Controllers, converters, loads in solar photovoltaic systems
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The main function of the controller in the photovoltaic power generation system is to control the charging/discharging of the battery according to the characteristics of the battery, so as to extend the service life of the battery. In various types of photovoltaic power generation systems, the controllers used are different, and their functions and complexity will also vary greatly, which need to be determined according to the requirements and importance of the power generation system. In the independent photovoltaic power generation system, because the investment of batteries (mainly lead-acid batteries) occupies a large proportion in the system, excessive charging and excessive discharging will greatly shorten the life of the battery, so the main function of the controller is to ensure The system can work normally and reliably, prolonging the service life of system components (especially the battery).
The controller should meet the following basic requirements.
① Directly or indirectly (through the inverter) from the solar cell components to supply power to the electrical appliances, and at the same time determine the best charging method, and store the surplus electrical energy in the storage battery to prepare for use when the solar power is insufficient (such as at night) Electrical power supply.
②In order to prevent the battery from overcharging and protect the cyclic charging/discharging performance of the battery, when the battery is overcharged, the charging circuit can be cut off in time, and the battery can be automatically restored to charge according to the preset protection mode.
③Provide the battery’s power supply path to various household appliances, and perform battery discharge management. In order to ensure the normal service life of the battery, when the battery is over-discharged, the discharge circuit can be cut off in time; when the battery is recharged, the power supply can be automatically restored.
④ When the electrical appliance fails or short circuit, it can automatically protect the safety of the controller and the system.
⑤It can withstand the breakdown protection, anti-reverse charging function and various operating status indication functions caused by lightning strikes in the multi-mine area.
The converter can be divided into a DC-AC (DC/AC) converter and a DC-DC (DC/DC) converter. Its function is to convert the DC power output by the solar cell and the storage battery into AC or DC power that matches the electrical appliances. The converter should meet the following basic requirements
①A stable AC/DC output voltage is required within the allowable fluctuation range of the specified input DC voltage and the rated load variation range. For example, when operating in a steady state, the voltage fluctuation does not exceed ±3% of the rated value; in dynamic conditions The voltage deviation does not exceed ±8% of the rated value
②In the case of sinusoidal inverter output, the total waveform distortion value of the output voltage should not exceed 5%. The rated output frequency should be a stable value, usually 50Hz, and the deviation should be within ±1% under normal working conditions.
③It has a certain overload capacity, and can generally withstand 150% overload.
④It has protection and display alarm functions such as overvoltage, overcurrent, and overheating.
⑤It has high conversion efficiency, and the full load efficiency is generally above 85%.
The output characteristics of the solar cell make the photovoltaic power generation system have certain requirements on the load. The start and stop of a load with a larger capacity will have a greater impact on the output of the photovoltaic power generation system, and in severe cases, it will cause the inductive load to fail to start normally. ; Under a certain light condition, the maximum power point of the solar cell output cannot be accurately matched with different loads, and the load needs to be adjusted to achieve the maximum photovoltaic output power.
Loads can be divided into three types: resistive, inductive and capacitive, and their impact on the photovoltaic system is also different. The resistive load has almost no impact on the system when it is started and disconnected; inductive loads and loads with large filter capacitors will produce “surge current” much larger than the rated current when they are disconnected. The freewheeling effect will produce induced overvoltages at both ends of the switch that are much higher than the operating voltage of the load. This type of load often threatens the voltage safety and current safety of the power electronic devices sensitive to overvoltage in the photovoltaic system converter.
The following principles should generally be followed for connecting loads in photovoltaic power generation systems.
① Due to the large investment in photovoltaic power generation systems, the load connected to the system must be restricted. High-power electrical appliances and high-energy-consuming loads must not be used, and energy-saving loads should be used as much as possible.
② Arrange the load power time scientifically and reasonably. High-power electrical appliances (such as water pumps, etc.) should be arranged as far as possible to be used at noon when the photovoltaic power generation capacity is strongest, and staggered as much as possible. In continuous rainy weather, use less electrical appliances as much as possible, reduce battery power consumption, and improve battery life and overall system efficiency.
The load characteristics of photovoltaic systems are divided into steady-state characteristics and dynamic characteristics. Steady-state characteristics include volt-ampere characteristics and power characteristics under normal operating conditions of the load; dynamic characteristics include start-up characteristics and stop characteristics, as well as the non-linear functional relationship between voltage and current during sudden changes in voltage and current.