1. What is the "counterflow" of the power system?
In the power system, the current flowing from the distribution transformer to the user's load is called the forward current. In a distributed photovoltaic system, when the photovoltaic power generation power is greater than the local load power, the excess power will flow into the grid, and this current direction is from the load side to the grid side, which is the opposite of the forward current, which is called counterflow. In the user's energy storage system, when the output of the energy storage converter is greater than the local load power, there will also be a situation where the power that cannot be absorbed will flow to the power grid, which is also a reverse flow.
2. Why should we prevent backflow?
As we all know, the problem of energy shortage and environmental pollution is becoming more and more severe, with the application of photovoltaic power generation and energy storage technology is becoming more and more extensive, the power security problems are becoming more and more prominent, especially the problem of "reverse current", because once there is a reverse current, there may be a series of serious problems such as power quality reduction, grid voltage fluctuations, and equipment damage caused by reverse charging.
In Chapter 8.5 of the GB/T 29319-2012 Technical Regulations for the Access of Photovoltaic Power Generation Systems to the Distribution Network, it is required that "when the photovoltaic power generation system is designed to be irreversibly connected to the grid, reverse power protection equipment shall be configured, and when the reverse current is detected to exceed 5% of the rated output, the photovoltaic power generation system shall automatically reduce the output or stop sending power to the grid line within 2s." ”
Chapter 5.3 of the T/CET 401-2022 Technical Specification for the Integration of Battery Energy Storage Systems: User-side Energy Storage requires: "The user's energy storage system shall be equipped with anti-islanding and reverse power protection devices, and the anti-islanding protection shall be matched with the protection of the power grid side, and the equipped reverse power protection equipment shall stop transmitting power to the grid line within 0.5s-2s when the reverse current is detected to exceed 5% of the rated output."
3. How to prevent backflow?
In order to reduce the harm caused by the reverse current in the new energy power generation, CET has actively responded to the national requirements and independently developed the PMC-350-A5 anti-backcurrent meter, which has the function of anti-backcurrent protection equipment, which can quickly detect the current backflow, so as to feed back to the energy management system combined with the gateway to adjust the inverter, reduce the inverter power within 2s required by the overall system, and realize the reverse power protection function.
In the photovoltaic power generation system, it is necessary to prevent the current output from the inverter from flowing to the power grid in view of the mode of spontaneous self-consumption surplus electricity is not connected to the grid; When the reverse current is detected to exceed 5% of the rated output, the anti-backcurrent meter detects the change of power within 100ms, and then the output of the inverter is adjusted by the gateway.
In the user's energy storage system, the charging pile is equipped with an energy storage cabinet, charging the energy storage cabinet when the electricity price is low, and then the energy storage cabinet supplies power to the charging pile when the electricity price is high, and uses the peak-to-valley electricity price difference to make a profit; When the energy storage cabinet supplies power to the charging pile, when it detects that there is a current flowing to the power grid, and the reverse current exceeds 5% of the rated current, the anti-backcurrent meter detects the change in the power direction, and the EMS energy management system adjusts the output of the converter in the energy storage cabinet to realize the anti-backflow function.
The PMC-350-A5 anti-backcurrent meter with fast computing power launched by CET has been applied to user-side energy storage systems and photovoltaic power generation systems, which can quickly detect changes in current and power within 100ms, communicate with the inverter/EMS energy management system, and realize the anti-backflow function.
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