1) Wind power charging control unit
The function of the wind power generation charging control unit is to control the charging of the battery by the wind power generator to ensure that the battery is not overcharged or overdischarged, so as to ensure the normal use of the battery and the reliable operation of the entire system. At present, the wind power generation control is equipped with an unloader, and its function is to discharge the electric energy generated by the wind turbine when the battery is fully charged.
The system block diagram of the wind turbine charging control module is shown in Figure 1. When the wind speed reaches the starting wind speed, the controller enters the working state. When the wind speed is lower than the rated wind speed, the controller tracks the power change of the wind turbine according to the power control method. At the rated wind speed, the speed limiting unit of the wind turbine limits the rotational speed of the wind turbine to make it close to constant power operation. The synchronizing pulse forming circuit generates the synchronizing pulse, which is added to the external interrupt pin of the microcontroller as a synchronizing signal. According to the charging characteristics of the battery, the conduction angle of the thyristor is controlled to trigger and control the output of the three-phase bridge rectifier circuit. When the battery is fully charged, the drive unloading circuit unloads the wind turbine to prevent the wind turbine from flying. In Figure 1, lun is the output current from the photovoltaic branch to realize the comprehensive charging of the battery.
The conventional control method of first constant current and then constant voltage can be used to charge the battery. Considering the limitation of the battery on the charging current and the switching of the charging control from constant current to constant voltage, the wind turbine charging control unit adopts the constant voltage limiter. The battery is charged and managed by the flow control method. The block diagram of the wind turbine charging control circuit is shown in Figure 2. At the given end of the current control link, there is a limiter link in front of it, which is used to limit the charging current. Due to the use of current closed-loop control, this limit value is the current given value for constant current charging.
During the initial charging, the battery voltage Vb’ is lower than the given voltage Vb* of the battery, so there is Vb-Vb’>0. Due to the function of the integral link in the PI regulator, the output reaches the maximum value, so the current gives The input is set to limit value to realize current-limited charging. When the charging voltage exceeds a given voltage, the charging current exits from the current-limiting state. The voltage of the battery is used as the outer loop of the charging control, and the charging voltage Vb, which is calculated by temperature compensation, is used as a given value. Due to the action of the voltage closed-loop, the voltage of the battery will never exceed this value. When the charging current decreases, the system automatically switches to the constant voltage charging state. As the battery voltage continues to increase, the charging current continues to decrease until it reaches zero. At this time, the battery voltage is equal to the given voltage.
2) Photovoltaic array charging control unit
The block diagram of the photovoltaic array charging control system is shown in Figure 3. It can be seen from the V-I output characteristic curve of the solar cell that the maximum power point is related to the sunlight and the temperature of the solar cell. In order to improve the power generation efficiency of the solar cell, a solar Battery peak power tracker, that is, a CVT-type MPPT tracker. Since the maximum power point of the solar cell is almost distributed on both sides of a vertical line, it can be assumed that the maximum power output point of the solar cell array roughly corresponds to a constant voltage, which greatly simplifies the control design of the system MPPT.
It is only necessary to obtain the maximum data from the manufacturer and to clamp the output voltage of the solar cell array at the maximum value Vmax. In fact, the MPPT control is simplified to the voltage regulation control, which constitutes the CTV-type MPPT control. By changing the pulse width of the switch tube, the current charged by the converter to the battery can be controlled to ensure that the battery has the maximum possible charging current, so as to achieve the purpose of maximum power point tracking. In order to ensure the effective charging of the battery, the feedback of the battery charging voltage and current should be increased in the control loop to realize overvoltage and overcurrent protection, as well as constant voltage charging control. At the same time, in order to adapt to the influence of temperature changes on the solar cell array, the output voltage of the solar cell array can be adjusted according to different temperatures or seasons. The output voltage at the maximum power point of the battery array.
3) Principle and implementation of double-standard three-stage charging
The lead-acid battery is the energy storage element of the wind-solar hybrid LED street lamp. The life of the lead-acid battery is a key factor affecting the life of the wind-solar hybrid LED street lamp. The control of the charge and discharge of the lead-acid battery directly affects the life of the lead-acid battery. Unreasonable charging Discharging will directly cause damage to the lead-acid battery. The intelligently controlled double-standard three-stage charging is used to optimize the charging process of the lead-acid battery. The double-standard three-stage charging process conforms to the characteristics of the lead-acid battery and can maintain the lead-acid battery well. The schematic diagram of the double-standard three-stage charging process is shown in Figure 4.
(1) The first stage (high current charging stage)
The voltage status of the lead-acid battery is obtained by the voltage sampling circuit. When the voltage is less than the standard open circuit voltage (VOC), the solar cell and wind turbine will charge the lead-acid battery with the maximum current that can be provided (the maximum current is used for systems with different powers). The value is different, it can be taken according to the C/5 charging rate, C is the capacity of the lead-acid battery), since the current of the solar cell and the wind turbine is related to the weather conditions, the value of the large current will be within a certain range. After maintaining high current charging to VOC, enter the second stage. Through the first stage, the charge level of the lead-acid battery can reach 70%~90%.
(2) The second stage (overvoltage constant charge stage)
Charge with a constant standard voltage (VC) until the charging current drops to lOC and then enter the third stage. Through the second stage, the lead-acid battery is nearly 100% charged.
(3) In the third stage (floating charge stage), the lead-acid battery is float-charged with a constant and accurate floating voltage Vf. After the lead-acid battery is fully charged, the terminal voltage of the lead-acid battery is maintained by floating charging. The choice of float voltage is particularly important for the life of lead-acid batteries. Even a 5% error will shorten the life of lead-acid batteries by half. The flow chart of the lead-acid battery charging state is shown in Figure 5. The intelligent control unit collects and judges the system state in real time, and controls the lead-acid battery charging state jointly with the input control and trigger signal.