One,Grounding Engineer Instruction
20MWSolar-Agriculture Complementary Project:this power plant is a 20MWSolar-Agriculture Complementary PV power plant .Due to the complex characteristics of the power station.(Higher soil moisture, many electricity facilities, needed fertilizer of greenhouse plants etc.)So there is a great difference of the structure environment and geology from the common PV power station.The corrosion of grounding material from the geological and environmental will be more serious.In order to ensure the safe operation of the power station, the grounding resistance value of the power station is required to be＜4Ω ,and the need high strength grounding material to fight against the serious corrosion of water,stray current and chemical fertilizer to make sure full-life grounding system.Thus make the following grounding engineer solutions.
Two, design base of grounding engineering
1, DL/T621 Grounding for AC electrical apparatus
2, DL/T620 Over voltage protection and insulation coordination of AC electrical apparatus
3, GB50169 Code for construction and acceptance of grounding device for installation of electrical equipment
4, GB 50057 Design code for protection of structures against lightning
5, State Power Grid Corp: Eighteen major anti accident measures of power grid
6, GB50065 Code for grounding design of AC electrical equipment
Three, project description
According to the design requirements of the national standard: The resistance of the grounding grid of power station must meet the requirements of ＜4Ω, because it is solar-fishing project, so the surrounding soil resistivity is relatively low. Soil resistivity ρ= 100Ωm, Then, use a similar traditional grounding method, such as : grounding area S=0.25·ρ2/R2，, the calculated S value is a very small number of square meters, so the resistance reduction for the power plant will not be a big problem. According to the actual situation of the surrounding terrain of the power station, a good potential and fault current drain channel should be considered after satisfying the requirement of the resistance of the grounding grid and to meet the design life of the use,.According to the GB50057 and GB50065 mentioned in the severe corrosion area , after economic analysis , copper plated steel can be used as a grounding material, so in our whole project in the grounding part, we use copper plated steel in line with the national standard of as a grounding material.And the vertical grounding grid and the underwater grounding device are used to realize resistance reduction and discharge channel.
Four, detailed construction scheme
(1)、Horizontal grounding line:according to the actual situation,can divide into two parts of the overall layout.,left is A, right is B.Outside of the plant to do a closed loop level equal potential .And ensure that each group of side support to reliably connected with equal potential grounding wire.The ground bus of A district can be divided into horizontal equal potential grounding wire and vertical grounding wire.Through the previous modular design,then to the factory to produce certain length of grounding unit according to the actual situation. Two adjacent areas are connected by connecting lines.In the same way, after the modular design of the B area, the factory will produce the grounding unit in the first time to meet the specific circumstances. Copper plated steel is used in all equal potential connection .Connection between round steels are all by exothermic welding.The connection between the round steel and bracket is by bolt connection.
(2)Grounding down conductor device:according to the drawing, we can manufacture in the factory in advance,customize different specifications and types (Φ8 copper plated round steel)down conductors.Ensure that each set of support and equipment is connected to the grounding grid to make sure the devices safety.
(3)Vertical grounding electrodes, because of the especial complexity of geological environment, we take the vertical grounding electrodes to reduce resistance and provide discharge channel for fault current.We want to achieve grounding resistance through the calculation ＜4Ω power requirements need to set up a total of 42 vertical grounding rods.Vertical grounding connect with periphery equal potential. According to the drawings , grounding evenly distributed in the periphery of equal potential connection line.Each inverter set vertical grounding electrodes in the diagonal position and connect with the equal potential ring of the inverter. The common vertical grounding electrodes using a 14/2.5 diameter meter copper plated steel grounding electrode, in order to achieve the effect of discharge current and resistance reduction.
(4)Specific construction site can be done appropriate changes according to the actual situation .
(5)The traditional galvanized flat steel has its own defects in the manufacturing process, lead to the coating adhesion is poor.Use welding process pick-up material connection process, high temperature of electric arc welding completely destroyed the junction of the coating, leading the connection to become the most fastest place to be corroded.And the traditional grounding materials using welding technology to connect, lack of experience lead to the difference of the length and the fullness of the welding joints.then in the welding joints will appear with slag, slipped welding, porosity etc. problems, so the conductivity and thermal stability of this connection is not reliable.And its 8-10 years of service life is difficult to meet the design life of 25 years of PV power plant.There will be 2-3 times risk of the transformation during the use of PV power plant.
The new material of round copper plated steel has almost same conductive properties and service life as copper.Therefore, the electrical properties of copper plated steel is 3 times the conductivity of galvanized steel,has better electrical conductivity to reduce the grounding resistance,and copper plating steel is used four dimensional continuous plating process,the thickness of joint coating based on the national standard UL467 is 0.254mm.The service life can reach more than forty years.Using exothermic welding to reach reliable connection, welding material itself is copper alloy and its corrosion resistance is better than the copper plated steel itself.Welding temperature can reach up to 2700 degrees Celsius, so the grounding material is fully integrated, completely solve the problem of thermal stability of the grounding grid.