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In a photovoltaic grid-connected system, components, inverters, and grids constitute an electrical system. The inverter will generate as much power as the solar radiation and the modules convert solar energy, so the grid-connected inverter has no special requirements for AC overload, because the output power of the off-grid inverter will basically not exceed the module power. In the photovoltaic off-grid system, components, batteries, inverters, and loads constitute the electrical system. The output power of the inverter is determined by the load. For some inductive loads, such as air conditioners, water pumps, etc., the motor inside, the starting power It is 3-5 times the rated power, so the off-grid inverter has special requirements for overload.
For off-grid inverters using high-frequency isolation technology, the peak power can reach twice the rated power; for off-grid inverters using power frequency isolation technology, the peak power can reach three times the rated power. Then, a 3kW high-frequency off-grid inverter can drive a 1P air conditioner (starting power is about 5.5kVA), and a 12kW power frequency off-grid inverter can drive a 6P air conditioner (starting power power about 33kVA). The inverter provides the load with start-up energy, part of which comes from the battery or photovoltaic modules, and the excess part is also provided by the inverter itself (internal energy storage elements—capacitors and inductors).
Capacitors and inductors are both energy storage components. The difference is that capacitors store electrical energy in the form of an electric field. The larger the capacity of the capacitor, the more electricity it stores. An inductor stores energy in the form of a magnetic field. The greater the permeability of the inductor core, the greater the inductance, and the more energy it can store.
The principle of the capacitor can be seen from its structure. There are two electrodes drawn from a metal plate on both sides, separated by an insulating material in the middle. When no external electric field is applied to both ends of the capacitor, the positive electrodes on the two plates are Negative charges are in a state of equilibrium.
When an external electric field is applied to both ends of the capacitor, positive charges begin to accumulate on one end of the plate, and negative charges accumulate on the other end of the plate. As the voltage across the capacitor continues to rise to the power supply voltage, the charging of the capacitor stops, and it is considered to be disconnected at this time. In an external circuit, the energy on the capacitor will not disappear, because the positive and negative charges have the characteristics of "same sex repels each other, opposite sex attracts each other", and the charges at both ends attract each other to form the function of storing energy.
A power frequency isolation transformer refers to a transformer with a frequency of power frequency (50HZ). The primary and secondary of the transformer have inductance, and the filter inductance in the inverter can store a certain amount of electric energy. When the inductance flows through the current, because the current will have a magnetic field, when the magnetic field of the current passes through the magnetic core, the current magnetic field will break the balance state of the "magnetic domain", so that the "magnetic domain" tends to the direction of the external magnetic field at the same time, resulting in The magnetic core will now exhibit a magnetic field to the outside. And the process of the core magnetic field from scratch is actually the process of the inductor storing the magnetic field.
Inductors are components formed by winding enameled wires on insulating skeletons or magnetic cores. When there is current passing through the coil, a certain magnetic field will be generated around it, and when the passing current contains AC components, the generated magnetic field will continue to change. According to the principle of electromagnetic induction, the changing magnetic force lines will generate induced electromotive force at both ends of the coil , but the direction of this electromotive force is opposite to the direction of the original electromotive force, and thus hinders the change of current.
It can be seen from this that the main function of inductance is to hinder the change of current. When the current increases, it will hinder the increase of the current, and at the same time store a part of the energy through the magnetic field; when the current decreases, it will prevent the decrease of the current in the circuit, and release the stored energy to maintain the current. It is precisely because the inductor has the characteristic of storing energy that it has functions such as filtering and delay.
For photovoltaic off-grid systems, the output power is determined by the load. When an inductive load such as a motor is started, a very large current is required for a short time, and this energy cannot be provided by photovoltaics, nor can the battery. If the lithium battery is overloaded for a short time, it will cause an explosion. However, the capacitors, inductors, and transformers in the off-grid inverter can store energy, and can also amplify the output several times in a short time without damage.
