control strategy in the charge / discharge process of the flywheel battery, combines the photovoltaic system with the flywheel battery, divides the combined system into four working models according to the supply and demand relationship between the output power of the photovoltaic system and the power required by the load, formulates the control method and strategy of the joint controller, and analyzes and optimizes the operating state of the system under each working model and the energy storage and release effect of the flywheel battery. The combined system can operate stably under each working model and between each working model; as an energy storage link, the flywheel battery has a high charge / discharge efficiency and discharge depth, and can respond quickly to the difference between the output of the photovoltaic system and the load power; and stores energy when there is surplus output of the photovoltaic system, and releases energy to make up for the needs when the output of the photovoltaic system is insufficient, effectively improving the quality of electric energy and reducing the waste of resources.

[0010] In order to achieve the above technical effects, the technical solutions adopted in this application are as follows:

[0011] An off-grid photovoltaic power generation system based on flywheel clean battery energy storage. The flywheel battery uses a permanent magnet synchronous motor and a PWM rectifier, and the charge / discharge equivalent model circuits under the clean model and the dq axis system are established respectively; the control method and control strategy of the flywheel battery charge / discharge process are analyzed and established, the photovoltaic system and the flywheel battery are combined, and the combined system is divided into four working models according to the supply and demand relationship between the output power of the photovoltaic system and the power required by the load, and the control method and strategy of the joint controller are formulated, and the operating state of the system and the energy storage and release effect of the flywheel battery under each working model are analyzed and optimized;

[0012] 1) Establish a flywheel battery energy storage optimization cleaning model, build an optimization model for photovoltaic cells, and analyze its IV and PV characteristics corresponding to different light intensities and temperatures. Under the same temperature and different light intensities, the maximum power of the photovoltaic cell increases with the increase of light intensity; when the light intensity is the same but the temperature is different, the maximum power of the photovoltaic cell decreases with the increase of temperature. Establish and optimize the flywheel battery power electronic conversion and motor linkage system, and use the conductivity increment method as the maximum power tracking method of the photovoltaic system;

[0013] 2) Analyze and establish a clean energy storage model for flywheel batteries. Establish a clean flywheel battery model consisting of a permanent magnet synchronous motor (PMSM) and a PWM converter. Analyze and model the motion equation, voltage equation, electromagnetic torque equation, and flux equation of the permanent magnet synchronous motor. Establish a four-quadrant operation model and a switching function model for the PWM converter. Build a charge / discharge equivalent model circuit for the flywheel battery in the dq axis.

[0014] 3) Formulate an optimized control strategy for flywheel battery charge / discharge, and analyze its charge / discharge model respectively, optimize the charge / discharge characteristics of the flywheel battery, ensure that the speed rises / falls smoothly, and outputs a given voltage smoothly during discharge. The speed of charge / discharge is related to the input / output power and the moment of inertia of the flywheel battery;

[0015] 4) The flywheel battery is used as an energy storage device and combined with the photovoltaic system. According to the relationship between the output power of the photovoltaic system and the power required by the load, the operating state of the combined system is set to four working models. The various working models of the combined system are analyzed, and the control strategy of the combined controller is formulated. The flywheel battery quickly switches the working model according to the difference between the output power of the photovoltaic system and the load power, stores electric energy when the photovoltaic system output is sufficient, and compensates for it when the photovoltaic system output is insufficient, providing stable electric energy for the load and improving energy utilization.

[0016] Preferably, a flywheel battery power electronic conversion and motor linkage system is constructed: charging and discharging control is implemented based on a PWM converter. In the charging mode, the PWM converter 1 works as a rectifier, and the PWM converter 2 works as an inverter. The AC power of the grid is changed into DC power through the PWM converter 1, and then converted into AC power with adjustable voltage and frequency through the PWM converter to drive the motor to work, and the flywheel stores energy; in the discharging mode, the PWM converter 1 works as an inverter, and the PWM converter works as a rectifier. The AC power of the flywheel battery is changed into DC power through the PWM converter 2, and then converted into AC power with adjustable voltage and frequency through the PWM converter 1 to be provided to the electrical equipment to release energy;

[0017] The frequency conversion system ensures that the motor speed changes smoothly, adjusts the motor speed, ensures the stability of the output power, and realizes two-way control of power flow; the flywheel battery motor adopts a permanent magnet synchronous motor to construct a flywheel battery power electronic conversion and motor linkage system.

[0018] Preferably, the permanent magnet synchronous motor cleaning model: the cross section of the permanent magnet synchronous motor is taken as a complex plane (coordinate R e -I m ), to represent the space vector inside the PMSM;

[0019] In the PMSM complex plane, select any spatial stationary complex plane R e -I m , if the real axis R e is the reference axis of the spatial coordinate system. Any vector R in the space is represented by R = |R|e jθ , |R| is the magnitude of the vector, and θ is the distance between the vector and the reference axis R e The spatial electrical angle, i.e., the spatial phase;

[0020] Let the a-axis of the three-phase stationary coordinate system (abc) and the real axis R e If they coincide, the spatial position angles of the b-axis and the c-axis are a=e j120° and a 2 =e j240° , a and a2 are spatial operators;

[0021] In the abc coordinate system, the effective number of turns of the single-axis coil is the number of turns of each phase line group. Then the stator current space vector equation is:

[0022]

[0023] In the formula, i a 、i b 、i cThe stator currents of the three phases a, b, and c are transformed from the three-phase stationary coordinates (abc) to the two-phase rotating coordinates (dq), and vector control is adopted to simplify the control system design;

[0024] The clean model of permanent magnet synchronous motor consists of flux equation, electromagnetic torque equation, voltage equation and motion equation, and makes the following assumptions: ① Ignore core saturation, hysteresis and eddy current losses; ② The conductivity of permanent magnet material is zero; ③ There is no damping winding on the rotor; ④ The waveform of the induced electromotive force in the phase winding is sinusoidal.

[0025] Preferably, the clean model of PMSM in a three-phase stationary coordinate system: s is the excitation space vector, which rotates with the rotor and its position in the three-phase stationary coordinate system (abc axis system) depends on θ;

[0026] According to the magnetic flux equation ψ=Li s +ψ s , the stator voltage vector equation is:

[0027]

[0028] Where:

[0029]

[0030] Among them, R s is the stator phase resistance, L is the equivalent synchronous inductance; L σs is the stator phase leakage inductance; L m1 is the magnetizing inductance of each phase; is the amplitude of the rotor flux, and the electromagnetic torque equation is:

[0031] T e =p n ψs×i s Formula 3

[0032] Where, T e is the electromagnetic torque, p n is the pole pair number;

[0033] After the positive direction of each physical quantity is given, the motion equation of the permanent magnet synchronous motor is expressed as:

[0034] T e =Jpω m +RΩω m Formula 4

[0035] In the formula, ω m is the mechanical rotation speed, ω m =pθ; p is the differential operator; J is the moment of inertia of the flywheel; R Ω is the friction coefficient, based on the flywheel bearing being suspended in a vacuum, RΩ Ignore.

[0036] Preferably, the clean model of PMSM in two-phase rotating coordinates: by Park transformation, equation 2 is transformed into a stator voltage equation in a two-phase rotating coordinate system (dq axis system), such as equation 5:

[0037]

[0038] The magnetic flux equation is:

[0039]

[0040] In the formula, u d 、u q 、i d 、i q is the voltage and current of the permanent magnet synchronous motor on the dq axis; L d =L σs +L md ; L q =L σs +L mq ; L d and L md , L q and L mq They are the self-inductance and excitation inductance of the stator coil on the dq axis: ω r is the electrical rotation speed;

[0041] Ignoring the effect of temperature on the permanent magnet in PMSM, ψ f Remain unchanged, for the surface mounted permanent magnet synchronous generator, L d =L q =L, then the voltage equation is:

[0042]

[0043] By s =ψ d +jψ q ;i s =i d +ji q , the electromagnetic torque equation is:

[0044] T e =p n (ψ d i q -ψ q i d ) Formula 8

[0045] Substituting equation 6 into equation 8, we get:

[0046] T e =pn [ψ f i q +(L d -L q )i d i q ]=p n ψ f i q Formula 9

[0047] The clean model of PMSM in two-phase rotating coordinates is obtained.

[0048] Preferably, the PWM rectifier four-quadrant operation model: the PWM rectifier realizes the two-phase flow of energy. When absorbing electric energy, the PWM converter is used as a rectifier; when transmitting electric energy to the outside, the PWM converter is used as an inverter. The PWM rectifier here is a four-quadrant operation converter with controllable AC and DC sides. According to the power balance, there is a relationship of formula 10:

[0049] iv=i dc U dc Formula 10

[0050] In the formula, i is the current on the AC side of the model circuit, v is the voltage on the AC side of the model circuit, i dc is the DC side current of the model circuit, U dc is the DC side voltage of the model circuit;

[0051] By controlling the AC side to realize the control of the DC side, the voltage vector formula of the AC side is obtained as follows:

[0052] E=V L +V Type 11

[0053] Where, E is the electromotive force vector of the power grid, V L is the inductor voltage vector; V is the AC side voltage vector. Assuming that the modulus of the current vector |I| remains unchanged, the modulus of the inductor voltage vector |V L |=ωL|I| also remains unchanged, that is, the motion trajectory of the endpoint V forms a circle with a radius of y; controlling I can realize the control of the four quadrants of the PWM rectifier.

[0054] Preferably, a clean model of the PWM rectifier:

[0055] 1) VSR cleaning model in three-phase stationary coordinate system

[0056] Let e a , e b , e c is the three-phase ideal sinusoidal electromotive force, the grid-side inductance L is linear, T1-T6 are ideal switching elements; R L and e Lare the DC load resistance and DC electromotive force respectively. L =0 and e L <U dc When the three-phase VSR is used as a rectifier; when e L >U dc When the three-phase VSR is used, it can be used as both a rectifier and an inverter;

[0057] The unipolar switching function is defined as:

[0058]

[0059] In the formula, k is the three phases a, b, and c, and the voltage equation of the phase a loop is:

[0060]

[0061] Where R is the combined resistance of the three-phase VSR power switch tube loss equivalent resistance and the inductor equivalent resistance. When T1 is turned on and T4 is turned off, s a =1,v aN =U dc ; When T1 is turned off and T4 is turned on, s a =0, there is v aN =U dc s a , put it into formula 13, we have:

[0062]

[0063] Similarly, the voltage equations for phases b and c are:

[0064]

[0065] According to the three-phase balance, we get:

[0066] e a +e b +e c =0,i a +i b +i c =0 Formula 16

[0067] Combining equation 13 and equation 16, we get:

[0068]

[0069] There are always three switch tubes turned on, and there are 8 switching modes in total, namely ①T1, T3, T5 are turned on, T4, T6, T2 are turned off; ②T1, T3, T2 are turned on, T4, T6, T5 are turned off; ③T1, T6, T5 are turned on, T4, T3, T2 are turned off; ④T1, T6, T2 are turned on, T4, T3, T5 are turned off; ⑤T4, T3, T5 are turned on, T1, T6, T5 are turned off; ⑥T4, T3, T2 are turned on, T1, T6, T5 are turned off; ⑦T4, T6, T5 are turned on, T1, T3, T2 are turned off; ⑧T4, T6, T2 are turned on, T1, T3, T5 are turned off, and the DC side current i dc It is expressed as:

[0070] i dc =i a s a +i b s b +i c s c Formula 18

[0071] At the positive pole of the DC capacitor:

[0072]

[0073] Introduce state variable X = [i a ,i b ,i c , U dc ], the state variable expression of the three-phase VSR unipolar switching function cleaning model is:

[0074]

[0075] Where:

[0076]

[0077] E=[e a ,e b , e c , e ] T aa21

[0078] The switching function model of VSR in the abc axis system is:

[0079]

[0080] 2) VSR cleaning model in two-phase rotating coordinate system

[0081] Since the variables on the AC side of the switch function model of the VSR in the three-phase stationary coordinate system are all time-varying variables, in order to facilitate the design of the control system, the time-varying AC quantities on the AC side of the VSR are transformed into DC quantities in the two-phase rotating coordinate system;

[0082] The cleaning model of VSR in the two-phase rotating coordinate system is:

[0083]

[0084] In the formula, e d and e q are the components of the motor electromotive force on the dq axis, i d and i q are the components of the PWM rectifier AC input current on the dq axis, let u d =U dc s d ,u q =U dc s q , then Equation 23 is expressed as:

[0085]

[0086] The VSR cleaning model in the two-phase rotating coordinate system is obtained.

[0087] Preferably, the flywheel battery charging equivalent model circuit is divided into a three-phase voltage type PWM rectifier, a three-phase voltage type PWM inverter bridge, a permanent magnet synchronous motor and a flywheel;

[0088] (1) Equivalent model circuit on the dq axis of a three-phase voltage-source PWM rectifier

[0089] The gyrator is introduced to establish the equivalent model circuit of the three-phase voltage-type PWM rectifier on the dq axis. The gyrator characteristic equation is:

[0090]

[0091] Where r is the rotation coefficient, is the peak value of the grid electromotive force, is the initial phase angle between the q axis in the dq axis system and the a axis in the abc axis system, is the phase angle of the electromotive force on the AC side, is the initial phase angle of the fundamental component of the switching function, is the fundamental peak value of the switching function;

[0092] (2) Equivalent model circuit of permanent magnet synchronous motor and flywheel on dq axis

[0093] The torque is used to replace the voltage, the speed is used to replace the current, the flywheel rotational inertia is used to replace the inductance, and the friction coefficient is used to replace the resistance. The three-phase PWM rectifier, the three-phase inverter bridge, the permanent magnet synchronous motor and the flywheel dq axis equivalent model circuit are connected to obtain the flywheel battery charging equivalent model circuit.

[0094] Preferably, the flywheel battery charging analysis optimization model:

[0095] (I) Flywheel battery charging process control strategy

[0096] The control system is a double closed-loop control system with a speed outer loop and a voltage inner loop. First, the rotor position, speed and stator current of the PMSM are detected by the detection device, and the stator current i dc Through coordinate transformation, it becomes i under the dq axis d and i q , then, the detected speed ω m With a given speed ω m * The difference is compared and the output current given value i is obtained after the speed regulator passes through it. q * , with i q For comparison, at the same time, i d With the given i d * =0, the output of both is decoupled by the circuit and the anti-Park conversion circuit to obtain the control voltage U α and U β , and then generate control signals through photovoltaic charging pulse width modulation to drive the PWM inverter to work;

[0097] (II) Photovoltaic charging pulse width modulation model

[0098] The signal is modulated in real time to work with the voltage vector u x The component u in the αβ coordinate system α and u β and PWM period T as input, u x The two adjacent space electric vectors u i and u j Synthesized, the selected working voltage vectors are expressed as:

[0099]

[0100] In the formula, θ i =(i-1)π / 3, i, j = 1, ..., 6, the operating voltage space vector is:

[0101]

[0102] In the formula, u mis the magnitude of the working voltage vector, and 0 is the phase angle;

[0103] According to the photovoltaic charging pulse width modulation rule, the total time constant rule and the symmetry of PWM, we get:

[0104]

[0105] Where t1, t2, T0, and T7 are the basic vectors u i and u j The action time with the zero vectors u0 and u7;

[0106] u x =u a +ju β , Adding it to equation 27 yields:

[0107]

[0108] In the above formula, the absolute value of sine / cosine can only have two values ​​when the sector changes. Variables X, Y, and Z are introduced to calculate the time t1 and t2, namely:

[0109]

[0110] The relationship between time t1, t2 and sector is obtained from formula 30;

[0111] Add constraints to the inverter output to prevent the voltage reference vector from exceeding the reference signal when the inverter outputs the maximum voltage. If t1+t2<T, t1 and t2 are the original values ​​and no correction is performed. If t1+t2>T, t1 and t2 need to be corrected. The specific correction is shown in formula 31:

[0112]

[0113] The determination of the sector where the reference voltage is located is completed by equation 32, where uα, u β They represent the components of the reference voltage vector corresponding to the αβ axes respectively;

[0114]

[0115] Definition rules: If v1>0, then A=1, otherwise A-0; if v2>0, then B=1, otherwise B=0; if v3>0, then C=1, otherwise C=0. The value of sector N is calculated based on the values ​​of the three binary numbers A, B, and C, and the result is:

[0116] N=4sgn(A)+2sgn(B)+sgn(c) Equation 33

[0117] The zero vector exists in each sector at the beginning and end of each modulation cycle, and the total zero state time is divided into two identical zero state times, namely:

[0118]

[0119] According to the rule of minimum number of switch switching, we get:

[0120]

[0121] The relationship between each sector and the pulse width of the three-phase PWM half cycle is thus determined.

[0122] Preferably, the flywheel energy storage photovoltaic combined system: the flywheel battery is used as an energy storage device and connected to the photovoltaic system to form a combined system. According to the relationship between the output power of the photovoltaic system and the load power, the combined system sets four working models to analyze and optimize the compensation effect of the flywheel battery on the output power of the photovoltaic system;

[0123] The combined system consists of four parts: photovoltaic system, flywheel battery, load, and combined controller. When the photovoltaic system has output, there are four relationships between the output power of the photovoltaic system and the load: (1) the output power of the photovoltaic system is greater than the power required by the load; (2) the output power of the photovoltaic system is less than the power required by the load; (3) the output power of the photovoltaic system is the same as the power required by the load; (4) the photovoltaic system has output power, and the load does not need power supply;

[0124] In response to these four different situations, the flywheel battery plays different roles, and the combined system has four working models:

[0125] 1) When P PV >P L When the flywheel battery is charged, the excess electric energy output by the photovoltaic system is absorbed, and the speed increases;

[0126] 2) When P PV <P L When the flywheel battery discharges, it makes up for the insufficient output power of the photovoltaic system, and the speed decreases;

[0127] 3) When P PV =P L When , there is no energy exchange between the photovoltaic system and the flywheel battery, the flywheel battery moves at a constant speed, and the speed remains unchanged;

[0128] 4) When P PV >0,P L =0, all the electric energy output by the photovoltaic system is stored in the flywheel battery, the flywheel battery is charged, and the speed increases.

[0129] Compared with the prior art, the innovations and advantages of this application are:

[0130] (1) This application divides the flywheel battery into two parts: a permanent magnet synchronous motor and a PWM rectifier, and establishes the charge / discharge equivalent model circuit under the clean model and the dq axis system respectively; analyzes and establishes the control mode and control strategy in the charge / discharge process of the flywheel battery, combines the photovoltaic system with the flywheel battery, divides the combined system into four working models according to the supply and demand relationship between the output power of the photovoltaic system and the power required by the load, formulates the control method and strategy of the joint controller, and analyzes and optimizes the operating state of the system under each working model and the energy storage and release effect of the flywheel battery. The combined system can operate stably under each working model and between each working model; as an energy storage link, the flywheel battery has a high charge / discharge efficiency and discharge depth, and can respond quickly to the difference between the output of the photovoltaic system and the load power; and stores energy when there is surplus output of the photovoltaic system, and releases energy to make up for the needs when the output of the photovoltaic system is insufficient, effectively improving the quality of electric energy and reducing the waste of resources.

[0131] (2) This application establishes a flywheel battery energy storage optimization cleaning model, builds an optimization model for photovoltaic cells, and analyzes its IV and PV characteristics corresponding to different light intensities and temperatures. Under the same temperature and different light intensities, the maximum power of the photovoltaic cell increases with the increase of light intensity; when the light intensity is the same but the temperature is different, the maximum power of the photovoltaic cell decreases with the increase of temperature. The flywheel battery power electronic conversion and motor linkage system is established and optimized, and the conductivity increment method is used as the maximum power tracking method of the photovoltaic system; the flywheel battery charge / discharge optimization control strategy is formulated, and its charge and discharge models are analyzed respectively to optimize the charge / discharge characteristics of the flywheel battery, so that the speed rises / falls smoothly and the given voltage is output smoothly during discharge. The speed of its charge and discharge is related to the input / output power and the rotational inertia of the flywheel battery; the flywheel energy storage device is designed in an off-grid photovoltaic power generation system to track the planned output and smooth the output to provide more stable and reliable electricity, enhance the flexibility and reliability of the system, optimize the system energy management path, and improve the freedom of energy trading.

[0132] (3) This application analyzes the establishment of a flywheel battery clean energy storage model, establishes a flywheel battery clean model composed of a permanent magnet synchronous motor (PMSM) and a PWM converter, analyzes and models the motion equation, voltage equation, electromagnetic torque equation and magnetic flux equation of the permanent magnet synchronous motor respectively, establishes a four-quadrant operation model and a switching function model of the PWM converter, and builds a charge / discharge equivalent model circuit of the flywheel battery in the dq axis; uses the flywheel battery as an energy storage device and combines it with a photovoltaic system. According to the relationship between the output power of the photovoltaic system and the power required by the load, the operating state of the combined system is set to four working models, analyzes the various working models of the combined system, and formulates a control strategy for the combined controller. The flywheel battery quickly switches the working model according to the difference between the output power of the photovoltaic system and the load power, stores electrical energy when the photovoltaic system outputs sufficient energy, and compensates for it when the photovoltaic system outputs insufficient energy, thereby providing stable electrical energy for the load, improving energy utilization, and effectively utilizing the excess electrical energy of the photovoltaic system, improving the problem of abandoned light, and being beneficial to the long-term operation of the photovoltaic system. It has a good application prospect. BRIEF DESCRIPTION OF THE DRAWINGS

[0133] Figure 1 It is a schematic diagram of the physical model of a two-pole surface-mounted PMSM in a spatial complex plane.

[0134] Figure 2 It is the equivalent model diagram of the four-quadrant operation of the PWM rectifier.

[0135] Figure 3 It is a topological diagram of a three-phase voltage-type PWM rectifier.

[0136] Figure 4 It is the equivalent model circuit diagram of VSR on the dq axis.

[0137] Figure 5 It is the equivalent model circuit diagram of PMSM on the dq axis.

[0138] Figure 6 It is the voltage vector synthesis diagram of the photovoltaic charging pulse width modulation model.

[0139] Figure 7 This is the relationship diagram between sector number and time of the photovoltaic charging pulse width modulation model.

[0140] Figure 8 This is the inverter action diagram of the bilateral space vector pulse width modulation in sector I.

[0141] Fig. 9 It is a curve diagram of the charging speed and energy change of the flywheel battery.

[0142] Fig.10 It is a framework diagram of the control strategy for the flywheel battery discharge process.

[0143] Fig.11 It is a schematic diagram of the change in discharge speed of the flywheel battery and the change in DC side voltage.

[0144] Fig.12 It is the structural block diagram of the joint system working model.

[0145] Fig.13 It is a schematic diagram of the four working modes of the joint system.

[0146] Fig.14 It is the control block diagram of the joint controller system. DETAILED DESCRIPTION

[0147] The following is a further description of the technical solution of the off-grid photovoltaic power generation system based on flywheel clean battery energy storage provided by the present application in conjunction with the accompanying drawings, so that those skilled in the art can better understand the present application and implement it.

[0148] This application divides the flywheel battery into two parts: a permanent magnet synchronous motor and a PWM rectifier, and establishes a clean model and a charge / discharge equivalent model circuit under a dq axis system respectively; analyzes and establishes the control method and control strategy during the charge / discharge process of the flywheel battery, combines the photovoltaic system with the flywheel battery, and divides the combined system into four working models according to the supply and demand relationship between the output power of the photovoltaic system and the power required by the load, formulates the control method and strategy of the joint controller, and analyzes and optimizes the operating status of the system under each working model and the energy storage and release effects of the flywheel battery.

[0149] The combined system can operate stably under and between various working models; the flywheel battery, as an energy storage link, has high charge / discharge efficiency and discharge depth, and can respond quickly to changes in the difference between the photovoltaic system output and the load power; it stores energy when there is surplus output from the photovoltaic system, and releases energy to make up for the needs when the output of the photovoltaic system is insufficient, effectively improving the quality of electric energy and reducing waste of resources.

[0150] 1. Flywheel battery energy storage optimization cleaning model

[0151] As an electromechanical energy conversion device, the flywheel battery uses a motor and its power electronic conversion device to change the kinetic energy of the flywheel, thereby storing and releasing energy. It has the characteristics of large specific energy, high efficiency, long life, and no pollution. In order to realize the two-way flow of flywheel battery energy, its suitable power electronic conversion device and efficient energy conversion motor are determined, a clean model of the flywheel battery is established, and its charge / discharge process is analyzed and optimized.

[0152] This application establishes and optimizes the flywheel battery power electronic conversion and motor linkage system, constructs a clean model of the flywheel battery and its charging and discharging equivalent circuit model under the dq axis, and lays the foundation for establishing and analyzing the flywheel battery charging and discharging control model.

[0153] (I) Flywheel battery cleaning model

[0154] The energy and energy storage density of a flywheel are related to its rotational speed and moment of inertia. When the flywheel is constant, its moment of inertia is constant, and the energy of the flywheel is only related to the rotational speed. The speed n of the flywheel battery is related to the frequency f of the power supply. The speed control can be achieved by changing the frequency. The control of the flywheel battery is based on changing the frequency.

[0155] 1. Build a flywheel battery power electronic conversion and motor linkage system

[0156] The charging and discharging control is realized based on the PWM converter. In the charging mode, the PWM converter 1 works as a rectifier and the PWM converter 2 works as an inverter. The AC power of the grid is changed into DC power through the PWM converter 1, and then converted into AC power with adjustable voltage and frequency through the PWM converter to drive the motor to work, and the flywheel stores energy. In the discharging mode, the PWM converter 1 works as an inverter and the PWM converter works as a rectifier. The AC power of the flywheel battery is changed into DC power through the PWM converter 2, and then converted into AC power with adjustable voltage and frequency through the PWM converter 1 to be provided to the electrical equipment to release energy.

[0157] The frequency conversion system ensures that the motor speed changes smoothly, adjusts the motor speed, ensures the stability of the output power, and realizes two-way control of power flow.

[0158] The flywheel battery motor adopts a permanent magnet synchronous motor to construct a flywheel battery power electronic conversion and motor linkage system. The topology of the flywheel battery is as follows: Figure 3-7 The operating characteristics are as follows: (1) Reversible, with two states: power generation and motoring; (2) Wide speed range and good regulation performance; (3) High-speed operation, storing more energy; (4) Large output power and torque; (5) No or very little loss when running at no load.

[0159] 2. Permanent magnet synchronous motor cleaning model

[0160] The cross section of the permanent magnet synchronous motor is taken as the complex plane (coordinate R e -I m ) to represent the space vector inside the PMSM, the physical model of the two-pole surface-mounted PMSM in the space complex plane Figure 1 shown.

[0161] In the PMSM complex plane, select any spatial stationary complex plane R e -Im , if the real axis R e is the reference axis of the spatial coordinate system. Any vector R in the space is represented by R = |R|e jθ , |R| is the magnitude of the vector, and θ is the distance between the vector and the reference axis R e The spatial electrical angle, i.e., the spatial phase;

[0162] Let the a-axis of the three-phase stationary coordinate system (abc) and the real axis R e If they coincide, the spatial position angles of the b-axis and the c-axis are a=e j120° and a 2 =e j240° , a and a2 are spatial operators.

[0163] In the abc coordinate system, the effective number of turns of the single-axis coil is the number of turns of each phase line group. Then the stator current space vector equation is:

[0164]

[0165] In the formula, i a 、i b 、i c The stator currents of the three phases a, b, and c are transformed from the three-phase stationary coordinates (abc) to the two-phase rotating coordinates (dq). Vector control is used to simplify the control system design.

[0166] The clean model of permanent magnet synchronous motor consists of flux equation, electromagnetic torque equation, voltage equation and motion equation, and makes the following assumptions: ① Ignore core saturation, hysteresis and eddy current losses; ② The conductivity of permanent magnet material is zero; ③ There is no damping winding on the rotor; ④ The waveform of the induced electromotive force in the phase winding is sinusoidal.

[0167] (1) Clean model of PMSM in three-phase stationary coordinate system

[0168] ψ s is the excitation space vector, which rotates with the rotor and its position in the three-phase stationary coordinate system (abc axis system) depends on θ;

[0169] According to the magnetic flux equation ψ=Li s +ψ s , the stator voltage vector equation is:

[0170]

[0171] Where:

[0172]

[0173] Among them, R s is the stator phase resistance, L is the equivalent synchronous inductance; Lσs is the stator phase leakage inductance; L m1 is the magnetizing inductance of each phase; is the amplitude of the rotor flux, and the electromagnetic torque equation is:

[0174] T e =p n ψ s ×i s Formula 3

[0175] Where, T e is the electromagnetic torque, p n is the pole pair number;

[0176] After the positive direction of each physical quantity is given, the motion equation of the permanent magnet synchronous motor is expressed as:

[0177] T e =Jpω m +R Ω ω m Formula 4

[0178] In the formula, ω m is the mechanical rotation speed, ω m =pθ; p is the differential operator; J is the moment of inertia of the flywheel; R Ω is the friction coefficient, based on the flywheel bearing being suspended in a vacuum, R Ω Ignore.

[0179] (2) Cleaning model of PMSM in two-phase rotating coordinates

[0180] Through Park transformation, Equation 2 is transformed into the stator voltage equation in the two-phase rotating coordinate system (dq axis system), as shown in Equation 5:

[0181]

[0182] The magnetic flux equation is:

[0183]

[0184] In the formula, u d 、u q 、i d 、i q is the voltage and current of the permanent magnet synchronous motor on the dq axis; L d =L σs +L md ; L q =L σs +L mq ; L d and L md , L q and L mqThey are the self-inductance and excitation inductance of the stator coil on the dq axis: ω r is the electrical rotation speed;

[0185] Ignoring the effect of temperature on the permanent magnet in PMSM, ψ f Remain unchanged, for the surface mounted permanent magnet synchronous generator, L d =L q =L, then the voltage equation is:

[0186]

[0187] By s =ψ d +jψ q ;i s =i d +ji q , the electromagnetic torque equation is:

[0188] T e =p n (ψ d i q -ψ q i d ) Formula 8

[0189] Substituting equation 6 into equation 8, we get:

[0190] T e =p n [ψ f i q +(L d -L q )i d i q ]=p n ψ f i q Formula 9

[0191] The clean model of PMSM in two-phase rotating coordinates is obtained.

[0192] 3. PWM rectifier cleaning model

[0193] (1) PWM rectifier four-quadrant operation model

[0194] The PWM rectifier realizes the two-phase flow of energy. When absorbing electric energy, the PWM converter acts as a rectifier; when transmitting electric energy to the outside, the PWM converter acts as an inverter. The PWM rectifier here is a four-quadrant converter with controllable AC and DC sides. Its equivalent model circuit diagram is shown in Figure 2 As shown. According to the power balance, there is a relationship in formula 10:

[0195] iv=i dc Udc Formula 10

[0196] In the formula, i is the current on the AC side of the model circuit, v is the voltage on the AC side of the model circuit, i dc is the DC side current of the model circuit, U dc is the DC side voltage of the model circuit;

[0197] By controlling the AC side to realize the control of the DC side, the voltage vector formula of the AC side is obtained as follows:

[0198] E=V L +V Type 11

[0199] Where, E is the electromotive force vector of the power grid, V L is the inductor voltage vector; V is the AC side voltage vector. Assuming that the modulus of the current vector |I| remains unchanged, the modulus of the inductor voltage vector |V L |=ωL|I| also remains unchanged, that is, the motion trajectory of the endpoint V forms a circle with a radius of y; controlling I can realize the control of the four quadrants of the PWM rectifier.

[0200] (2) Clean Model of PWM Rectifier

[0201] The topology of the three-phase voltage-type PWM rectifier is established as follows Figure 3 shown.

[0202] 1) VSR cleaning model in three-phase stationary coordinate system

[0203] Let e a , e b , e c is the three-phase ideal sinusoidal electromotive force, the grid-side inductance L is linear, T1-T6 are ideal switching elements; R L and e L are the DC load resistance and DC electromotive force respectively. L =0 and e L <U dc When the three-phase VSR is used as a rectifier; when e L >U dc When the three-phase VSR is used, it can be used as both a rectifier and an inverter;

[0204] The unipolar switching function is defined as:

[0205]

[0206] In the formula, k is the three phases a, b, and c, and the voltage equation of the phase a loop is:

[0207]

[0208] Where R is the combined resistance of the three-phase VSR power switch tube loss equivalent resistance and the inductor equivalent resistance. When T1 is turned on and T4 is turned off, s a =1,v aN =U dc ; When T1 is turned off and T4 is turned on, s a =0, there is v aN =U dc s a , put it into formula 13, we have:

[0209]

[0210] Similarly, the voltage equations for phases b and c are:

[0211]

[0212] According to the three-phase balance, we get:

[0213] e a +e b +e c =0,i a +i b +i c =0 Formula 16

[0214] Combining equation 13 and equation 16, we get:

[0215]

[0216] There are always three switch tubes turned on, and there are 8 switching modes in total, namely ①T1, T3, T5 are turned on, T4, T6, T2 are turned off; ②T1, T3, T2 are turned on, T4, T6, T5 are turned off; ③T1, T6, T5 are turned on, T4, T3, T2 are turned off; ④T1, T6, T2 are turned on, T4, T3, T5 are turned off; ⑤T4, T3, T5 are turned on, T1, T6, T5 are turned off; ⑥T4, T3, T2 are turned on, T1, T6, T5 are turned off; ⑦T4, T6, T5 are turned on, T1, T3, T2 are turned off; ⑧T4, T6, T2 are turned on, T1, T3, T5 are turned off, and the DC side current i dc It is expressed as:

[0217] i dc =i a s a +i b s b +i c s c Formula 18

[0218] At the positive pole of the DC capacitor:

[0219]

[0220] Introduce state variable X = [i a ,i b ,i c , U dc ], the state variable expression of the three-phase VSR unipolar switching function cleaning model is:

[0221]

[0222] Where:

[0223]

[0224] E=[e a , e b , e c , e L ] T Formula 21

[0225] The switching function model of VSR in the abc axis system is:

[0226]

[0227] The VSR cleaning model in the three-phase stationary coordinate system is obtained.

[0228] 2) VSR cleaning model in two-phase rotating coordinate system

[0229] Since the variables on the AC side of the switch function model of the VSR in the three-phase stationary coordinate system are all time-varying variables, in order to facilitate the design of the control system, the time-varying AC quantities on the AC side of the VSR are transformed into DC quantities in the two-phase rotating coordinate system;

[0230] The cleaning model of VSR in the two-phase rotating coordinate system is:

[0231]

[0232] In the formula, e d and e q are the components of the motor electromotive force on the dq axis, i d and i q are the components of the PWM rectifier AC input current on the dq axis, let u d =U dc s d ,u q =U dc s q , then Equation 23 is expressed as:

[0233]

[0234] The VSR cleaning model in the two-phase rotating coordinate system is obtained.

[0235] (III) Flywheel battery equivalent model circuit

[0236] 1. Flywheel battery charging equivalent model circuit

[0237] It is divided into three-phase voltage type PWM rectifier, three-phase voltage type PWM inverter bridge and permanent magnet synchronous motor and flywheel.

[0238] (1) Equivalent model circuit on the dq axis of a three-phase voltage-source PWM rectifier

[0239] The gyrator is introduced to establish the equivalent model circuit of the three-phase voltage-type PWM rectifier on the dq axis. The gyrator characteristic equation is:

[0240]

[0241] Where r is the rotation coefficient, and the equivalent model circuit of VSR on the dq axis is as follows: Figure 4 As shown;

[0242] is the peak value of the grid electromotive force, is the initial phase angle between the q axis in the dq axis system and the a axis in the abc axis system, is the phase angle of the electromotive force on the AC side, is the initial phase angle of the fundamental component of the switching function, is the fundamental peak value of the switching function.

[0243] (2) Equivalent model circuit of permanent magnet synchronous motor and flywheel on dq axis

[0244] Using torque instead of voltage, speed instead of current, flywheel moment of inertia instead of inductance (energy storage), and friction coefficient instead of resistance, the equivalent model circuit of PMSM on the dq axis can be expressed as: Figure 5 The three-phase PWM rectifier, three-phase inverter bridge, permanent magnet synchronous motor and flywheel dq axis equivalent model circuit are connected to obtain the flywheel battery charging equivalent model circuit.

[0245] 2. Flywheel battery discharge equivalent model circuit

[0246] When the flywheel battery is discharged, the current direction on the DC bus is opposite to that when it is charged. The three-phase PWM rectifier, the three-phase inverter bridge, the permanent magnet synchronous motor and the flywheel dq axis equivalent model circuit are connected to obtain the equivalent model circuit of the flywheel battery when it is discharged.

[0247] 2. Flywheel battery charging analysis and optimization model

[0248] (I) Flywheel battery charging process control strategy

[0249] The control system is a double closed-loop control system with a speed outer loop and a voltage inner loop. First, the rotor position, speed and stator current of the PMSM are detected by the detection device, and the stator current i dc Through coordinate transformation, it becomes i under the dq axis d and i q , then, the detected speed ω m With a given speed ω m * The difference is compared and the output current given value i is obtained after the speed regulator passes through it. q * , with i q For comparison, at the same time, i d With the given i d * =0, the output of both is decoupled by the circuit and the anti-Park conversion circuit to obtain the control voltage U α and U β , and then the photovoltaic charging pulse width modulation generates a control signal to drive the PWM inverter to work.

[0250] (II) Photovoltaic charging pulse width modulation model

[0251] The signal is modulated in real time to work with the voltage vector u x The component u in the αβ coordinate system α and u β and PWM period T as input, u x The two adjacent space electric vectors u i and u j Synthesis, such as Figure 6 The selected working voltage vectors are expressed as:

[0252]

[0253] In the formula, θ i =(i-1)π / 3, i, j = 1, ..., 6, the operating voltage space vector is:

[0254]

[0255] In the formula, u m is the magnitude of the working voltage vector, and 0 is the phase angle;

[0256] According to the photovoltaic charging pulse width modulation rule, the total time constant rule and the symmetry of PWM, we get:

[0257]

[0258] Where t1, t2, T0, and T7 are the basic vectors ui and u j The action time with the zero vectors u0 and u7;

[0259] u x =u a +ju β , Adding it to equation 27 yields:

[0260]

[0261] In the above formula, the absolute value of sine / cosine can only have two values ​​when the sector changes. Variables X, Y, and Z are introduced to calculate the time t1 and t2, namely:

[0262]

[0263] The relationship between time t1, t2 and sector is obtained from formula 30: Figure 7 .

[0264] Add constraints to the inverter output to prevent the voltage reference vector from exceeding the reference signal when the inverter outputs the maximum voltage. If t1+t2<T, t1 and t2 are the original values ​​and no correction is performed. If t1+t2>T, t1 and t2 need to be corrected. The specific correction is shown in formula 31:

[0265]

[0266] The determination of the sector where the reference voltage is located is completed by equation 32, where u α 、u β They represent the components of the reference voltage vector corresponding to the αβ axes respectively;

[0267]

[0268] Definition rules: If v1>0, then A=1, otherwise A-0; if v2>0, then B=1, otherwise B=0; if v3>0, then C=1, otherwise C=0. The value of sector N is calculated based on the values ​​of the three binary numbers A, B, and C, and the result is:

[0269] N=4sgn(A)+2sgn(B)+sgn(c) Equation 33

[0270] The zero vector exists in each sector at the beginning and end of each modulation cycle, and the total zero state time is divided into two identical zero state times, namely:

[0271]

[0272] According to the minimum number of switch switching rules, Figure 6It can be seen that the inverter switching signal of the double-sided space vector pulse width modulation of the voltage space vector in the I sector is as follows: Figure 8 As shown, we get:

[0273]

[0274] The relationship between each sector and the pulse width of the three-phase PWM half cycle is thus determined.

[0275] (III) Optimization analysis of flywheel battery charging process

[0276] A simulation model of the flywheel battery charging process was built in Matlab / Simulink software. The initial speed of the flywheel was set to 500r / min, and the speed regulator used a PID controller. To verify the effect of the flywheel battery charging control strategy, the flywheel battery was charged with an input power of 1kW and 5kW respectively. The setting time was 10s. The speed change of the flywheel battery is as follows: Fig. 9 .

[0277] Assume that when the flywheel battery is charged at 1kW, the slope of the flywheel battery speed change is k, and when the flywheel battery is charged at 5kW, the slope of the flywheel battery speed change is k, k2>k1, indicating that the speed of charging the flywheel battery at 5kW is faster, that is, when the flywheel battery is charged, the speed of the speed increase is related to the input power. The greater the input power, the faster the speed increases. The relationship between the two satisfies

[0278] 3. Flywheel battery discharge analysis and optimization model

[0279] (I) Flywheel battery discharge process control strategy

[0280] The feedforward decoupling control strategy is used as the control strategy of the PWM rectifier. The discharge control block diagram is shown in Fig.10 The control system is a double closed-loop control system with a DC voltage outer loop and an AC current inner loop. First, the detected AC current i abc and AC voltage e abc Perform coordinate transformation to obtain i under the dq axis d 、i q and e d 、e q , the detected DC side voltage U dc Compared with the given value U dc * The difference is passed through the PI controller to output the d-axis current given value i d * , let i q * =0, and compare the two given values ​​with the measured i d 、i qBy comparison, the control voltage u is obtained through the PI controller and the decoupling circuit. d 、u q , the control voltage U of photovoltaic charging pulse width modulation is obtained through Park inverse transformation α , U β , driving PWM operation, the control system ensures the voltage stability on the DC side, and the inner loop current is determined by the outer loop voltage, so the current has a good tracking effect on the voltage.

[0281] (II) Optimization analysis of flywheel battery discharge process

[0282] A simulation model of the flywheel battery charging process was built in Matlab / Simulink software, and the initial speed of the flywheel was set to 28000r / min. In order to verify the effect of the flywheel battery discharge control strategy, the DC side voltage was set to 48V and 110V, and the power was released at 1kW and 5kW respectively for comparison, and the simulation time was set to 10s. The simulation results are shown in Figure 2. Fig.11 .

[0283] When the flywheel battery is discharged at 1kW, the absolute value of the slope of the flywheel battery speed change is k1. When it is discharged at 5kW, the absolute value of the slope of the flywheel battery speed change is k2. k2>k1. The flywheel battery discharges faster at 5kW. That is, when the flywheel battery is discharged, the speed of speed decrease is related to the output power. The greater the output power, the faster the speed decreases. The relationship between the two satisfies When the flywheel battery is discharged, it tracks the DC side voltage quickly, and it only takes 0.5-1s to stabilize to the given voltage value; the overshoot is small, and when the DC side voltage is set to 48V ( Fig.11 (b)), the overshoot is about 8.3%. When the DC side voltage is set to 110V ( Fig.11 (c)), the overshoot is about 10%.

[0284] The speed change during the charge / discharge process of the flywheel battery is related to its input / output power. When the flywheel battery is discharged, it tracks the DC side voltage quickly, has a small overshoot and good dynamic performance.

[0285] 4. Flywheel Energy Storage Photovoltaic Combined System

[0286] Flywheel batteries have good charging and discharging characteristics, and their rotation speed can track the changes in input / output power, which can well compensate for the randomness and volatility of the output power of the photovoltaic system. The flywheel battery is used as an energy storage device and connected to the photovoltaic system to form a joint system. According to the relationship between the output power of the photovoltaic system and the load power, the joint system sets four working models to analyze and optimize the compensation effect of the flywheel battery on the output power of the photovoltaic system.

[0287] 1. Analysis of the Joint System Working Model

[0288] The combined system is divided into four parts: photovoltaic system, flywheel battery, load, and combined controller. Its structure diagram is as follows: Fig.12 shown.

[0289] When the PV system has output, there are four relationships between the output power of the PV system and the load: (1) the output power of the PV system is greater than the power required by the load; (2) the output power of the PV system is less than the power required by the load; (3) the output power of the PV system is the same as the power required by the load; (4) the PV system has output power and the load does not need power supply.

[0290] In response to these four different situations, the flywheel battery plays different roles, and the combined system has four working models:

[0291] 1) When P PV >P L When the flywheel battery is charged, the excess electric energy output by the photovoltaic system is absorbed, and the speed increases;

[0292] 2) When P PV <P L When the flywheel battery discharges, it makes up for the insufficient output power of the photovoltaic system, and the speed decreases;

[0293] 3) When P PV =P L When , there is no energy exchange between the photovoltaic system and the flywheel battery, the flywheel battery moves at a constant speed, and the speed remains unchanged;

[0294] 4) When P PV >0,P L =0, all the electric energy output by the photovoltaic system is stored in the flywheel battery, the flywheel battery is charged, and the speed increases;

[0295] The schematic diagram of the four working modes of the joint system is as follows Fig.13 shown.

[0296] (II) System Joint Optimization Analysis

[0297] Based on four different working models, the control strategy of the system joint controller is analyzed. The joint controller uses the difference △P between the photovoltaic system output power and the load power as input to judge the working state of the flywheel battery. When the speed of the flywheel battery reaches the highest / lowest speed, the flywheel battery ends charging / discharging to prevent the flywheel battery from overcharging / overdischarging. The control block diagram is shown in Fig.14 shown.

[0298] The various working modes appear alternately. In the time period of 0:00-6:00, △P=0, the photovoltaic system has no output and the load does not need energy supply. The system is in working model 3, and the speed and energy of the flywheel battery remain basically unchanged; in the time periods of 6:00-7:30 and 16:00-24:00, △P<0, the output of the photovoltaic system is less than the energy required by the load, the system is in working model 2, the flywheel battery is discharged, and its speed and energy decrease; in the time period of 7:30-16:00, △P>0, the output of the photovoltaic system is greater than the energy required by the load, the system is in working model 1, the flywheel battery is charged, and its speed and energy increase. The combined system switches accurately between the models. As an energy storage link, the flywheel battery can respond quickly to changes in the difference between the output of the photovoltaic system and the load power.

Claims

1. An off-grid photovoltaic power generation system based on flywheel clean battery energy storage, characterized in that: The flywheel battery uses a permanent magnet synchronous motor and a PWM rectifier, and establishes the charge / discharge equivalent model circuit under the clean model and the dq axis system respectively; analyzes and establishes the control mode and control strategy during the charge / discharge process of the flywheel battery, combines the photovoltaic system with the flywheel battery, and divides the combined system into four working models according to the supply and demand relationship between the output power of the photovoltaic system and the power required by the load. The control method and strategy of the joint controller are formulated, and the operating state of the system and the energy storage and release effect of the flywheel battery under each working model are analyzed and optimized; 1) Establish a flywheel battery energy storage optimization cleaning model, build an optimization model for photovoltaic cells, and analyze its IV and PV characteristics corresponding to different light intensities and temperatures. Under the same temperature and different light intensities, the maximum power of the photovoltaic cell increases with the increase of light intensity; when the light intensity is the same but the temperature is different, the maximum power of the photovoltaic cell decreases with the increase of temperature. Establish and optimize the flywheel battery power electronic conversion and motor linkage system, and use the conductivity increment method as the maximum power tracking method of the photovoltaic system; 2) Analyze and establish a clean energy storage model for flywheel batteries. Establish a clean flywheel battery model consisting of a permanent magnet synchronous motor (PMSM) and a PWM converter. Analyze and model the motion equation, voltage equation, electromagnetic torque equation, and flux equation of the permanent magnet synchronous motor. Establish a four-quadrant operation model and a switching function model for the PWM converter. Build a charge / discharge equivalent model circuit for the flywheel battery in the dq axis. 3) Formulate an optimized control strategy for flywheel battery charge / discharge, and analyze its charge / discharge model respectively, optimize the charge / discharge characteristics of the flywheel battery, ensure that the speed rises / falls smoothly, and outputs a given voltage smoothly during discharge. The speed of charge / discharge is related to the input / output power and the moment of inertia of the flywheel battery; 4) The flywheel battery is used as an energy storage device and combined with the photovoltaic system. According to the relationship between the output power of the photovoltaic system and the power required by the load, the operating state of the combined system is set to four working models. The various working models of the combined system are analyzed, and the control strategy of the combined controller is formulated. The flywheel battery quickly switches the working model according to the difference between the output power of the photovoltaic system and the load power, stores electric energy when the photovoltaic system output is sufficient, and compensates for it when the photovoltaic system output is insufficient, providing stable electric energy for the load and improving energy utilization.

2. The off-grid photovoltaic power generation system based on flywheel clean battery energy storage according to claim 1 is characterized in that: Construct a flywheel battery power electronic conversion and motor linkage system: charging and discharging control is implemented based on PWM converter. In charging mode, PWM converter 1 works as a rectifier and PWM converter 2 works as an inverter. The AC power of the grid is converted into DC power through PWM converter 1, and then converted into AC power with adjustable voltage and frequency through PWM converter to drive the motor to work, and the flywheel stores energy. In discharging mode, PWM converter 1 works as an inverter and PWM converter works as a rectifier. The AC power of the flywheel battery is converted into DC power through PWM converter 2, and then converted into AC power with adjustable voltage and frequency through PWM converter 1 to provide to the electrical equipment to release energy. The frequency conversion system ensures that the motor speed changes smoothly, adjusts the motor speed, ensures the stability of the output power, and realizes two-way control of power flow; the flywheel battery motor adopts a permanent magnet synchronous motor to construct a flywheel battery power electronic conversion and motor linkage system.

3. The off-grid photovoltaic power generation system based on flywheel clean battery energy storage according to claim 1 is characterized in that: Permanent magnet synchronous motor cleaning model: The cross section of the permanent magnet synchronous motor is taken as a complex plane (coordinate R e -I m ), to represent the space vector inside the PMSM; In the PMSM complex plane, select any spatial stationary complex plane R e -I m , if the real axis R e is the reference axis of the spatial coordinate system. Any vector R in the space is represented by R = |R|e jθ ,|R| is the magnitude of the vector, θ is the distance between the vector and the reference axis R e The spatial electrical angle, i.e., the spatial phase; Let the a-axis of the three-phase stationary coordinate system (abc) and the real axis R e If they coincide, the spatial position angles of the b-axis and the c-axis are a=e j120° and a 2 =e j240° , a, a 2 is a spatial operator; In the abc coordinate system, the effective number of turns of the single-axis coil is the number of turns per phase line group. Then the stator current space vector equation is: In the formula, i a 、i b 、i c The stator currents of the three phases a, b, and c are transformed from the three-phase stationary coordinates (abc) to the two-phase rotating coordinates (dq), and vector control is adopted to simplify the control system design; The clean model of permanent magnet synchronous motor consists of flux equation, electromagnetic torque equation, voltage equation and motion equation, and makes the following assumptions: ① Ignore core saturation, hysteresis and eddy current losses; ② The conductivity of permanent magnet material is zero; ③ There is no damping winding on the rotor; ④ The waveform of the induced electromotive force in the phase winding is sinusoidal.

4. The off-grid photovoltaic power generation system based on flywheel clean battery energy storage according to claim 3 is characterized in that: Clean model of PMSM in three-phase stationary coordinate system: ψ s is the excitation space vector, which rotates with the rotor and its position in the three-phase stationary coordinate system (abc axis system) depends on θ; According to the magnetic flux equation ψ=Li s +ψ s , the stator voltage vector equation is: Where: Among them, R s is the stator phase resistance, L is the equivalent synchronous inductance; L σs is the stator phase leakage inductance; L m1 is the magnetizing inductance of each phase; is the amplitude of the rotor flux, and the electromagnetic torque equation is: T e =P n ψ s ×i s Formula 3 Where, T e is the electromagnetic torque, p n is the pole pair number; After the positive direction of each physical quantity is given, the motion equation of the permanent magnet synchronous motor is expressed as: T e =Jpω m +R Ω oh m formula 4 In the formula, ω m is the mechanical rotation speed, ω m =pθ; p is the differential operator; J is the moment of inertia of the flywheel; R Ω is the friction coefficient, based on the flywheel bearing being suspended in a vacuum, R Ω Ignore.

5. The off-grid photovoltaic power generation system based on flywheel clean battery energy storage according to claim 3 is characterized in that: Clean model of PMSM in two-phase rotating coordinates: Through Park transformation, Equation 2 is transformed into the stator voltage equation in the two-phase rotating coordinate system (dq axis system), as shown in Equation 5: The magnetic flux equation is: In the formula, u d 、u q 、i d 、i q is the voltage and current of the permanent magnet synchronous motor on the dq axis; L d =L σs +L md ; L q =L σs +L mq ; L d and L md , L q and L mq They are the self-inductance and excitation inductance of the stator coil on the dq axis: ω r is the electrical rotation speed; Ignoring the effect of temperature on the permanent magnet in PMSM, ψ f Remain unchanged, for the surface mounted permanent magnet synchronous generator, L d =L q =L, then the voltage equation is: By s =ψ d +jψ q ;i s =i d +ji q , the electromagnetic torque equation is: T e =p n (ψ d I q -ψ q I d ) formula8 Substituting equation 6 into equation 8, we get: T e = p n [ψ f i q +(L d - L q )i d i q = p n ψ f i q Equation 9 The clean model of PMSM in two-phase rotating coordinates is obtained.

6. The off-grid photovoltaic power generation system based on flywheel clean battery energy storage according to claim 1 is characterized in that: PWM rectifier four-quadrant operation model: PWM rectifier realizes the two-phase flow of energy. When absorbing electric energy, the PWM converter acts as a rectifier; when transmitting electric energy to the outside, the PWM converter acts as an inverter. The PWM rectifier here is a four-quadrant converter with controllable AC and DC sides. According to the power balance, there is a relationship of formula 10: iv = i dc U dc Equation 10 In the formula, i is the current on the AC side of the model circuit, v is the voltage on the AC side of the model circuit, i dc is the DC side current of the model circuit, U dc is the DC side voltage of the model circuit; By controlling the AC side to realize the control of the DC side, the voltage vector formula of the AC side is obtained as follows: E=VL+V Formula 11 Where, E is the electromotive force vector of the power grid, V L is the inductor voltage vector; V is the AC side voltage vector. Assuming that the modulus of the current vector |I| remains unchanged, the modulus of the inductor voltage vector |V L |=ωL|I| also remains unchanged, that is, the motion trajectory of the endpoint V forms a circle with a radius of y; controlling I can realize the control of the four quadrants of the PWM rectifier.

7. The off-grid photovoltaic power generation system based on flywheel clean battery energy storage according to claim 1 is characterized in that: Clean model of PWM rectifier: 1) VSR cleaning model in three-phase stationary coordinate system Let e a , e b , e c is the three-phase ideal sinusoidal electromotive force, the grid-side inductance L is linear, T1-T6 are ideal switching elements; R L and e L are the DC load resistance and DC electromotive force respectively. L =0 and e L <U dc When the three-phase VSR is used as a rectifier; when e L >U dc When the three-phase VSR is used, it can be used as both a rectifier and an inverter; The unipolar switching function is defined as: In the formula, k is the three phases a, b, and c, and the voltage equation of the phase a loop is: Where R is the combined resistance of the three-phase VSR power switch tube loss equivalent resistance and the inductor equivalent resistance. When T1 is turned on and T4 is turned off, s a =1,v aN =U dc ; When T1 is turned off and T4 is turned on, s a =0, there is v aN =U dc s a , put it into formula 13, we have: Similarly, the voltage equations for phases b and c are: According to the three-phase balance, we get: e a +e b +e c =0,i a +i b +i c =0 Equation 16 Combining equation 13 and equation 16, we get: There are always three switch tubes turned on, and there are 8 switching modes in total, namely ①T1, T3, T5 are turned on, T4, T6, T2 are turned off; ②T1, T3, T2 are turned on, T4, T6, T5 are turned off; ③T1, T6, T5 are turned on, T4, T3, T2 are turned off; ④T1, T6, T2 are turned on, T4, T3, T5 are turned off; ⑤T4, T3, T5 are turned on, T1, T6, T5 are turned off; ⑥T4, T3, T2 are turned on, T1, T6, T5 are turned off; ⑦T4, T6, T5 are turned on, T1, T3, T2 are turned off; ⑧T4, T6, T2 are turned on, T1, T3, T5 are turned off, and the DC side current i dc It is expressed as: i dc = i a s a + i b s b + i c s c Formula 18 At the positive pole of the DC capacitor: Introduce state variable X = [i a ,i b ,i c , U dc ], the state variable expression of the three-phase VSR unipolar switching function cleaning model is: Where: The switching function model of VSR in the abc axis system is: 2) VSR cleaning model in two-phase rotating coordinate system Since the variables on the AC side of the switch function model of the VSR in the three-phase stationary coordinate system are all time-varying variables, in order to facilitate the design of the control system, the time-varying AC quantities on the AC side of the VSR are transformed into DC quantities in the two-phase rotating coordinate system; The cleaning model of VSR in the two-phase rotating coordinate system is: In the formula, e d and e q are the components of the motor electromotive force on the dq axis, i d and i q are the components of the PWM rectifier AC input current on the dq axis, let u d =U dc s d ,u q =U dc s q , then Equation 23 is expressed as: The VSR cleaning model in the two-phase rotating coordinate system is obtained.

8. The off-grid photovoltaic power generation system based on flywheel clean battery energy storage according to claim 1 is characterized in that: Flywheel battery charging equivalent model circuit: divided into three-phase voltage type PWM rectifier, three-phase voltage type PWM inverter bridge and permanent magnet synchronous motor and flywheel; (1) Equivalent model circuit on the dq axis of a three-phase voltage-source PWM rectifier The gyrator is introduced to establish the equivalent model circuit of the three-phase voltage-type PWM rectifier on the dq axis. The gyrator characteristic equation is: Where r is the rotation coefficient, is the peak value of the grid electromotive force, is the initial phase angle between the q axis in the dq axis system and the a axis in the abc axis system, is the phase angle of the electromotive force on the AC side, is the initial phase angle of the fundamental component of the switching function, is the fundamental peak value of the switching function; (2) Equivalent model circuit of permanent magnet synchronous motor and flywheel on dq axis The torque is used to replace the voltage, the speed is used to replace the current, the flywheel rotational inertia is used to replace the inductance, and the friction coefficient is used to replace the resistance. The three-phase PWM rectifier, the three-phase inverter bridge, the permanent magnet synchronous motor and the flywheel dq axis equivalent model circuit are connected to obtain the flywheel battery charging equivalent model circuit.

9. The off-grid photovoltaic power generation system based on flywheel clean battery energy storage according to claim 1, characterized in that: Flywheel battery charging analysis and optimization model: (I) Flywheel battery charging process control strategy The control system is a double closed-loop control system with a speed outer loop and a voltage inner loop. First, the rotor position, speed and stator current of the PMSM are detected by the detection device, and the stator current i dc Through coordinate transformation, it becomes i under the dq axis d and i q , then, the detected speed ω m With a given speed ω m * The difference is compared and the output current given value i is obtained after the speed regulator passes through it. q * , with i q For comparison, at the same time, i d With the given i d * =0, the output of both is decoupled by the circuit and the anti-Park conversion circuit to obtain the control voltage U α and U β , and then generate control signals through photovoltaic charging pulse width modulation to drive the PWM inverter to work; (II) Photovoltaic charging pulse width modulation model The signal is modulated in real time to work with the voltage vector u x The component u in the αβ coordinate system α and u β and PWM period T as input, u x The two adjacent space electric vectors u i and u j Synthesized, the selected working voltage vectors are expressed as: In the formula, θ i =(i-1)π / 3, i, j = 1, ..., 6, the operating voltage space vector is: In the formula, u m is the magnitude of the working voltage vector, and 0 is the phase angle; According to the photovoltaic charging pulse width modulation rule, the total time constant rule and the symmetry of PWM, we get: Where t1, t2, T0, and T7 are the basic vectors u i and u j The action time with the zero vectors u0 and u7; u x =u α +ju β , Adding it to equation 27 yields: In the above formula, the absolute value of sine / cosine can only have two values ​​when the sector changes. Variables X, Y, and Z are introduced to calculate the time t1 and t2, namely: The relationship between time t1, t2 and sector is obtained from formula 30; Add constraints to the inverter output to prevent the voltage reference vector from exceeding the reference signal when the inverter outputs the maximum voltage. If t1+t2<T, t1 and t2 are the original values ​​and no correction is performed. If t1+t2>T, t1 and t2 need to be corrected. The specific correction is shown in formula 31: The determination of the sector where the reference voltage is located is completed by equation 32, where uα, u β They represent the components of the reference voltage vector corresponding to the αβ axes respectively; Definition rules: If v1>0, then A=1, otherwise A-0; if v2>0, then B=1, otherwise B=0; if v3>0, then C=1, otherwise C=0. The value of sector N is calculated based on the values ​​of the three binary numbers A, B, and C, and the result is: N=4sgn(A)+2sgn(B)+sgn(c) Equation 33 The zero vector exists in each sector at the beginning and end of each modulation cycle, and the total zero state time is divided into two identical zero state times, namely: According to the rule of minimum number of switch switching, we get: The relationship between each sector and the pulse width of the three-phase PWM half cycle is thus determined.

10. The off-grid photovoltaic power generation system based on flywheel clean battery energy storage according to claim 1, characterized in that: Flywheel energy storage photovoltaic combined system: The flywheel battery is used as an energy storage device and connected to the photovoltaic system to form a combined system. According to the relationship between the output power of the photovoltaic system and the load power, the combined system sets four working models to analyze and optimize the compensation effect of the flywheel battery on the output power of the photovoltaic system; The combined system consists of four parts: photovoltaic system, flywheel battery, load, and combined controller. When the photovoltaic system has output, there are four relationships between the output power of the photovoltaic system and the load: (1) the output power of the photovoltaic system is greater than the power required by the load; (2) the output power of the photovoltaic system is less than the power required by the load; (3) the output power of the photovoltaic system is the same as the power required by the load; (4) the photovoltaic system has output power, and the load does not need power supply; In response to these four different situations, the flywheel battery plays different roles, and the combined system has four working models: 1) When P PV >P L When the flywheel battery is charged, the excess electric energy output by the photovoltaic system is absorbed, and the speed increases; 2) When P PV <P L When the flywheel battery discharges, it makes up for the insufficient output power of the photovoltaic system, and the speed decreases; 3) When P PV =P L When , there is no energy exchange between the photovoltaic system and the flywheel battery, the flywheel battery moves at a constant speed, and the speed remains unchanged; 4) When P PV >0,P L =0, all the electric energy output by the photovoltaic system is stored in the flywheel battery, the flywheel battery is charged, and the speed increases.

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