In a photovoltaic system, the flow direction of energy is photovoltaic module-inverter-load-grid, while in a grid system, the flow direction of energy is grid-load, and if it does not match, it is countercurrent. The anti-reverse flow of photovoltaic systems that we often talk about actually includes two aspects: one is that if there is a short circuit or grounding fault in the DC system, it can prevent the current of other components from flowing back and cause accidents; flow to the grid.
一.Anti-backflow at the DC end
The anti-reverse current of the DC terminal is mainly realized by anti-reverse diodes. Using the unidirectional conductivity of the diode, an anti-reverse diode is connected in series with the anode of each string. The main functions are: to prevent the current backflow caused by the reverse connection of the positive and negative poles of the photovoltaic modules and to burn the photovoltaic modules; to prevent the current reverse transmission caused by the voltage difference between the branches of the photovoltaic module array, that is, the circulation; In the event of a fault, it acts as a disconnection point, effectively isolating it from the system, and providing convenience for maintenance while protecting the faulty string. This function is mainly used in photovoltaic systems of centralized inverters. The photovoltaic system of the string Growatt inverter has an MPPT boost circuit at the front end, and the power switch tube has unidirectional conductivity, so it does not need to be equipped with an anti-reverse diode.
- Prevent reverse connection of positive and negative poles of photovoltaic modules
Due to the large amount of wiring work during the construction process, it is inevitable that the positive and negative lines will be confused and reversed when the photovoltaic modules are connected in series to the combiner box. After the anti-reverse diode is installed, in the case of reverse operation of the positive and negative poles, the reversed string is isolated from the system, which plays a good protective role.
- Prevent circulation between strings and improve power generation efficiency
The output voltage of this string is lower than that of other strings due to PV module failure or shadow shading. There is a certain voltage difference due to the difference in the cleanliness, heat dissipation effect and loss of PV modules. The voltage difference will cause the current of the high-voltage branch to flow to the low-voltage branch through the busbar in the combiner box or the busbar at the upper level of the combiner box, thereby generating a circulating current inside the string. When circulating current, the low-voltage string acts as the load of the high-voltage string, and the high-voltage string reduces the voltage and loses power. When the high-voltage string voltage decreases, the output power and power generation efficiency also decrease. The lost electrical energy will be converted into heat to increase the temperature of the photovoltaic module, which not only reduces the power generation efficiency, but also accelerates the hot spot effect. The existence of anti-reverse diodes can isolate each string under any circumstances to prevent mutual interference. Therefore, the circulation between strings and the reduction of power generation efficiency can be effectively avoided.
- Easy maintenance
When a string fails, the anti-reverse diode can isolate the faulty string from the system, which not only protects the faulty string, but also prevents the faulty string from interfering with other normal strings, preventing the expansion of the fault range. Maintenance is carried out without affecting the normal operation of other equipment, reducing the scope of power outages and improving the power generation efficiency of the system. For unattended or few-attended power stations, the fault response time is long, and the string works for a long time with illness, and the anti-reverse diode plays a very good protective role.
二.Anti-backflow at the AC side
The anti-reverse flow of the AC side is mainly realized by the current detection device and the Growatt inverter at the grid connection point. In the power system, the distribution transformer is generally used to transmit power to each load in the power grid, which is called forward current. After the photovoltaic power station is installed, when the power of the photovoltaic system is greater than the power of the local load, the power that cannot be consumed must be sent to the grid. Since the current direction is different from the conventional one, it is called countercurrent. At present, the power departments in some places only allow photovoltaic systems to be integrated into the mains grid, but do not allow the surplus power to be fed to the large grid through distribution transformers. The anti-backflow device is created to solve this kind of problem.
At present, there are two anti-reverse current solutions. One is a single-machine anti-reverse current solution. One inverter is equipped with a two-way digital meter. The Growatt inverter and the meter communicate through the 485 interface. The two-way meter is installed at the grid-connected point. When the inverter detects If the electric meter has current flowing to the grid, the working mode is changed immediately, from the MPPT maximum power tracking working mode to the control output power working mode, and the output power of the inverter is nearly equal to the load power. This method is suitable for the stand-alone mode where the current of the grid connection point is below 100A. The wiring is simple, the cost is low, it is convenient and reliable, and the inverter is required to have a 485 interface. If the photovoltaic installation place has many loads and the current at the grid connection point is greater than 100A, a matching current transformer is required.
If there are more than one inverter, it is recommended to use a multi-machine anti-backflow solution. As shown in the figure below, multiple inverters are connected in series through the 485 interface and connected to the data collector. The current transformer detects the current of each phase, and the signal is transmitted to the electricity meter. , then connect to the data collector, and then connect to the Ethernet through the router. Through remote operation, set the server address and anti-backflow parameters. This method is suitable for multi-machine mode. The wiring and debugging are more complicated, but the function is more powerful and the capacity is larger. It requires the Growatt inverter to have 485 interface and the installation site to have Ethernet.
三.Frequently Asked Questions
After installing the anti-backflow system, common problems:
- The inverter reports 111 fault
The reason for this fault is that the Growatt inverter is set with anti-reverse current power, but the communication line between the inverter and the anti-reverse current device is not connected. As long as the connection is made, the fault will be eliminated.
- Growatt inverter output power becomes smaller
This is the most common mistake made by first-time installers. The reasons are as follows: First, the current detection device is in the wrong position. The anti-reverse current system requires the photovoltaic system to enter the customer’s load first, and then enter the anti-reverse current meter or current transformer. This requires an anti-reverse current meter or The current transformer should be as close as possible to the power meter on the grid side. If the power of the Growatt inverter enters the anti-reverse current meter first, and then enters the load, the output power of the inverter will become smaller. Second, the load has not been fully started.