Working in solar power generation, we inevitably come into contact with terms and terms related to electricity, and often have a lot of confusion about electricity. The same component, the same installation method, what factors cause the different power generation? Why do fires start with electricity? Why does the isolation transformer consume power when the grid-connected inverter does not work at night?

Many often heard, seemingly simple, and not easy to explain the question, through the study of this chapter will have a clear answer.

1. Composition and function of the circuit

The path through which the current flows is called a circuit. It is composed of four basic parts: power supply, load, switch and connecting conductor. A power supply is a device that converts non-electrical energy into electrical energy and provides external electrical energy. Common power sources include photovoltaic power generation systems, batteries and generators. A load is a general term used in a circuit to convert electrical energy into other forms of energy. Such as the electric lamp to convert electricity into light energy; The soldering iron converts electrical energy into heat energy; An electric motor converts electrical energy into mechanical energy. Switches belong to control appliances, used to control the circuit on or off. The connecting wire connects the power supply and the load, and is responsible for the transmission and distribution of electric energy. The current direction of the circuit is from the positive pole of the power supply through the load to the negative pole of the power supply, inside the power supply, the current flows from the negative pole to the positive pole, forming a closed path. 

 The power supply and load can also be transformed. For example, the power grid is generally the power supply, but in the photovoltaic grid-connected system, the power supply is the module and inverter, and the power grid is the load. The battery is a load when charging and a power source when discharging.

2. Three states of the circuit

There are three states of a circuit: path, open circuit and short circuit. Access means that the circuit is connected everywhere. 

A pathway circuit is also called a closed circuit, or closed circuit for short. Only in the case of the pathway, the circuit has a normal working current, open circuit is somewhere in the circuit is disconnected, there is no path of the circuit, open circuit is also called open circuit, at this time there is no current in the circuit;  Short circuit means that the two ends of the power supply or load are connected together by wires, which are called power short circuit or load short circuit respectively. When the power supply short circuit, the current provided by the power supply is many times larger than the current provided by the path. It is usually harmful and very dangerous, so the short circuit of the power supply is generally not allowed.

In the photovoltaic system, when the inverter is working, the components are in the path state, and the voltage of the components is the working voltage. When the inverter is not working, the component is in the open state, and the voltage of the component is the open circuit voltage, which is generally 19% higher than the working voltage. Due to component is a current source, internal resistance is higher, the component when the short circuit current is not big, about 1.25 times of working current, so the single way of short circuit components, fuse not fuse, the inside of the inverter is the main purpose of fuse when there are multiple access, any link to prevent other groups will flow along the way, you need more than three road with fuse.

3. Ohm's Law

The so-called resistive circuit refers to the external circuit including the power supply. Ohm's law of resistive circuit is that the current intensity flowing through the conductor is proportional to the voltage at both ends of the conductor. Is inversely proportional to the resistance of the Yin conductor.Its mathematical expression is:

I -- current in A conductor, (A);

U -- voltage at both ends of the conductor, (V);

R -- the resistance of the conductor, omega.

1) When the whole circuit is in the path state, the terminal voltage is: U= E-IR. According to the formula, the external circuit voltage decreases with the increase of the current. The greater the internal resistance of the power supply, the more the external circuit voltage decreases. Constant voltage supply is required for DC loads, so it is always desirable to have as little internal resistance as possible.

2) When the whole circuit is in the state of open circuit, it is equivalent to the external circuit resistance value tends to infinity, at this time, the circuit current is zero, the open circuit resistance voltage is zero, the external circuit voltage is equal to the source electromotive force.

3) When the whole circuit is in short circuit state, the resistance value of the external circuit is close to zero, and the circuit current is called short circuit current. Because the internal resistance of the power supply is small, the short circuit current is large. Short circuit when the external circuit voltage is zero, the internal circuit resistance voltage is equal to the source electromotive force.

4. Series and parallel circuits

On a circuit, several resistors or power sources are connected end to end in order to form a circuit without branching, called a series circuit. Two or more resistors or two ends of the power supply are connected to the same two nodes in the circuit. This connection is called a parallel circuit.

The application of resistors in parallel is as extensive as that of resistors in series. Such as:

1) Because the total resistance of the resistance parallel is less than any resistance in the parallel circuit, the method of resistance parallel can be used to obtain the resistance with smaller resistance value.

2) Because the voltage at both ends of each branch of the parallel resistance is equal, therefore, loads with the same working voltage, such as motors and lights, are used in parallel, and the working state of any load is not affected by other loads, nor does it affect other loads. In the parallel circuit, the number of loads increases, the total resistance of the circuit decreases, the current increases, the load from the power source to take more energy, the load becomes heavier; The number of loads decreases, the total resistance of the circuit increases, the current decreases, the load takes less energy from the power supply, and the load becomes lighter. Thus, one can start or stop loads used in parallel as required by the job.

3) In the electrical measurement of the application of resistance parallel shunt method to expand the range of ammeter.