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Analysis on Off-Grid Inverter Overload Capacity

In the solar PV grid system, the solar module, inverter and power grid can constitute the electrical system. The component will convert the solar energy based on the solar radiation and then inverter will output related power. Thus, an on grid inverter does not have special demand on the AC overload because basically the inverter output power will not exceed component power. In the solar PV off-grid system, component, battery, inverter and load constitute the electrical system. The output power of the inverter is decided by the load. The start power of motor of some inductive-load devices like air conditioner or water pump is 3-5 times of the rated power. Thus, the off grid inverter has special requirements on the overload situation.

Off grid inverter in home solar system

The peak power of the off grid inverter that adopts high-frequency insulation technology can be 2 times of the rated power. Peak power of the off grid inverter that adopts power-frequency isolation technology can be 3 times of the rated power. A set of 3kW high frequency off grid inverter can drive a set of 1P air conditioner (The start power is about 5.5kVA). A 12kW power-frequency off grid inverter can drive a set of 6P air conditioner (The start power is about 33kVA). Some energy provided by the inverter to drive the load is from the battery or solar PV module. If it is not enough, the excess part is from the energy storage elements (capacitors and inductors) of the inverter.

Analysis the Off Grid Inverter Overload Capacity from Circuit

Capacitors and inductors are both energy storage components. The difference is that the capacitor store power energy in electric field form. The larger capacitor capacity is, the larger the stored power will be. The inductor stores power energy in magnetic field form. The larger the permeability of the inductor core is, the larger the inductance and the stored power energy will be.

The work principle of capacitor can be discovered from its structure. As shown in the following diagram, the two sides are separately equipped with a mental plate to lead out two electrodes. At the same time, the middle place is isolated by the insulation material. Under the situation that no external electric field is added on the 2 capacitors terminals, the positive and negative charges on the two electrode plates are in balance status.

Capacitor principle of off-grid inverter

As shown in the above photo, it can be found that when external electric field is added on the 2 capacitors terminals, the positive charges are accumulated on one electrode plate and the negative charges are accumulated on the other electrode plate. When the voltage at the 2 capacitors terminals continuously increases and reaches the power voltage, the capacitor will stop charging. Under this situation, the capacitor energy will not disappear even if the external circuit connection is cut off. This is affected by the rule that like electric charges repel and unlike ones attract. The charges at two terminals attract each other to store the energy.

Power-frequency isolation transformer refers to the transformer with 50Hz frequency. Primary and secondary transformers all have inductors which can store certain power energy, just like the filter inductor of the off grid inverter. When the inductor passes through the current, the current has the magnetic field. When the current magnetic field passes by the magnetic core, the current magnetic field will break the balance status of the magnetic domain and drives the magnetic domain to the external magnetic field direction. Thus, the magnetic core will create an external magnetic field. The external magnetic field generation process is actually the process of how the magnetic field is stored by the inductor.

The inductor is a component made of enameled wire and it is added on the insulating framework or magnetic core. When the current passes the coil, the magnetic field will be generated around it. When the passed current contains AC, the generated magnetic field will change frequently. In accordance with the electromagnetic induction principles, the changing magnetic line will generate induction electromotive force at two coil terminals. However, the direction of this induction electromotive force is opposite with the direction of the original induction electromotive force, which can prevent the current change.

Inductor principle of off-grid inverter

It can be concluded that inductor is mainly used to prevent the current change. When the current increases, it will hinder the current increase and will store some energy through the magnetic field. When the current decreases, it will hinder the decrease of the current in the circuit and will release some stored power to maintain the current. Due to its energy storage characteristics, the inductor can realize the filter and delay functions.

Summary

In solar PV off grid system, the output power is determined by the load. When motor or other devices with inductive load are started, the large current will be required within short time. The PV system and battery are not able to provide this energy. In addition, if the lithium battery outputs excessively in short time, it may explore. However, the capacitor, inductor, transformer of the off grid inverter can store power and will not be damaged even if it amplifies the output for several times in a short time. Therefore, off grid inverters can be overloaded several times.

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