1. The current has only one direction 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 unique, that is to say, the current cannot both flow out and flow in at the same point. We take the user-side meter as a node. At the same time, the current has only one direction, either the photovoltaic current goes to the grid, or the grid current goes to the load. Therefore, there is no situation in which photovoltaic power is fed into the grid at the same time, and the load takes power from the grid.
2. Why should photovoltaic power generation be used first?
Photovoltaic power generation is a kind of power source, it can output electric energy, and can only output electric energy, and the grid is a special kind of power source, it can not only provide electric energy to the load, but also can receive electric energy as the load, according to the current, it is from the high voltage. The principle that places flow to places with low voltage, when photovoltaic power generation, from the load point of view, the voltage of the grid-connected inverter is always a little higher than the voltage of the grid, so the load preferentially uses photovoltaic power generation, only when the photovoltaic power is smaller than the load. After the power is supplied, the voltage at the grid connection point will drop, and the grid will supply power to the load. Distributed photovoltaics, self-generated and self-used, and the remaining amount is connected to the Internet. Generally, two meters are installed, one meter is installed out of the inverter to record photovoltaic power generation, and a two-way meter is installed at the grid-connected point on the user side to record the transmission of photovoltaic power to the grid and the user’s transmission to the grid. Buy electricity from the grid.
In the grid-connected feeder system, the commercial power is mainly used to construct the voltage/frequency/phase of the grid. The grid-connected inverter does not output voltage, but tracks the phase and waveform of the grid, and only outputs current to the grid, because the voltage of the photovoltaic inverter is higher than the grid voltage. According to the circuit principle, the current flows from the place where the voltage is high to the place where the voltage is low. Therefore, as long as the photovoltaic can generate electricity, it will be sent to the load first.
From the perspective of the load, the load consumes current and obtains the current from the current source closest to itself. Taking the rooftop system as an example, the grid-connected inverters are all after the mains transformer, of course, the grid-connected inverters feed power and are used first.
3. Frequent switching between photovoltaic power generation and commercial power will not affect the inverter or equipment?
Due to the unstable illumination, the power of photovoltaic power generation is also unstable from time to time, so the electricity used for the load may sometimes be photovoltaic power, sometimes it may be commercial power, and sometimes it may be both photovoltaic power and commercial power supply. In fact, this is the surface Up. Theoretically, the electricity used by the user is the electricity from the grid, because the inverter has a function that can turn the electricity from the components into the same electricity as the grid, with the same voltage, the same frequency, and the same phase. This switching process does not actually exist.
4. How does the user distinguish the electricity being used, whether it is solar photovoltaic electricity or grid electricity?
From the point of view of power quality, users cannot tell where the electricity they use comes from. In fact, there is no need to distinguish which kind of electricity is used. As shown in the figure below, the grid connection point is installed in the substation, and the grid connection point is installed with a two-way meter. The current has two directions, which can measure the electricity generated by the photovoltaic, how much the load uses, and how much is sent to the Internet. However, in plants 1 to 3, the current has only one direction, so it is impossible to measure how much electricity is used by photovoltaics in which plant and how much electricity is used by the grid.