In photovoltaic power station system, grounding design is a crucial part of electrical design, related to the safety of plant equipment and personnel. Excellent grounding design can ensure a safe operating environment for the power station for a lengthy time, reduce the frequency of failures, and improve the overall operation efficiency of the power station.
Therefore, in the process of power plant construction, it is necessary to ensure that the grounding resistance meets the standard specifications. Then what are some common ways to reduce ground resistance? First of all, let's look at the type of grounding and the corresponding grounding resistance requirements:
First, lightning protection and grounding. Including lightning rod (belt), lead down, grounding body, etc., the ground resistance is required to be less than 10 ohms, and it is better to consider setting a separate grounding body. There's an 8 awg pv wire.
Two, safety protection grounding, working grounding, shielding grounding, etc. The ground resistance must be no greater than 4 ohms. When safety 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 a separate grounding device is configured for lightning protection, the remaining three types of grounding devices should share the same grounding device, and the grounding resistance should not be greater than the minimum value.
Knowing the requirements of standard grounding resistance, let's talk about six ways to reduce grounding resistance in the construction process of photovoltaic power stations.
1. Land shift method
To meet the requirements of low grounding resistance, a grounding network composed of multiple grounding bodies in parallel is commonly used in sandy soil and rock surface soil with large land resistivity. However, large photovoltaic power stations require a lot of steel materials, and the ground area is large, so it is usually difficult to reach the specified ground resistance. At this time, we can try to reduce the resistivity of soil near the grounding body, so as to achieve the purpose of reducing the grounding resistance.
2. Add resistance reducing agent
The resistance reducer is composed of a variety of components, including fine graphite, bentonite, curing agent, lubricant, conductive cement, etc., typically gray black, is a excellent electrical conductor. When the drag reducer is used between the grounding body and the soil, on the one hand, it can make close contact between the drag reducer and the metal grounding body to form a large sufficient current flow surface; on the other hand, it can penetrate into the surrounding soil and reduce the soil resistivity, forming a gently changing low resistance area around the grounding body.
The resistance reducing agent has excellent conductivity and strong electrolyte, and is not easy to be lost with groundwater and rainwater, so it can maintain excellent conductivity for a long time. When it is used for limited area of centralized grounding and tiny ground grid, the resistance reducing effect is the most significant.
3. External grounding method
This is a grounding method to reduce the power frequency grounding impedance of the grounding device and connect the grounding device with a distant natural grounding pole (body) or an artificial auxiliary grounding pole. In some hill power stations, when the requirement of ground resistance value is tiny and it is difficult to reach in situ, the simplest solution is to expand the area of the ground network or find a place near the built ground network with low resistivity to build a fresh ground network, and then connect the two ground networks to reduce the ground resistance of the ground network.
Practice has proved that this is a highly effective method, but it should be noted that the external grounding device should avoid the pedestrian passage, to prevent stepping voltage shock; if crossing the highway, the buried depth of the outer 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 be measured by about 30%. This method is commonly used in lightning protection grounding devices. In order to additionally reduce the impact grounding resistance value, a needle grounding pole can 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, and reduce the impact grounding resistance value.
5. Add salt method
Adding salt, cinder, charcoal, furnace ash and coke ash to the soil around the grounding body is the most commonly used method to improve the electrical conductivity of the soil, because salt has a favorable effect on improving the soil resistivity, is subject to small seasonal shift, and the cost is low. This method can reduce the grounding resistance to the original (1/6~1/8) for sandy soil, and to the original (2/5~1/3) for sandy clay. Because charcoal is a solid conductive body, will not be dissolved, penetration and corrosion, therefore, the synchronous addition of a certain amount of charcoal, the effective time is longer, the effect is better. For flat steel, round steel and other parallel grounding bodies, the above method can also get better results.
However, this method also has disadvantages: for example, it has small effect on the rock and the soil with additional stone, reduces the stability of the grounding body, accelerates the corrosion of the grounding body, and makes the grounding resistance gradually become larger because of the gradual melting and loss of salt, so it needs to be processed again about two years.
6. Deep drilling method
This method is suitable for crowded buildings or narrow ground grid laying areas and other occasions. In these cases, it is difficult to find the proper location of the ground pole by the traditional method, and the safe distance cannot be guaranteed. The length of the vertical grounding body used in this method is typically 4~6 meters, depending on the geological conditions, beyond which the effect is not obvious and the construction is difficult. The grounding body is normally round steel with a diameter of 20 to 75 mm. Round steel with different diameters has slight influence on the grounding resistance value. Because the sand-bearing layer is mostly located within 3 meters of the surface layer, and the soil resistivity is low in the depth of the stratum, the deep burial method is the most effective for the sand-bearing soil. In addition, the method can also be applied to rocky plate areas.
The grounding body constructed by deep burying method is fewer affected by seasons 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 extremely vital. According to the standard grounding design specification, the protection grounding resistance value of the electrical equipment in the photovoltaic power station field is required to be no more than 4 ohms. In installation scenarios with complex geological conditions, you can use the preceding methods to meet the grounding standards and protect the electrical equipment, buildings, and personnel of the photovoltaic power station.