1. There is only one direction of current at the same moment
From the circuit principle, the current is flowing from the place of high voltage to the place of low voltage, at the same moment, the direction of the current is unique, that is to say, the current in the same point can not be both outflow and inflow. We take the customer side of the meter as the node, at the same time, the current has only one direction, either the PV current to the grid, or the grid current to the load. So there is no the same moment PV electricity into the grid, the load from the grid to take power situation.
2、Why PV power can be used in priority?
Photovoltaic power generation is a power source, it can output electricity, and can only output electricity, while the grid is a special power source, it can provide electricity to the load, but also as a load to receive electricity, according to the current are from the high voltage flow to the low voltage place this principle, when photovoltaic power generation, from the load, the voltage of the grid-connected inverter is always a little higher than the voltage of the grid, so the load priority Only when the power of PV is less than the load power, the voltage of the grid-connected network will drop and the grid will supply power to the load. Distributed PV, self-generation, surplus online, generally to install two meters, the inverter out of a meter to record PV power, the user side of the grid connection point to install a two-way meter, used to record the PV to the grid power delivery and users to buy electricity from the grid.
In a grid-connected feeder system, the utility power is mainly used to build 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 PV inverter is higher than the grid voltage, and according to the circuit principle, current flows from a place with high voltage to a place with low voltage, so as long as the PV can generate electricity, it will definitely be sent to the load side first.
From the load's point of view, the load consumes current and gets it from the current source closest to itself. Take the rooftop system as an example, the grid-connected inverters are after the utility transformer, so of course the grid-connected inverter feeds the power and is utilized first.
3、 Will frequent switching between PV power and utility power affect the inverter or equipment?
Due to the unstable light, when big time, the power of photovoltaic power generation is also unstable, so the load with electricity, sometimes may be photovoltaic electricity, sometimes may be utility power, sometimes may be photovoltaic electricity and utility power supply at the same time, in fact, this is the surface. 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 module into the exact same electricity as the grid, with the same voltage, the same frequency, and the same phase. This switching process is actually not stored.
4, how do users distinguish between the electricity being used, solar photovoltaic electricity or grid electricity?
From the point of view of power quality, users can not distinguish where the electricity they are using comes from. In fact, there is no need to distinguish which kind of electricity is being used. As shown in the figure below, the parallel point is installed in the substation room, and the parallel point is installed with a two-way meter, the current has two directions, which can measure the electricity sent from the PV, how much the load uses, and how much is sent online. But plant 1 to 3, the current only one direction, it can not be measured separately which plant PV used how much electricity, how much electricity used by the grid.
