“Outage of Solar Plant due to earthing may result into 100% revenue loss”
What is Electrical Earthing?
Any metal body which is not intended to carry electric current such as metal enclosure, equipment body is connected to earth (ground) through conducting wire is called earthing or grounding.
Why we should do earthing of electrical system?
The basic purpose of electrical earthing is to avoid any electric shock to human life by keeping the equipment and electrical system safe from electrical hazards. Electrical earthing keeps the non-conducting part of equipment body at ground potential and pass on any unwanted electricity discharge directly to the ground.
Importance of Earthing
All electrical systems are designed to operate with protection system in place. Without proper earthing of electrical system, all electrical equipment and appliances does not operate.
The earthing of electrical system helps to achieve following objectives.
- To protect human life from an electric shock.
- To keep the equipment body and neutral of electric system at ground potential.
- To provide shortest least resistance path to electric current in case of insulation failure.
- To avoid malfunctioning of electrical protection system
Types of Earthing
Basically, there are two types of earthing.
- Equipment body earthing
All electrical equipment, machinery and appliances are earthed to avoid any electrical fire and shock hazards. There is deliberate safety interlock provided to this equipment not to operate without proper earthing. The non-current carrying parts of the equipment like metal body is connected to the earth with the help of conducting wire. If any leakage current flows through the metal body due to insulation failure or short circuit, the fault current passes to the ground through earthing wire.
- Neutral Earthing
Any electrical system with star winding, neutral is to be grounded to avoid vector shift and unbalancing of voltage. Neutral earthing is also called as system earthing. Neutral earthing is provided in generator, motor, transformer, and electrical distribution system.
Components of earthing
Earthing Electrode
The earthing electrode type and size depends upon the voltage level and fault current of the system. Low tension system having voltage less than 440V should have earthing electrode with 4 feet length either GI pipe or solid copper rod.
HT system having voltage above 440V up to 33000V should have earthing electrode with 10 feet length of GI pipe.
Electrodes are either flat plate, Pipe or Rod. Selection of earth electrodes are done based upon soil condition (soil earth or rock earth), soil moisture, voltage, and fault current level.
Size of Earth Electrode
Resistance of an Electrode R=Kx ρ/L
Where ρ=Resistivity of earth in ohm-m
L = Length of electrode, d = diameter of electrode, K = Material constant
For GI material,
K =0.75 if 25 < L/d < 100
K= 1 if 100 < L/d < 300
K= 1.2 if 300 < L/d < 600
Length of ground electrode – Double the length, reduce ground resistance up to 40%
Diameter of electrode – Double the diameter, reduce ground resistance up to 10%
Earthing wire
Proper selection of earthing wire size and material should be done to maintain the effectiveness of earthing. Size of wire depends on maximum fault current it is intended to carry. Normally Copper wire is used in hose hold appliances. Aluminum or GI conductor is used in electrical distribution system.
Size of Earthing conductor
The size of earthing conductor can be determined by following formula.
Cross section of conductor (mm2) = [Fault Current (I) x Fault duration (t)]/ [Material Constant (K)]
Fault current (I) is given on equipment name plate. It is generally in KA.
Fault duration of electrical system is generally from 1 sec to 3 sec (time required by protection system to act and isolate the fault).
Material constant (K) for Galvanized Iron (GI) is 80, Copper – 205, Aluminum – 126.
Consider 1% degradation of material every year due to corrosion up to 20 years with safety factor of 1.5.
Required conductor size = Cross sectional area x 20% degradation allowance x safety factor.
Chemical Earthing
The earthing resistance differs with location and mainly depends upon soil resistivity. The fault current dissipation will be quick if the soil is less resistive. It is a general practice to add chemical in the backfill soil of the earthing pit to improve its conductivity. Salt and Charcoal is used to improve the soil condition with regular watering. This is the old method. With improvement in the technology, there is maintenance free earthing available in the market. As per IEEE 80 14.5 (d), there are certain compounds which has resistivity less than 0.12-ohm meter. Nowadays Bentonite powder is used as chemical for improving soil condition.
Factors affecting earthing resistance
- Resistivity of the soil- High resistive soil requires deeper earthing (longer length of earthing rod) with bigger cross-sectional area of electrode.
- Material of earthing wire and electrode – GI, Copper and Aluminum are the best earthing material.
- No. of electrodes – More the electrodes, lesser will be the earthing resistance
- Nature of soil – Rocky and concrete soil offers highest resistivity to fault current. A suitable location should be chosen for proper earthing.
- Distance of earthing pit – Earthing pit should be as close as to the equipment to reduce earthing wire resistance.
Importance of earthing in Solar Plant
Solar plant is combination of DC power as well as AC power. Solar PV panels generate DC power which is more fatal if not handled properly. AC power from electricity distribution company is fed up to Inverter.
Inverter Earthing
Inverter is the brain of the Solar PV plant. It handles both AC and DC power. Hence, earthing of Inverter with proper conductor size is prime necessity. In fact, inverter is designed in such a way that without proper earthing it does not start. Inverter earthing should be separate. Size of earthing wire is determined by KW rating of Inverter.
PV Panel Earthing
Each PV panel operates at certain voltage as per its wattage rating. It is a good installation practice to connect each PV panel with green color 4 sq mm2 copper wire with each other. A separate earthing wire should be brought down to ground after connecting each PV panel together. Its earthing should not be mixed with Inverter earthing or Lightening Arrestor earthing.
Lightening Arrestor Earthing
Lightening is the common cause of failures in Solar PV plant. A high voltage surge can occur from lightning that strikes from a long distance. Most lightening damage is preventable.
The earthing of lightening arrestor is foremost neglected part in PV plant. In many cases, under cost cutting, LA is not installed for smaller plants. However, for bigger plant, it is made mandatory by legal authority.
Lightening arrestor carries safely the high lightening discharge current to the ground without affecting the Solar PV plant component. The earthing wire and earthing electrode of LA should be separate and of appropriate size.
Color coding
Earthing wire should be green in color which is universally recognized. It gives clear identification out of many wires running parallel in same cable tray.
Conclusion
Earthing is most important part of Solar Plant system. Solar PV panel, Inverter and Lightening Arrestor earthing should be separately done. Earthing should be done in a technically correct method to avoid any unwanted shutdown.
