Oneintroduction

The purpose of grounding is to ensure the safety of personnel and equipment, as well as the normal operation of equipment. According to the power station site geological investigation report .The geological condition of mountain power station is relatively poor, soil resistivity value (recommended for soil resistivity report), area for ground net , in accordance with the calculation of soil resistivity, to achieve the most safe ground resistance value.

TwoReference standard 

1.1  GB50169-2006 Specification for construction acceptance of grounding device              China National Standard

1.2  GB/T50065-2011 Code for grounding design of AC electrical equipment

China National Standard

1.3  DL/T621-1997 Grounding for AC electrical apparatus

China electric power industry standard

1.4  DL/T5161、6-2002 Construction quality inspection of grounding device

China electric power industry standard

1.5  ANSI/IEEE Std80-2000 Safety guidelines for AC substations

American national standard

ThreeCharacteristics of Flat Ground Power Station Project

With the increasing scale of solar PV power station, most of the PV power station choose  unused wasteland etc. , mainly because in the rock layer, resistance reduction is difficult , construction is also difficult

The grounding system of PV power station has a direct impact on the safety of personal and equipment. To choose a proper grounding method, a safe protection method and the grounding device is the key to ensure the long-term safe operation of the power system of the PV power station. 

In order to ensure the safe, stable and reliable operation of the PV power station, we must focus on solving the following five aspects.

①Grounding resistance problem

Because it is directly related to the power frequency grounding short circuit 

and the ground potential rise of lightning current.

② Pressure equalization problem 

Especially the partial discharge of grounding grid easily cause a counterattack

 to the cables in cable trench.

③ Equipment grounding problem 

Particularly serious is some lightning protection equipment, such as lightning protection line, grounding is not good, will produce very high residual pressure and counter pressure

④ Thermal stability of ground wire 

If the thermal stability of the ground wire is not up to the requirements, when the grounding short circuit current pass by, will burn off the ground wire , thereby causing the device casing to live, also easy to produce counter attack of high pressure to the control line.

⑤ Anti corrosion  problem of grounding grid

Due to the grounding device for a long time is in the underground operation, operating condition is poor. In particular, some grounding grid of the PV power station is in some wet or acidic soil where the corrosion most likely to occur. The electrical parameters of the corroded grounding grid are often changed. Even cause the electrical open circuit between the grounding of the electrical equipment , the grounding grid and the various parts of grounding grid, thus lead to electrical faults. 

Fourtechnical requirement

According to the provisions of the DL/T 621 specification ,normally , the grounding resistance of the power plant and substation in the effective grounding and low resistance grounding system should meet the following requirements: 

                    R≤2000/IG

R-the maximum ground resistance of taking into account the seasonal variation,Ω;

I-The input short-circuit current of the grounding device used for calculation, A.

In the substation, the grounding short-circuit current I= KA

So the grounding resistance R≤2000/I=2000/  KA= Ω;

Because of the bad grounding condition of the power station, the grounding resistance is difficult to meet. R= 4Ω. Under normal circumstances, the grounding resistance of photovoltaic power plants required not more than 4Ω, however, the soil resistivity of the mountain station is high, also, ground resistance is difficult to meet R no greater than 4Ω.

According to the design specification: when the grounding resistance of the grounding device does not meet the requirements, At the same time, the step potential and contact potential should meet the relevant provisions.

FiveDesign parameters

1, according to the geotechnical investigation report, the value of soil resistivity :Ω.m.

2, Available grounding area : (m)2

3, the annual average temperature ℃, annual icing period; annual rainfall mm, annual average relative humidity%,  rainfall mainly concentrated months, monthly rainfall mm; relative humidity of each month%, so the change of resistivity of the soil in one year can be concluded.

Sixscheme plan

In the design of the grounding grid of the flat ground power station, enlarge the area of the grounding grid of the power station will lead to large investment , In the limited area, the surface layer is used to lay the horizontal network as the equal potential. a long vertical grounding rod is added on the drilling hole. Each earth electrode matched with the resistance reducing agent scheme to design method of grounding resistance reduction. 

Taking into account such as the service life of the grounding grid, the grounding material, the resistance, the geological condition and the humidity temperature of the earth, and so on, In this kind of grounding, the copper plated steel is a good choice. 

On the basis of the design of modular material selection, modular material selection is based on the requirements of the design phase, to choose the grounding material fitting to the project itself. And then to make a customized production requirements of the grounding unit to the factory. On the basis of the full use of copper plated steel, we will prefabricate the grounding bus, up conductors and other units in the factory, processed into different specifications and models, and then make the material numbered, to do the preliminary work for the next assembly construction. 

    

 copper plated steel

By using exothermic welding in construction process,we can realize the molecular connection between the grounding wires. Effectively eliminate the problems of voltage equalization,thermal stability and spot corrosion etc.By assembly and full mechanization of the construction process, trenching and laying the grounding wires can be finished at same time.The construction speed is 5 times than that of traditional process.Also,we effectively avoid the unsafe hidden dangers caused by manual operation.

    

exothermic welding and construction drawing

According to the specification, the grounding resistance of the power station is less than 4Ω; By the way to improve the surface contact resistance, use a long vertical grounding body. After checking, after taking these measures, can meet the requirements of equal potential average voltage ,contact potential and step potential. the grounding resistance is less than 4Ω. 

6.1 Laying a horizontal grounding grid in the power plant, the outer edge of the grounding grid should be closed.

6.2 To set vertical grounding electrode In the connection point of the horizontal grounding grid and around the grid for the discharge and reducing the grounding resistance, at the same time to prevent step voltage and contact potential .

6.4 To fix copper nose and the support bracket bolt in the connection mode of the component support bracket. 

6.3 Exothermic welding is used in the connection between copper plated steel and copper plated steel, copper plated steel and copper plated steel grounding  electrode.


Seven, compare of cost and performance between new grounding system and traditional grounding system

Eight, conclusion

The grounding resistance of grounding grid is calculated by using this method. R< 4Ω, It can meet the requirements in flat ground power plant of contact potential and step potential. The level grid using a 8 mm diameter copper round wire, vertical grounding electrode using a 16 mm diameter copper plated steel wire. connection technology use heat welding. Therefore, the grounding grid can be guaranteed to be under safe operation in the substation, maintenance-free use for more than 30 years. Truly to make sure safe operation in the entire life of the grounding system.