1. There is only one direction of current at the same time
From the circuit principle, the current flows from the place with high voltage to the place with low voltage. At the same time, the direction of the current is the only, that is, the current cannot flow out and in at the same point at the same time. We take the user side meter as the node. At the same time, the current has only one direction, either the photovoltaic current to the grid or the grid current to the load. Therefore, there is no situation that photovoltaic power is sent to the power grid and load takes power from the power grid at the same time.
2. Why can photovoltaic power generation be used preferentially?
Photovoltaic power generation is a kind of power supply, which can output electric energy, and can only output electric energy. Power grid is a special power supply, which can not only provide electric energy to the load, but also receive electric energy as a load. According to the principle that the current flows from the place with high voltage to the place with low voltage, when photovoltaic power generation, from the perspective of load, the voltage of grid connected inverter is always higher than that of power grid, Therefore, the load gives priority to photovoltaic power generation. Only when the photovoltaic power is less than the load power, the voltage of the merging point will drop, and the power grid will supply power to the load. Distributed photovoltaic, for self use and surplus Internet access, generally need to install two meters, one meter out of the inverter to record photovoltaic power generation, and a two-way meter is installed at the user side and the network to record photovoltaic power transmission to the power grid and users’ power purchase from the power grid.
In the grid connected feed system, the mains power is mainly used to construct the voltage / frequency / phase of the power grid. The grid connected inverter does not output voltage, but tracks the phase and waveform of the power grid and only outputs current to the power grid. Because the voltage of the photovoltaic inverter is higher than the power grid voltage, according to the circuit principle, the current flows from the place with high voltage to the place with low voltage. Therefore, as long as the photovoltaic power generation can be generated, it will be sent to the load end first.
From the perspective of load, the load consumes current, which is obtained from the nearest current source. Taking the roof system as an example, the grid connected inverters are fed by the grid connected inverter after the mains transformer, which is first used.
3. Will frequent switching between photovoltaic power generation and commercial power affect the inverter or equipment?
As the light is unstable, the power of photovoltaic power generation is also unstable in large time. Therefore, the power used by the load may sometimes be photovoltaic power, sometimes municipal power, and sometimes photovoltaic power and municipal power may be supplied at the same time. In fact, this is superficial. Theoretically, the electricity used by users is the electricity of the power grid, because the inverter has a function to turn the electricity generated by components into exactly the same electricity as the power grid, with the same voltage, the same frequency and the same phase. This switching process does not actually exist.
4. How can users distinguish the electricity they are using, solar photovoltaic electricity or grid electricity?
From the perspective of power quality, users can’t tell where their electricity comes from. In fact, there is no need to distinguish which kind of electricity is used. As shown in the figure below, the parallel network is installed in the substation, and the parallel network is installed with two-way electricity meters. The current has two directions, which can measure the electricity generated by photovoltaic, how much load is used and how much is sent online. However, for plants 1 to 3, the current has only one direction, so it is impossible to measure how much electricity is used for photovoltaic in which plant and the power grid separately.
