Control method of direct-current converter for wind-solar dual-storage micro-grid

Abstract

The invention relates to the technical field of micro-grid dual energy storage system control, in particular to a control method of a direct current converter for a wind-solar dual-storage micro-grid, and provides dual closed-loop converter control, a linear quadratic regulator is used for current inner loop control, and sliding mode control is used for voltage outer loop control. In the working mode control of the buck-boost converter, a four-switch switching circuit is triggered by power deviation polarity, so that stable and rapid switching of energy storage charging and discharging is realized; and the overshoot of system control is less, and the robustness and the disturbance resistance are better. According to the invention, in the operation process of the micro-grid dual-energy-storage system, the system can still operate stably when the power generation supply end and the load demand end have relatively high power fluctuation at any or at the same time and a charging and discharging circuit needs to be switched.

Description

Technical Field

[0001] The present invention relates to the technical field of microgrid dual energy storage system control, and in particular to a control method for a direct current converter used in a wind-solar dual energy storage microgrid. Background Art

[0002] Clean energy represented by wind and solar energy has developed rapidly in recent years. With the increasing environmental pollution and depletion of traditional energy in the world today, it is urgent to replace traditional fuels with secondary energy. Wind power and photovoltaic power have become the most commercially promising and dynamic renewable energy sources due to their clean use, low cost and inexhaustible advantages.

[0003] However, wind power and photovoltaic power are also facing many problems. First of all, wind power generation is intermittent and unstable, and the power generation is difficult to predict and extremely unstable. When the wind speed is too low or too high, the wind turbine may not be able to operate normally. If it is connected to the grid, it will have a very large impact on the power grid, causing a series of problems such as grid voltage fluctuations.

[0004] Similarly, photovoltaic power generation also has problems of intermittency and uncertainty, and the photovoltaic conversion efficiency of photovoltaic panels is very limited. In ordinary wind, solar and energy storage microgrid off-grid systems, a storage battery and a bidirectional DC / DC converter or a storage battery, a supercapacitor and a bidirectional DC / DC converter are generally used to complete the bus power throughput. Both of the above systems are inseparable from bidirectional DC / DC converters. In actual use, bidirectional DC / DC converters face the following problems:

[0005] 1. High circuit complexity and high control difficulty. Bidirectional DC / DC converters have more complex circuits, more power devices, control circuits and protection circuits, which also requires more sophisticated control strategies;

[0006] 2. Low efficiency. Although bidirectional DC / DC converters can work in both directions, their overall efficiency is lower than that of unidirectional converters. Especially under light load conditions, the efficiency drop is more obvious. Especially when the switching frequency is high, this loss is more obvious;

[0007] 3. Electromagnetic interference. Since bidirectional DC / DC converters require frequent power switching, they are more likely to generate electromagnetic interference. Therefore, in order to avoid the problems caused by bidirectional DC / DC converters, a Buck circuit and a Boost circuit are used to exchange power between the energy storage battery and the DC bus.

[0008] In response to these problems, some scholars have used a new type of quasi-Z source high-gain boost converter, which has a certain effect, but this converter is more suitable for low-power occasions. The interleaved Boost circuit solves the problem of large switching noise and current ripple of the traditional Boost circuit under high current, but the circuit is more complex and the control is more cumbersome. With the development of neural networks and prediction algorithms, many scholars use neural networks combined with prediction algorithms to push the control of Boost circuits to a new level of development. Summary of the invention

[0009] The purpose of the present invention is to solve the shortcomings of the prior art, and a control method for a DC converter for a wind-solar dual-storage microgrid is proposed, which uses dual energy storage batteries to ensure power throughput, and uses power absorption circuit A and power release circuit B to ensure that the bus voltage can remain stable when any or all of the wind, light, and load power fluctuate greatly. In addition, a four-switch switching circuit is used to ensure that the dual energy storage battery switches quickly and stably between the Buck and Boost circuits, that is, the charging and discharging states.

[0010] In order to achieve the above object, the present invention adopts the following technical solutions:

[0011] A control method for a DC converter for a wind-solar-storage dual-storage microgrid comprises the following steps:

[0012] Step 1: The microgrid system includes a power generation system consisting of two output modules, wind power and photovoltaic power generation, and an energy regulation system consisting of a dual energy storage battery system and multiple converters. The wind power output voltage controls the bus voltage through an AC / DC rectifier; the dual energy storage battery system includes a power absorption circuit, a power release circuit and two sets of energy storage batteries; the DC converter A of the power absorption circuit works in Buck mode, and the DC converter B of the power release circuit works in Boost mode; by detecting the fluctuation of the DC bus voltage, the DC converter A and DC converter B are triggered to work in rotation, and both DC converter A and DC converter B work under the dual closed-loop control of external voltage and internal current;

[0013] Step 2: The Boost converter uses sliding mode control to optimize the PI control of the voltage outer loop, stabilize the output voltage and output the precision reference current I ref , use the linear quadratic regulator LQR to replace the current inner loop, output the switch PWM duty cycle required to smooth the bus voltage change, so that the output power of the energy storage battery can achieve the ideal supply and demand compensation effect;

[0014] Step 2.1: Sliding mode control uses voltage deviation and voltage deviation change rate as variables to establish a sliding surface In the formula, c is used to adjust the dynamic performance of the system. The c value can affect the response speed and stability of the system. The ultimate goal of the design is to make the voltage deviation 0 and the voltage deviation change rate 0. The constant speed approaching law is selected: Further optimize the system's adjustment time;

[0015] Step 2.2: Linear Quadratic Regulator LQR for Second-Order Linear System Design a linear feedback controller u = -Kx, where the optimization of the control strategy is based on the cost function This function is used to measure the performance of the system; the control goal is to minimize the cost function J by selecting the values ​​of Q and R under the premise of satisfying system stability. The matrices Q and R are two diagonal matrices, which are used to adjust the weights of the state vector x and the input vector u in the optimization; the state vector x is the state space variable of the Boost circuit, and the control input u represents the duty cycle D of the Boost circuit.

[0016] Step 3: Under the condition of reasonable energy storage capacity configuration, use the ampere-hour integration method to predict and calculate the battery SOC. The basic principle of the ampere-hour integration method is expressed as follows: Among them, SOC0 represents the initial state of charge of the battery, C represents the total capacity of the battery, η is the charge and discharge efficiency, and I is the inductor current. After obtaining the battery SOC, the battery overcharge and overdischarge protection interval is set, and the four-switch charge and discharge switching circuit and dual energy storage battery logic design are performed according to this interval.

[0017] In step 1, the specific method includes the following: According to the permanent magnet synchronous generator power formula P = T m ×ω, it can be seen that the power of the permanent magnet synchronous generator is related to the torque and angular velocity. Torque T m =0.5ρπd 2 v 3 c p / n, speed Reference speed n ref is the speed corresponding to the maximum torque of the fan, and n is controlled according to the optimal tip speed ratio ref =30λv / πd, c p By the relation: Calculated. In the formula, T m represents the torque of the permanent magnet synchronous generator, ρ is the gas density, d is the impeller diameter, v is the wind speed, c pis the wind energy utilization coefficient, n is the fan speed, ω is the fan angular velocity, d is the fan diameter, β is the fan pitch angle, and λ is the optimal tip speed ratio of the fan. In this system, the fan speed outer loop and 0d axis current inner loop double closed loop control are used to control the fan speed and output voltage. The speed difference Δn is determined by the reference speed n ref The actual speed of the permanent magnet synchronous generator output is obtained by subtracting it, and the q-axis current reference value I is obtained after passing through the PI regulator. sq * , and the actual value of the q-axis current I obtained by the coordinate change of the three-phase stator current output by the permanent magnet synchronous generator sq The error of the q-axis current is obtained by difference, and the q-axis voltage is obtained through the PI regulator. Similarly, the d-axis voltage can also be obtained. After obtaining the dq-axis voltage, the coordinates are changed again to transform the dq-axis voltage to the αβ-axis. Finally, the PWM wave is obtained through space vector PWM (SVPWM) modulation, which is used to control the rectifier to output a stable DC voltage.

[0018] In step 2.1, the following method is specifically included: Calculate the voltage error e=U ref -U o , then the sliding surface Inductor current reference value I ref =I c +I l , I c Output from the ammeter, Right now At this point, the sliding mode control ends, the output voltage is stabilized and the current reference value is output to the subsequent LQR regulator to control the current.

[0019] In step 2.2, the specific method includes the following: Since the target of the linear quadratic regulator must be linear, the small signal modeling of the Boost circuit is first performed, and the transfer function of the small signal model of the Boost circuit is written according to the established small signal model: According to the transfer function, the state space equation of the Boost circuit is obtained by the state space averaging method: According to the input and output relationship of the Boost circuit voltage Rewrite the state space equations Regular expression middle, u=1-D. Another key to the linear quadratic regulator is the selection of weight matrices Q and R. Q and R represent the importance of the state vector x and the control input u to the cost function J, respectively. After selecting the appropriate Q and R, the gain matrix K is solved by the Matlab function lqr(A, B, Q, R). The state space equation is output to the observer to obtain the updated value of the state at a certain moment. The observer is played by the Kalman filter, which is mainly divided into three steps: the first is the prediction of the state in is the prior state estimate (prediction value) at time k, where A1 is the transfer state matrix, is the posterior state estimate (updated value) at time k-1, B1 is the control input matrix, u k-1 is the control input vector. Next, the error covariance is predicted, P k∣k-1 =A1P k-1∣k-1 A1 T +q, where P k∣k-1 is the prior error covariance matrix at time k, P k-1∣k-1 is the posterior error covariance matrix at time k-1, and q is the process noise covariance matrix. The last step is the update step, which combines the measured values ​​to correct the predicted state and error covariance. Kalman gain K k =P k∣k-1 H T (HP k∣k-1 H T +R) -1 , where K k is the Kalman gain matrix at time k, H is the measurement matrix, R is the measurement noise covariance matrix, and finally the state is updated to obtain the state at time k. in, is the posterior state estimate at time k, z k is the measured value at time k. After obtaining the state at time k, it is multiplied by the gain matrix K to obtain the control law u = -Kx. That is, u = -Kx = 1-D, then D = 1 + Kx. The output duty cycle D is passed through a PWM generator to generate the corresponding PWM wave to control the power release circuit to output the power to smooth the bus voltage fluctuation;

[0020] In step 3, the following method is specifically adopted: First, determine whether to use a power absorption circuit (Buck circuit) or a power output circuit (Boost circuit) based on the sign of the difference ΔP between the total power output from the energy output end and the power consumed by the load end. When ΔP > 0, it indicates that energy overflows at this time, and the energy storage battery needs to absorb power. The energy flows from the bus to the energy storage battery, and the Buck circuit starts to work. Then, according to the battery SOC obtained by the ampere-hour integration method, to prevent overcharging and over-discharging, the working range of the battery is set to 20 < SOC < 80. Determine whether the SOC of the first battery is greater than 80. If not, the switch S1 is closed, and the first battery is charged; if so, the switch S1 is disconnected, and S3 is closed, and the second battery is charged. When Δp < 0, it indicates that energy is insufficient at this time, and the energy storage battery needs to release power. The energy flows from the energy storage battery to the bus, and the Boost circuit starts to work. Determine whether the SOC of the first battery is less than 20. If not, the switch S4 is closed, and the first battery discharges; if so, the switch S4 is disconnected, and S2 is closed, and the second battery discharges.

[0021] Compared with the prior art, the present invention has the following beneficial effects:

[0022] 1. The present invention enables the permanent magnet synchronous generator to operate at the maximum operating point through the optimal tip speed ratio control, and uses the outer speed and inner 0d-axis current double closed-loop control to make the fan output a stable voltage when the wind speed is constant. Then, the bus voltage is preliminarily stabilized through the Buck circuit; the photovoltaic panel is made to operate at the maximum operating point through the conductance increment method to obtain the maximum operating efficiency of the photovoltaic panel.

[0023] 2. The present invention uses a Boost circuit improved based on the sliding mode control and LQR algorithm for the energy flow between the DC bus and the energy storage battery, enabling the energy storage battery to respond quickly and work when the power at the energy output end and the load power change suddenly, and having a good regulation time and overshoot.

[0024] 3. The present invention uses two energy storage batteries, and one Buck and one Boost circuit are used for the energy exchange between the DC bus and the energy storage battery, effectively avoiding a series of problems caused by a single battery with insufficient capacity and using a bidirectional DC / DC converter.

[0025] 4. The present invention uses a four-switch switching circuit to ensure the rapid switching of the two energy storage batteries between the charging and discharging conditions, and reduces the dead time and overshoot caused by the condition switching. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a flowchart of the present invention

[0027] Figure 2 is a schematic diagram of the hardware structure of the wind-solar dual energy storage battery in the present invention;

[0028] Figure 3 The result diagram of the present invention using the classic dual-loop PI control, the dual closed-loop control based on the LQR improved inner loop, and the improved Boost circuit based on the sliding mode control and the LQR controller;

[0029] Figure 4 The results of the three methods for stabilizing the bus voltage when the system power fluctuates and switches in the present invention are shown;

[0030] Figure 5 It is a schematic block diagram of the optimal tip speed ratio control and the external speed and internal 0d-axis current double closed-loop control of the fan system in the present invention. DETAILED DESCRIPTION

[0031] The technical solutions in the embodiments of the present invention will be described clearly and completely below in conjunction with the accompanying drawings, so that those skilled in the art can better understand the advantages and features of the present invention, thereby making a clearer definition of the protection scope of the present invention. The embodiments described in the present invention are only a part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of the present invention.

[0032] like Figure 1 As shown, a control method for a DC converter for a wind-solar dual-storage microgrid comprises the following steps:

[0033] 1) The power generation system includes two output modules: wind power and photovoltaic power generation. The wind power output voltage controls the bus voltage through the AC / DC rectifier. Photovoltaic uses the conductance increment method to find the maximum power point to ensure that the photovoltaic panel works at the maximum power;

[0034] 1.1) The power generation system uses a permanent magnet synchronous generator as the fan. First, the optimal blade tip speed ratio control is used to obtain the maximum torque, and then the outer speed loop and the inner 0d axis current loop double closed loop control are used to initially stabilize the fan output voltage, and then the AC-DC converter is used to further stabilize the fan output voltage. According to the permanent magnet synchronous generator power formula p = T m ×ω, it can be seen that the power of the permanent magnet synchronous generator is related to the torque and angular velocity. Torque T m =0.5ρπd 2 v 3 c p / n, angular velocity Reference speed n ref is the speed corresponding to the maximum torque of the fan, and n is controlled according to the optimal tip speed ratio ref =30λv / πd, c p By the relation: In this system, the fan speed outer loop and the 0d axis current inner loop are used for double closed-loop control to achieve the purpose of controlling the fan speed and stabilizing the voltage. The speed difference Δn is determined by the reference speed n ref The speed difference Δn is obtained by subtracting the actual speed output by the permanent magnet synchronous generator. After passing through the PI regulator, the q-axis current reference value I is obtained. sq * , and the actual value of the q-axis current I obtained by the coordinate change of the three-phase stator current output by the permanent magnet synchronous generator sq The difference of q-axis current is obtained by difference, and the q-axis voltage is obtained through PI regulator. Similarly, the d-axis voltage can also be obtained. After obtaining the dq-axis voltage, the coordinates are changed again to transform the dq-axis voltage to the αβ-axis. Finally, the PWM wave is obtained through space vector PWM (SVPWM) modulation, which is used to control the rectifier to output a stable DC voltage.

[0036] The meanings of the letters mentioned above are as follows: m represents the torque of the permanent magnet synchronous generator, ρ is the gas density, d is the impeller diameter, v is the wind speed, c p is the wind energy utilization coefficient, n is the fan speed, π is the fan angular velocity, d is the fan diameter, β is the fan pitch angle, and λ is the optimal tip speed ratio of the fan.

[0037] 1.2) Use the conductance increment method to find the maximum power point of the photovoltaic panel. The output voltage and output current of the photovoltaic panel are U pv ,I pv . Take the derivative of both sides of P=UI with respect to U, ( From the power-voltage curve, we can see that when (dP / dU) = 0, it is at the maximum power point, that is, Right now Since the formula contains the denominator dU, consider the case where dU=0. When dU=0, if dI=0, then the system is at the maximum power point. If dI>0, it is on the left side of the maximum power point, otherwise it is on the right side of the maximum power point. When dU≠0, compare and The magnitude of is the negative of the conductivity g. At this moment, it is on the left side of the maximum power point, otherwise it is on the right side of the maximum power point;

[0038] 2) The dual energy storage battery system includes a power absorption circuit and a power release circuit. The DC converter A of the power absorption circuit works in Buck mode, and the DC converter B of the power release circuit works in Boost mode. By detecting the fluctuation of the DC bus voltage, the two DC converters A and B are triggered to work in rotation. Both converters A and B work under the dual closed-loop control of external voltage and internal current. The Boost converter uses sliding mode control to optimize the PI control of the voltage outer loop, so that the output reference current Iref The value is as accurate as possible. The LQR algorithm is used to replace the traditional current inner loop, and the output duty cycle changes with the power demand, so that the output power of the energy storage battery can achieve the best control effect;

[0039] 2.1) The sliding mode control system uses voltage deviation and voltage deviation change rate as variables to establish a sliding surface The ultimate goal is to make the voltage deviation 0 and the voltage deviation change rate 0. In the step, the selection of the sliding mode control reaching law is based on the three common reaching laws: the constant speed reaching law is as follows The exponential convergence law is as follows k>0 and the power reaching law Select the constant speed reaching law with the shortest adjustment time as the reaching law of this sliding mode control. Calculate the voltage error e = U ref -U o , then the sliding surface Inductor current reference value I ref =I c +I l , I c Output from the ammeter, Right now At this point, the sliding mode control ends and the output current reference value I ref To the subsequent LQR regulator, the current is controlled.

[0040] 2.2) Linear Quadratic Regulator (LQR) for Second-Order Linear Systems Design a linear feedback controller u = -Kx. The optimization of the control strategy is based on the cost function This function is used to measure the performance of the system. The control objective is to minimize the cost function J by reasonably selecting the values ​​of Q and R while ensuring system stability. Matrices Q and r are two diagonal matrices, which are used to adjust the weights of the state vector x and the input vector u in the optimization. In the system studied in this paper, the state vector x is the state space variable of the Boost circuit, and the control input u represents the duty cycle D of the Boost circuit. Since the target of the linear quadratic regulator is linear, the Boost circuit is first modeled with small signals, and the transfer function of the small signal model of the Boost circuit is written based on the established small signal model: According to the transfer function, the state space equation of the Boost circuit is obtained by the state space averaging method: According to the input and output relationship of the Boost circuit voltage Rewrite the state space equations point Then the expression middle, u=1-D. Another key to the linear quadratic regulator is the selection of weight matrices Q and R. Q and R represent the importance of the state vector x and the control input u to the cost function J, respectively. After selecting the appropriate Q and R, the gain matrix K is solved by the Matlab function lqr(A, B, Q, R). The state space equation is output to the observer to obtain the updated value of the state at a certain moment. The observer is played by the Kalman filter, which is mainly divided into three steps: the first is the prediction of the state in is the prior state estimate (prediction value) at time k, where A1 is the transfer state matrix, is the posterior state estimate (updated value) at time k-1, B1 is the control input matrix, u k-1 is the control input vector. Next, the error covariance is predicted, P k∣k-1 =A1P k-1∣k-1 A1 T +q, where P k∣k-1 is the prior error covariance matrix at time k, P k-1∣k-1 is the posterior error covariance matrix at time k-1, and q is the process noise covariance matrix. The last step is the update step, which combines the measured values ​​to correct the predicted state and error covariance. Kalman gain K k =P k∣k-1 H T (HP k∣k-1 H T +R) -1 , where K k is the Kalman gain matrix at time k, H is the measurement matrix, R is the measurement noise covariance matrix, and finally the state is updated to obtain the state at time k. in, is the posterior state estimate at time k, z k is the measured value at time k. After obtaining the state at time k, it is multiplied by the gain matrix K to obtain the control law u = -Kx. That is, u = -Kx = 1-D, then D = 1 + Kx. The output duty cycle D is passed through a PWM generator to generate the corresponding PWM wave to control the power release circuit to output the power to smooth the bus voltage fluctuation;

[0041] 3) Under the condition of reasonable energy storage capacity configuration, the battery SOC is predicted and calculated using the ampere-hour integration method. The basic principle of the ampere-hour integration method is expressed as follows: Among them, SOC0 represents the initial state of charge of the battery, C represents the total capacity of the battery, η is the charge and discharge efficiency, and I is the inductor current. When the energy storage capacity is reasonably configured, η = 100%. After obtaining the battery SOC, set the overcharge and over-discharge protection intervals of the battery, and design the four-switch charge and discharge switching circuit and the dual energy storage battery logic according to this interval. First, determine whether to use the power absorption circuit (Buck circuit) or the power output circuit (Boost circuit) according to the positive or negative of the difference ΔP between the total power output from the energy output end and the power consumed by the load end. When ΔP > 0, it means that energy overflows at this time, and the energy storage battery needs to absorb power, and the Buck circuit starts to work. Then, according to the battery SOC obtained by the ampere-hour integration method, to prevent overcharge and over-discharge, the working interval of the battery is set to 20 < SOC < 80. Judge whether the SOC of the first battery is greater than 80. If not, close the switch S1, and the first battery is charged; if so, disconnect the switch S1 and close the switch S3, and the second battery is charged. When ΔP < 0, it means that the energy is insufficient at this time, and the energy storage battery needs to release power, and the Boost circuit starts to work. Judge whether the SOC of the first battery is less than 20. If not, close the switch S4, and the first battery discharges; if so, disconnect the switch S4 and close the switch S2, and the second battery discharges.

[0042] Figure 2 The effect of the Boost circuit in Figure 3 is shown as follows. The present invention is to achieve the stability of the bus voltage by using the energy storage battery when the dual wind-solar energy storage batteries are in state switching or there are large fluctuations in power, and finally achieve the stability of power.

[0043] For Figure 3 analysis, there are mainly three criteria for judging the boosting effect of the Boost circuit: the regulation time t r , the overshoot B, and the bus voltage ripple ΔU. Now, it is desired to output a stable direct current of 800V. Figure 3 The Boost simulation parameters in in are set as the input voltage V p = 300V, L = 0.00026H, C = 0.0002F, R = 20Ω, the switching frequency f = 20kHz, and the outer-loop PI parameters obtained after parameter tuning of the dual-loop PI are k i = 0.6, k p = 30, and the inner-loop PI parameters are k i = 0.6, k Figure 3 It can be seen that although the classic voltage outer-loop current inner-loop dual-loop PI control has a good overshoot, its regulation time reaches t r=0.1s, voltage ripple ΔU=1.25%. Due to the disadvantages of dual-loop control, the regulation effect of the current inner loop is relatively poor. Therefore, the LQR regulator is used to improve the current inner loop, making the final control more accurate. The specific parameters of the LQR regulator are set as follows: Matrix C=[1 0],D=1, According to Matlab's built-in algorithm lqr(A, B, Q, R), the feedback gain matrix K = [3.0967 3.1589] is calculated. Then, according to the real-time state x observed by the Kalman filter, the real-time control input u = -Kx is obtained. The controller has obvious advantages in voltage ripple, and the voltage ripple ΔU is controlled at 0.2%, but there is a large overshoot, and the adjustment time has not improved. Therefore, the sliding mode control is used to optimize the voltage outer loop, so that the entire controller has a great improvement in reducing overshoot, shortening adjustment time and suppressing voltage ripple. The specific performance is t r =0.01s, ΔU=0.2%, B=1.25. Compared with the above two methods, the controller proposed in the present invention has significantly better effect.

[0044] This controller is used in a wind-solar dual energy storage system and simulated in simulink. The basic settings of the wind-solar dual energy storage system are as follows: the wind speed of the wind turbine is 5m / s to cut in and 12m / s to cut out, the voltage fluctuation range without voltage reduction is 850-1500V, and the output power range is 50-300KW. The photovoltaic panel uses a photovoltaic array of 15 columns and 10 rows. Each photovoltaic panel reaches the maximum power point at 25°C and the light intensity is 1000Lx. The maximum output power of a single photovoltaic panel is 600W, the open circuit voltage at the maximum power point is 40V, and the short-circuit current is 15A. It is now desired to stabilize the DC bus voltage at 800V, and when either or both of the wind speed or light intensity changes, the load demand power changes, and the bus voltage can still remain stable. The improved LQR dual closed-loop controller based on sliding mode control proposed in the present invention has a significantly shorter adjustment time when the wind turbine is reduced once. Using the ramp function, the wind speed and light intensity are controlled to decrease slowly, and the resistance of the pure resistive load decreases slowly to simulate the situation of weak wind and dim light at night and the power demand increases. At 1.8s, the power provided by the energy output end is not enough to maintain the power consumed by the load, the bus voltage begins to drop, and the dual energy storage system starts to work, providing power to maintain the bus voltage stable. The classic dual closed-loop PI controller has a short adjustment time, but the adjustment time for the entire system is very long, and the voltage ripple after adjustment is large, ΔU = 1.25%. Using LQR to improve the dual-loop control of the current inner loop has a significant improvement in the bus voltage ripple, ΔU = 0.2%, but at the same time there is a non-negligible overshoot B. o=30. The LQR double closed-loop controller based on sliding mode control proposed in the present invention not only adjusts the time t r =0.095s, and voltage ripple ΔU = 0.2%, overshoot B o =10, indicating that the controller has good sensitivity and robustness.

[0045] In summary, the present invention uses a Boost circuit based on sliding mode control and an LQR algorithm for energy flow between a DC bus and an energy storage battery, so that when the power at the energy output end and the load power suddenly change, the energy storage battery can react quickly and work, and has good robustness and rapidity; dual energy storage batteries, a Buck and a Boost circuit are used for energy exchange between the DC bus and the energy storage battery, effectively avoiding a series of problems caused by the use of a bidirectional DC / DC converter due to insufficient capacity of a single battery, and when dealing with power fluctuations in the power grid, the adjustment time is fast and the overshoot is small; a four-switch switching circuit is used to ensure that the dual energy storage battery switches quickly between charging and discharging conditions, and reduce the dead time and overshoot caused by the switching of conditions.

[0046] The description and practice disclosed in the present invention are easy to think and understand for ordinary technicians in the technical field, and several improvements and modifications can be made without departing from the principles of the present invention. Therefore, modifications or improvements made without departing from the spirit of the present invention should also be regarded as the protection scope of the present invention.

Claims

1. A control method for a DC converter of a wind-solar dual-storage microgrid, characterized in that: The method includes the following steps: Step 1: The microgrid system includes a power generation system composed of two output modules of wind power generation and photovoltaic power generation, and an energy regulation system composed of a dual energy storage battery system and multiple converters. The output voltage of the wind power is used to control the bus voltage through an AC / DC rectifier. The dual energy storage battery system includes a power absorption circuit, a power release circuit, and two groups of energy storage batteries. The DC converter A of the power absorption circuit operates in the Buck mode, and the DC converter B of the power release circuit operates in the Boost mode. By detecting the fluctuation of the DC bus voltage, the DC converter A and the DC converter B are triggered to work alternately, and both the DC converter A and the DC converter B operate under the double closed-loop control of external voltage and internal current. Step 2: The Boost converter uses sliding mode control to optimize the PI control of the voltage outer loop, stabilize the output voltage and output a high-precision reference current I ref , use the linear quadratic regulator LQR to replace the current inner loop, output the switch PWM duty cycle required to smooth the bus voltage change, so that the output power of the energy storage battery can achieve the ideal supply and demand compensation effect; Step 2.1: Sliding mode control uses voltage deviation and voltage deviation change rate as variables to establish a sliding surface In the formula, c is used to adjust the dynamic performance of the system. The c value can affect the response speed and stability of the system. The ultimate goal of the design is to make the voltage deviation 0 and the voltage deviation change rate 0. The constant speed approaching law is selected: Further optimize the system's adjustment time; Step 2.2: Linear Quadratic Regulator LQR for Second-Order Linear System Design a linear feedback controller u = -Kx, where the optimization of the control strategy is based on the cost function This function is used to measure the performance of the system; the control objective is to minimize the cost function J by selecting the values ​​of Q and R under the premise of satisfying system stability; the matrices Q and R are two diagonal matrices, which are used to adjust the weights of the state vector x and the input vector u in the optimization respectively; the state vector x is the state space variable of the Boost circuit, and the control input u represents the duty cycle D of the Boost circuit; Step 3: Under the condition of reasonable energy storage capacity configuration, use the ampere-hour integration method to predict and calculate the battery SOC. The basic principle of the ampere-hour integration method is expressed as follows: Among them, SOC0 represents the initial state of charge of the battery, C represents the total capacity of the battery, η is the charge and discharge efficiency, and I is the inductor current. After obtaining the battery SOC, the battery overcharge and overdischarge protection interval is set, and the four-switch charge and discharge switching circuit and dual energy storage battery logic design are performed according to this interval.

2. A control method for a DC converter for a wind-solar-storage dual-grid according to claim 1, characterized in that: In step 1, the following method is specifically included: According to the permanent magnet synchronous generator power formula P = T m ×ω, it can be seen that the power of the permanent magnet synchronous generator is related to the torque and angular velocity; the torque T m =0.5ρπd 2 v 3 c p / n, angular velocity Reference speed n ref is the speed corresponding to the maximum torque of the fan, and n is controlled according to the optimal tip speed ratio ref =30λv / πd, c p By the relation: Calculated, where T m represents the torque of the permanent magnet synchronous generator, ρ is the gas density, d is the impeller diameter, v is the wind speed, c p is the wind energy utilization coefficient, n is the fan speed, ω is the fan angular velocity, β is the fan pitch angle, and λ is the optimal tip speed ratio of the fan; the fan speed outer loop and 0d axis current inner loop double closed loop control are used to control the fan speed and output voltage; the speed difference Δn is determined by the reference speed n ref The actual speed of the permanent magnet synchronous generator output is obtained by subtracting it, and the q-axis current reference value I is obtained after passing through the PI regulator. sg * , and the actual value of the q-axis current I obtained by the coordinate change of the three-phase stator current output by the permanent magnet synchronous generator sq The error of the q-axis current is obtained by making a difference, and the q-axis voltage is obtained through the PI regulator. Similarly, the d-axis voltage can also be obtained. After obtaining the dq-axis voltage, the coordinates are changed again to transform the dq-axis voltage to the αβ-axis. Finally, the PWM wave is obtained through space vector PWM modulation, which is used to control the rectifier to output a stable DC voltage.

3. The control method of a DC converter for a wind-solar dual-storage microgrid according to claim 1 is characterized in that: In step 2.1, the following method is specifically included: Calculate the voltage error e=U ref -U o , then the sliding surface Inductor current reference value I ref =I c +I l , where I c Output from the ammeter, Right now At this point, the sliding mode control ends, the output voltage is stabilized and the output current reference value I ref To the subsequent LQR regulator, the current is controlled.

4. The control method of a DC converter for a wind-solar-storage dual-grid according to claim 1 is characterized in that: In step 2.2, the following method is specifically included: Since the target of the linear quadratic regulator must be linear, the small signal modeling of the Boost circuit is first performed, and the transfer function of the small signal model of the Boost circuit is written according to the established small signal model: According to the transfer function, the state space equation of the Boost circuit is obtained by the state space averaging method: According to the input and output relationship of the Boost circuit voltage Rewrite the state space equations Then the expression middle, u=1-D; Another key to the linear quadratic regulator is the selection of weight matrices Q and R, which represent the importance of the state vector x and the control input u to the cost function J respectively; After selecting the appropriate Q and R, the gain matrix K is solved by the Matlab function lqr(A, B, Q, R), and the state space equation is output to the observer to obtain the updated value of the state at a certain moment; The observer is played by the Kalman filter, which is divided into three steps: the first is the prediction of the state in is the prior state estimate at time k, where A1 is the transfer state matrix, is the posterior state estimate at time k-1, B1 is the control input matrix, u k-1 is the control input vector; followed by the prediction of the error covariance, P k|k--1 =A1P k-1|k--1 A1 T +q, where P k|k-1 is the prior error covariance matrix at time k, P k-1|k-1 is the posterior error covariance matrix at time k-1, and q is the process noise covariance matrix; Finally, there is the update step, which combines the measured values ​​to correct the predicted state and error covariance; the Kalman gain K k =P k|k-1 H T (HP k|k-1 H T +R) -1 , where K k is the Kalman gain matrix at time k, H is the measurement matrix, R is the measurement noise covariance matrix, and finally the state is updated to obtain the state at time k. in, It is the a posteriori state estimate at time k, zk is the measured value at time k, and after obtaining the state at time k, it is multiplied by the gain matrix K to obtain the control law u=-Kx, that is, u=-Kx=1-D, then D=1+Kx, and the output duty cycle D is passed through a PWM generator to generate the corresponding PWM wave to control the power release circuit to output power to smooth the bus voltage fluctuation.

5. The control method of a DC converter for a wind-solar-storage dual-grid according to claim 1 is characterized in that: In step 3, the following method is specifically adopted: First, according to the positive or negative of the difference ΔP between the total power output from the energy output end and the power consumed by the load end, select whether to use the power absorption circuit or the power output circuit. When ΔP>0, it means that energy overflows at this time, and the energy storage battery needs to absorb power, and the Buck circuit starts to work, and the energy flows from the bus into the energy storage battery. Then, according to the SOC of the battery obtained by the ampere-hour integration method, to prevent overcharging and over-discharging, the working range of the battery is set to 20<SOC<80. Judge whether the SOC of the first battery is greater than 80. If not, the switch S1 is closed, and the first battery is charged; if so, the switch S1 is disconnected, and S3 is closed, and the second battery is charged. When ΔP<0, it means that energy is insufficient at this time, and the energy storage battery needs to release power, and the energy flows from the energy storage battery into the bus, and the Boost circuit starts to work. Judge whether the SOC of the first battery is less than 20. If not, the switch S4 is closed, and the first battery discharges; if so, the switch S4 is disconnected, and S2 is closed, and the second battery discharges.

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