Anti-backflow equipment in photovoltaic systems

In the photovoltaic system, the energy flows from photovoltaic components to inverters, loads and grids, while in the grid system, the energy flows from grids to loads. If this is not the case, it is called reverse flow. The reverse flow prevention of photovoltaic systems, which we often talk about, actually includes two aspects: one is to prevent the current of other components from flowing back and causing accidents if there is a short circuit or grounding fault in the DC system; the other is that some places only allow photovoltaic power generation for self-use to prevent the current from flowing into the grid.

1. Anti-reverse flow at the DC end

The reverse current protection at the DC end is mainly achieved by using a reverse current protection diode. Using the unidirectional conductivity of the diode, an anti-reverse current protection diode is connected in series to the positive pole of each string. The main functions are: to prevent the photovoltaic components from being burned due to the reverse connection of the positive and negative poles of the photovoltaic components; to prevent the voltage difference between the branches of the photovoltaic component array from causing the current to be reversed, that is, the circulation; when the string fails, it acts as a disconnection point and is effectively isolated from the system, which provides convenience for maintenance while protecting the faulty string. This function is mainly used in the photovoltaic system of the centralized inverter. The photovoltaic system of the string inverter has an MPPT boost circuit at the front end, and the power switch tube has unidirectional conductivity, so there is no need to equip it with a reverse current protection diode.

01. Prevent the positive and negative poles of photovoltaic modules from being reversed

Due to the large amount of wiring work during the construction process, it is inevitable that the positive and negative lines of the photovoltaic modules are confused and connected in reverse when they are connected in series to the combiner box. After installing the anti-reverse diode, in the case of reverse connection of the positive and negative poles, the reversed strings are isolated from the system, which plays a good protective role.

02. Prevent circulation between strings and improve power generation efficiency

The failure or shadow of the photovoltaic module makes the output voltage of the string lower than that of other strings. There is a certain voltage difference due to the different cleanliness, heat dissipation effect and loss degree of the photovoltaic module. 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 above the combiner box, thereby generating a circulation inside the string. During the circulation, the low-voltage string acts as the load of the high-voltage string, and the high-voltage string not only reduces the voltage but also loses electric energy. When the voltage of the high-voltage string is reduced, the output power and power generation efficiency are also reduced. The lost electric energy will be converted into heat energy to increase the temperature of the photovoltaic module. The increase in the temperature of the photovoltaic module will not only reduce the power generation efficiency, but also accelerate the hot spot effect. The existence of anti-reverse diodes can isolate each string under any circumstances to prevent mutual interference. Therefore, the situation of circulating current and reduced power generation efficiency between strings can be effectively avoided.

03.Easy maintenance

When a string fails, the anti-reverse diode can isolate the faulty string from the system, not only protecting the faulty string, but also preventing the faulty string from interfering with other normal strings and preventing the scope of the fault from expanding. Maintenance can be 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 unmanned or undermanned power stations, the fault response time is long and the string works for a long time with faults. The anti-reverse diode plays a good protective role.

2. Anti-backflow at the AC end

The reverse current protection at the AC end is mainly achieved by the current detection device and inverter at the grid connection point. In the power system, the distribution transformer generally supplies 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 unabsorbed power must be sent to the power grid. Since the current direction is different from the conventional one, it is called reverse current. At present, some local power departments only allow photovoltaic systems to be connected to the mains power grid, but do not allow the remaining power to be fed to the large power grid through the distribution transformer. The reverse current protection device is created to solve this kind of problem.

There are currently two anti-backflow solutions. One is a single-machine anti-backflow solution. One inverter is equipped with a two-way digital meter. The inverter and the meter communicate through the 485 interface. The two-way meter is installed at the grid connection point. When the inverter detects that the meter has current flowing to the grid, it immediately changes the working mode from the MPPT maximum power tracking working mode to the output power control working mode. The output power of the inverter is nearly equal to the load power. This method is suitable for single-machine mode with a grid connection point current below 100A. It has simple wiring, low cost, convenient and reliable, and requires the inverter to have a 485 interface. If the photovoltaic installation place has a lot of loads and the grid connection point current 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 meter, which is then connected to the data collector and then connected to the Ethernet through the router. The server address and anti-backflow parameters are set through remote operation. This method is suitable for multi-machine mode, and the wiring and debugging are more complicated, but it is more powerful and has a larger capacity. It requires the inverter to have a 485 interface and Ethernet at the installation site.

3. Frequently Asked Questions

After installing the anti-backflow system, common problems:

01. The inverter reports a 111 fault

The cause of this fault is that the inverter is set to prevent reverse flow, but the communication line between the inverter and the reverse flow device is not connected. As long as the connection is made, the fault will be eliminated.

02. The inverter output power becomes smaller

This is the most common mistake made by beginners. The reasons are: First, the current detection device is not in the right position. The anti-reverse flow system requires the photovoltaic system to enter the customer's load first, and then enter the anti-reverse flow meter or current transformer. This requires the anti-reverse flow meter or current transformer to be as close to the meter on the grid side as possible. If the power of the inverter enters the anti-reverse flow meter first and then enters the load, the inverter output power will be reduced. Second, the load has not been fully started.