How does the inverter achieve load priority in using photovoltaic power generation?

100 Questions on Distributed Photovoltaic Power Generation (Part 25)

Q: Friends who are new to photovoltaics may have this question: how can we ensure that the electricity generated by photovoltaics is used first by the user's load, rather than the photovoltaic electricity being sent to the grid and the load taking electricity from the grid? Because the price of photovoltaic electricity sold to the grid is cheap, only about 0.4 yuan/kWh, while the price sold by the grid to users is more expensive, generally about 0.6-1.1 yuan/kWh.

A: From the circuit principle, the current flows from the high voltage to the low voltage. At the same time, the current has only one direction, that is, the current cannot flow out and flow 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 where photovoltaic power is sent to the grid and the load draws power from the grid at the same time.

Q: Why is photovoltaic power generation given priority?

Answer: Photovoltaic power generation is a power source that can output electrical energy, and can only output electrical energy, while the power grid is a special power source that can both provide electrical energy to the load and receive electrical energy as a load. Based on the principle that current flows from high voltage to low voltage, when photovoltaic power generation is in progress, from the perspective of the load, the voltage of the grid-connected inverter is always slightly higher than the voltage of the grid. Therefore, the load gives priority to photovoltaic power generation. Only when the photovoltaic power is less than the load power, the voltage at the grid-connected point will drop, and the grid will supply power to the load.

Distributed photovoltaic power generation is for self-use and the surplus power is connected to the grid. Generally, two electricity meters are installed. One meter is installed after the inverter to record the photovoltaic power generation, and a two-way meter is installed at the user-side grid connection point to record the power supply from photovoltaic power generation to the grid and the power purchase from the user to the grid.

When the photovoltaic power is less than the load, it is self-generated and self-used

Because the voltage of the photovoltaic inverter is higher than the grid voltage, according to the circuit principle, the current flows from the high voltage to the low voltage, so as long as the photovoltaic power can generate electricity, it will be sent to the load end first.

Things to note: The photovoltaic grid connection point should be installed on the inside of the user's meter, close to the load end. The power grid is equivalent to an infinite power source, and photovoltaic power generation accounts for a small proportion. Therefore, no matter how photovoltaic power generation changes, the power grid can always remain stable.

The editor has something to say: In 2013, the China Renewable Energy Association organized and compiled "100 Questions and Answers on Distributed Photovoltaic Power Generation in China", which popularized the basic knowledge of photovoltaics to the general public, had a great impact on the photovoltaic industry, and was widely welcomed by everyone. Due to the limitations at the time, the 100 questions compiled in 2013 did not explore the technology in depth enough. The times are progressing and technology is developing. The editor will select some questions and do some in-depth discussions. Welcome to leave a message in the background for communication.