Principles and Solutions for Backflow Protection in Photovoltaic Inverters
Generally speaking, the electricity generated by a photovoltaic system is primarily used to power the load. When the amount of electricity generated exceeds the load's consumption, electricity flows in the opposite direction into the grid, known as "reverse power flow." The grid has strict regulations regarding the feeding of photovoltaic power into the grid, and unauthorized reverse power flow may result in penalties. At the same time, for photovoltaic projects that do not feed power into the grid, anti-reverse flow protection is key to achieving self-sufficiency in green energy.
So what is backflow prevention? How does it work, and what are the solutions?
01. What is Backflow Prevention?
In a photovoltaic (PV) system, the direct current (DC) output from the PV modules is converted into alternating current (AC) by the inverter for use by the load. When the power generated by the PV system exceeds the load power, the load cannot consume all of the electricity generated by the PV system, and the excess power flows back into the grid, creating "backflow." A PV system equipped with anti-backfeed functionality can promptly reduce the inverter's output power when the generation capacity exceeds the load capacity. This reduces the system's overall power output, ensuring that the electricity generated by the PV system is used solely by the load and preventing excess power from flowing into the grid.
02. Why is a backflow prevention device required?
Generally speaking, there are several reasons for installing a backflow prevention device:
Grid policy restrictions. In some regions, grid connection is not permitted due to policies or grid capacity limitations; feeding power back into the grid without authorization may result in penalties;
Grid connection power limits. The grid imposes strict limits on grid-connected power; if excess power is fed directly into the grid without control, it may cause a surge;
The principle of self-generation for self-consumption, with surplus power not fed into the grid. Photovoltaic systems must ensure that the electricity generated is prioritized for local consumption. If local loads cannot absorb the excess, a backflow prevention device is required to prevent surplus power from flowing back into the grid.
03. How Backflow Prevention Works
In practical applications, a backflow prevention meter and CT (current transformer) installed on the busbar at the service entrance are used to measure the real-time power, current magnitude, and direction of the line. When current flowing toward the grid (reverse current) is detected, the backflow prevention meter transmits the reverse power data to the inverter via RS485 communication. Upon receiving the command, the inverter responds within seconds by reducing its output power, ensuring that the current flowing from the PV system to the grid remains close to zero. This achieves backflow prevention, preventing excess electricity from being fed into the grid.
04. Anti-Reverse Flow Solutions
(1) Single-Unit, Single-Phase Anti-Reverse Flow System Solution
Required equipment: Grid-tied PV inverter, anti-reverse flow meter, and a communication cable between the meter and the inverter. This solution is suitable only for residential PV applications.
(2) Standalone Three-Phase Anti-Reverse Flow System Solution
For small-capacity residential grid-connected inverters, a DC anti-reverse flow meter can be used directly. The inverter's AC output terminals are connected directly to the meter, and the output from the meter is then connected to the grid connection point to prevent reverse flow; For high-power grid-connected inverters, current on the grid-connected busbar must be detected via current transformers (CTs). After the current is proportionally reduced by the transformers, it is fed into the anti-backfeed meter to enable current and power metering at the grid connection point.
(3) Multi-Inverter Anti-Reverse Flow System Solution
Multiple inverters are connected in series via communication interfaces and linked to a data logger, making this solution ideal for multi-inverter configurations. It offers enhanced functionality and greater capacity.
Anti-reverse flow solutions effectively meet the policy requirements of "grid connection without power export" in certain regions. Furthermore, anti-reverse flow technology not only ensures the stable operation of the power grid and enhances system security, but also optimizes economic performance, improves energy efficiency, and adapts to technological advancements and policy changes.
