How to reduce ground resistance for solar power system

In the photovoltaic power station system, the grounding design is a crucial part of the electrical design, which is related to the safety of the power station equipment and personnel. Good grounding design can ensure that the power station is in a safe operating environment for a long time, reduce the frequency of faults, and improve the overall operating efficiency of the power station.

Therefore, during the construction of the power station, it is necessary to ensure that the grounding resistance meets the standard specifications. So what are the common methods to reduce grounding resistance? First, let's take a look at the type of grounding and the corresponding grounding resistance requirements:

1. Lightning protection grounding. Including lightning rod (belt), down-conductor, grounding body, etc., the grounding resistance is required to be less than 10 ohms, and it is best to consider setting the grounding body separately.

2. Safety protection grounding, working grounding, shielding grounding, etc. It is required that the grounding resistance should not be greater than 4 ohms. When the safety protection grounding, working grounding, shielding grounding and lightning protection grounding share a set of grounding devices, the grounding resistance shall be determined according to the minimum value of 4 ohms; if the lightning protection grounding has been set separately, the other three groundings should be shared For a group of grounding devices, the grounding resistance should not be greater than the minimum value.

Knowing the requirements of standard grounding resistance, let's talk about six methods to reduce grounding resistance during the construction of photovoltaic power plants.

1. Soil replacement method
In sandy, rocky and other soils with high soil resistivity, in order to meet the requirements of low grounding resistance, a grounding grid composed of multiple grounding bodies in parallel is often used. However, large-scale photovoltaic power plants require a lot of steel materials and a large grounding area, so it is often difficult to achieve the specified grounding resistance. At this time, you can try to reduce the resistivity of the soil near the grounding body, so as to achieve the purpose of reducing the grounding resistance.

2. Add drag reducing agent
The resistance reducing agent is composed of various components, including fine graphite, bentonite, curing agent, lubricant, conductive cement, etc. It is generally gray-black and is a good electrical conductor. The resistance reducing agent is used between the grounding body and the soil. On the one hand, the resistance reducing agent can be in close contact with the metal grounding body to form a large enough current flow surface; on the other hand, it can penetrate into the surrounding soil and reduce the soil resistivity. Create a gently varying low resistance area around the ground plane.
The resistance reducing agent has good conductivity and strong electrolyte, and it is not easy to be lost with groundwater and rainwater, so it can maintain a good electrical conductivity for a long time.

3. External grounding method
This is a grounding method that reduces the power frequency grounding impedance of the grounding device and uses a grounding electrode to connect the grounding device to a remote natural grounding electrode (body) or an artificially laid auxiliary grounding electrode. In some hilly power stations, when the grounding resistance value is small and difficult to achieve in situ, the simplest solution is to expand the area of the grounding grid or find a place with lower resistivity near the built grounding grid. Build a new ground grid, and then connect the two ground grids to reduce the grounding resistance of the ground grid.
Practice has proved that this is a very effective method, but it should be noted that the external grounding device should avoid pedestrian passages to prevent electric shock from step voltage; if crossing the road, the buried depth of the external lead should be greater than or equal to 0.8 meters.

4. Use conductive concrete
When carbon fiber is mixed into cement, its power frequency grounding resistance (compared with ordinary concrete) can usually be reduced by about 30%. This method is often used in lightning protection grounding devices. In order to further reduce the impact grounding resistance value, a needle-shaped grounding electrode can also be embedded in the conductive concrete at the same time, so that the discharge corona can continuously spread from the needle tip to the carbon fiber, reducing the impact grounding resistance value.

5. Add salt
Add salt, cinder, charcoal powder, furnace ash, coke ash, etc. to the soil around the grounding body to improve the conductivity of the soil. Because the salt has a better effect on improving the soil resistivity, it is less affected by seasonal changes and the cost is low. , so adding table salt is the most common method. This method can reduce the grounding resistance to the original (1/6~1/8) for sandy soil, and to about 2/5~1/3 for sandy clay. Because charcoal is a solid conductor, it will not be dissolved, penetrated and corroded. Therefore, adding a certain amount of charcoal at the same time will have a longer effective time and better effect. For parallel grounded bodies such as flat steel and round steel, the above method can also obtain better results.
However, this method also has disadvantages: for example, it has little effect on rocks and soils containing more rocks, reduces the stability of the grounding body, accelerates the corrosion of the grounding body, and gradually increases the grounding resistance due to the gradual melting and loss of salt, so the two It needs to be reprocessed every year or so.

6. Drilling deep burial method
This method is suitable for situations such as crowded buildings or narrow areas where grounding grids are laid. In these occasions, it is difficult to find a suitable location for burying the ground electrode using traditional methods, and the safety distance cannot be guaranteed. The length of the vertical grounding body used in this method is generally 4 to 6 meters depending on the geological conditions. If the length is exceeded, the effect is not obvious and the construction is difficult. The grounding body is usually made of round steel of Φ20~75 mm, and the round steel of different diameters has little effect on the grounding resistance value. Because most of the sand-bearing layers are in the surface layer within 3 meters, and the soil resistivity in the deep stratum is low, the deep burial method is most effective for sand-bearing soils. In addition, the method also applies to stony rock areas.
The grounding body constructed by the deep burial method is less affected by the season and can obtain a stable grounding resistance value. At the same time, deep burial can also reduce the step voltage and ensure personal safety.

No matter what kind of photovoltaic power station, grounding is of great significance. According to the standard grounding design specification, the protective grounding resistance value of the electrical equipment in the photovoltaic power station field area is required to be no more than 4 ohms. In some installation scenarios with complex geological conditions, the above methods can meet the requirements of grounding standards and protect the personal safety of electrical equipment, buildings and staff of photovoltaic power plants.