Download - Earthing Practices

Earthing Practices

By Er. J. M. Pardhi

Superintending Engineer,Testing Circle, MSEDCL,

Nagpur.

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Objectives of Earthing

• To dissipate electric currents into the earth without exceeding operating & equipment limits.

• To ensure human safety from electric shock.• To prevent hazardous voltage for reducing risk

of fires and personal injuries.

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Earthing Systems

• Basically 6 grounding systems in use:– Equipment grounds.– Static grounds.– Systems grounds.– Maintenance grounds.– Electronic grounds.– Lightning grounds.

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Equipment Grounds

• It is the physical connection to earth of non current carrying metal parts.

• All metal parts of equipments that personal may come into contact to be at zero potential with respect to ground.

• All metal parts must be interconnected and grounded to ensure path of lowest impedance for flow of ground fault current.

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Equipment Grounds

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Static Grounds

• Connection made between a piece of equipment and earth for the purpose of draining off electricity charges before a flashover potential is reached.

• Utilized in dry materials handling, flammable liquid pumps and delivery equipments, plastic piping and explosive storage facilities.

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System Grounds

• Refers to the point in an electrical circuit that is connected to earth through electrical neutral.

• The sole purpose is to protect equipment. • Provides low impedance path for fault currents

improving ground fault coordination. • Ensures longer insulation life of motors,

transformers and other system components.

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TYPES OF GROUNDING SYSTEMS

• Ungrounded System:The ungrounded system is one that has no intentional connection between the neutral or any phase and ground. Please note that an ungrounded system is grounded through the concept of capacitively coupling. The neutral potential of an ungrounded system, with balanced loading will be close to ground potential due to the capacitance between each phase conductor and ground.

Low ground fault current.Very high voltages to ground potential on unfaulted phases.Sustained faults lead to system line-to-line voltages on unfaulted line.Insulation failure.Failure due to restrike ground faults.

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TYPES OF GROUNDING SYSTEMS• Solidly Grounded System:

The solidly grounded system is one that has the neutral connected to ground without an intentional impedance. In contrast to the ungrounded system the solidly grounded system will result in a large magnitude of current to flow (Aids in coordination), but has no increase in voltage on unfaulted phases.

Low initial cost to install and implement, but stray currents then become a possible consequence.Common in low voltage distribution systems, such as overhead lines.typically feeds to transformer primary with high side fuse protection.Not preferred for industrial or commercial facilities due to high magnitude fault currents.

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Normal Pipe type Earthing for trans. line

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Distribution Transformer Double Pole Structure.

- as per REC Construction Standard.

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Typical arrangement of earthing to railway

traction system.

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HT Metering Cubicle

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TYPES OF GROUNDING SYSTEMS• Low Resistance Grounded System:

The low resistance grounded system is one that has the neutral connected to ground through a small resistance that limits the fault current. The size of the grounding resistor is selected to detect and clear the faulted circuit..

The resistor can limit ground currents to a desired level based on coordination requirement or relay limitations.Limits transient overvoltages during ground faults.Low resistance grounding is not recommended for low voltage systems due to the limited ground fault current. This reduced fault current can be insufficient to positively operate fuses and/or series trip units.Ground fault current typically in the 100 – 600 Amp range.


TYPES OF GROUNDING SYSTEMS• High Resistance Grounded System:

The high resistance grounded system is one that has the neutral connected to ground through a resistive impedance whose resistance is selected to allow a ground fault current through the resistor equal to or slightly more that the capacitive charging current of the system.

The resistor can limit ground currents to a desired level based on coordination requirement or relay limitations.Limits transient overvoltages during ground faults.Physically large resistor banks.Very low ground fault current, typically under 10 Amps.Special relaying methods utilized to detect and remove ground faults.High resistance grounding is typically applied to situations where it is essential to prevent unplanned outages.Recent trend has been to utilize high resistance grounding methods on 600 volt systems and lower.

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Components of earthing system in Sub-stn.

Sub Station Earth Mesh

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Maintenance Grounds• Utilized for safe work practices.• It is a temporary ground.

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Electronic Grounds• Equipment ground and system ground are combined and

applied in unity.• Must not only provide a means of stabilizing input voltage

levels, but also act as zero voltage reference point. • Must be able to provide effective grounding and bonding

functions well into the high frequency MHz range.


System GroundsMethods of System

GroundingCharacteristics Ungrounde

dSolid

GroundLow Resistance

GroundHigh Resistances

Ground

Susceptible to Transient overvoltages

WORST GOOD GOOD BEST

Under fault conditions (line-to-ground) increase of voltage stress

POOR BEST GOOD POOR

Arc Fault Damage WORST POOR GOOD BEST

Personnel Safety WORST POOR GOOD BEST

Reliability WORST GOOD BETTER BEST

Economics' (Maintenance costs) WORST POOR POOR BEST

Plant continues to operates under single line-to-ground fault

FAIR POOR POOR BEST

Ease of locating ground faults (time) WORST GOOD BETTER BEST

System coordination NOT POSSIBLE

GOOD BETTER BEST

Upgrade of ground system WORST GOOD BETTER BEST

Two voltage levels on same system NOT POSSIBLE

POSSIBLE

NOT POSSIBLE NOT POSSIBLE

Reduction in number of faults WORST BETTER GOOD BEST

Initial fault current Into ground system

BEST WORST GOOD BETTER

Potential flashover to ground POOR WORST GOOD BEST

Lightning Grounds• Depends upon the structure, equipment to be protected and

the level of lightning protection required of desired.

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• Straight risers• Separate earth rod per

phase• Rods connected to each

other & to Earth-grid.• No G.I. pipe for riser.

Earthing of LA

Earthing in a EHV SubstationCEA Regulation 2010 (I.E.Rules 1956)

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Earthing in a EHV SubstationCEA Regulation 2010 (I.E.Rules 1956)

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Earth Mat Design

• Earthing system in a Sub Station comprises of;– Earth mat or grid.– Earth electrode.– Earthing conductor.– Earth connectors.

• Primary requirement of earthing is to have a low earth resistance.

• To consider safe limit of step/ touch/ transfer potential .

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Surface potentials in a Sub-station• 1)Step potential: The difference in surface

potential experienced by a person bridging a distance of 1 meter with the feet, without contacting any grounded object .

• 2)Touch potential: The Potential difference between the Ground Potential Rise (GPR) & the surface potential at the point where a person is standing while at the same time having a hand in contact with a grounded structure.

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Ground Potential Rise (GPR)

• The maximum electrical potential that a sub-station grounding grid may attain relative to a distant grounding point assumed to be at the potential of remote earth. This voltage is equal to:

G.P.R.= Ig x Rg where, Ig= Maximum grid current Rg= Grid resistance (Grid means ‘Earth grid’ i.e. Earthing system )

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Step & Touch Voltages

Tolerable Step & Touch voltages• Estep = 0.116 (Rb + 2Rf) / √ts = 0.116 (1000 + 2 x 3 ρ) / √ts = 0.116 (1000 + 6 ρ)/ √ts where, ρ= Resistivity of soil.

• Etouch = 0.116 ( 1000+ Rf/2)/ √ts = 0.116 ( 1000 + 1.5 ρ)/ √ts

• While designing earthing system, care is taken to ensure that actual Step & Touch voltages are well within tolerable Step & Touch Voltages.

• Note that ‘tolerable touch voltage’ is much less than tolerable step voltage. Hence, is critical.

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METHOD OF MEASURING SOIL RESISTIVITY:

• Geological information and soil samples• Variation of depth method• Two point method• Four point method - Equally spaced Wenner arrangement - Unequally spaced or Schlumberger - Palmer arrangement

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WENNER FOUR ELECTRODE METHOD:

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UNEQUALLED SPACED OR SCHLUMBERGER – PALMER ARRANGEMENT:

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EQUIPMENT USED FOR MEASUREMENT OF SOIL RESISTIVITY:

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FACTORS AFFECTING THE SOIL RESISTIVITY (ρ):

• Type of the soil.• Moisture.• Dissolved salt in water.• Temperature.• Grain size and its distribution.• Seasonal variation.• Artificial treatment.

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Effect of temperature on soil resistivity

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0° C 50°C-15° C

RESISTIVITY

TEMPERATURE

Transferred Potential• It is a special case of the ‘touch voltage’ where a

voltage is transferred into or out of the sub-station from or to a remote point external to the sub-station site.

• A person standing in a sub-station coming in contact with say rails/water pipeline/neutral coming from an adjacent sub-station at the time of occurrence of earth-fault at that sub-station gets exposed to the transferred potential which equals difference in GPRs of the two sub-stations.

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Transferred Potential

Factors influencing Earth Mat Design

• Magnitude of Fault Current.• Duration of Fault.• Soil resistivity.• Resistivity of Surface Material.• Shock Duration• Material of Earth Mat Conductor.• Earthing Mat Geometry.

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Calculation of size of earth grid conductor.

Conductor area A = I .

root[ T cap x 10-4 x ln Ko + Tm ]

tc – αr x Pv Ko + Ta

Simplified formula for steel grid conductorArea A = I x √t where; A =area of earth conductor in mm2

K I = short circuit current in kA t = duration of short circuit current in

sec. K factor = 80 for steel

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Calculation of size of earth grid conductor.

• If mat is made up of Flat then; A = Area of FlatGive allowance of 35% for corrosion

• If it is a conductor; A = ∏ r2 , where r = √(A) / ∏

• STANDARD FLATs in mm10 x 6, 20x6, 30x6, 40x6, 50x6, 60x6, 50x8, 65x10, 75x10

• ROD: 40mm mild steel rod

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Maintenance Schedule

• Watering of earth pit - Daily• Measurement of earth resistance of individual earth pit - Half yearly• Measurement of combined earth resistance at all the pits - Half yearly• Checking of inter connections

between earth pits and tightnessof bolts and nuts - Quarterly

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Case Study

• Fatal Electrical Accident to farmer at Kholapur (Amravati) dtd:06/06/2012.

• Bird fault occurred at 11kV cut point.• Support insulator not provided for cut point

jumpers.• High resistance arcing fault.• Sufficient fault current not developed.• Feeder protection not operated.• High soil resistivity and high impedance.• Electric pole, stay and soil acted as load.

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Earth fault current- two return paths

•

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Parallel path for fault current through Ground Wire of trans. line

Grid current Ig through earth

Source

Fault

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Electric currents & their effects on Human body

Sr.No A.C. Current range (mA)

Called as Effect

1 1.0 Threshold Tingling

2 1 to 6 Let-go Can hold & release

3 9 to 25 >More than 25

>

Painful breathlessne

ss.

Difficult to release

4 116 alarm Ventricular fibrillation,

heart failure.

Tolerable Body current• Dr. Dalziel observed that the limiting current a

human body weighing 50Kg can tolerate without ventricular fibrillation is given by:

Ib = 0.116/√ts where, ts=duration of shock

current• Similarly, Ib = 0.157/√ts for 70 Kg. weight.• If ts = 1second, Ib=116 milli-amps or 157 ma.• With numerical relays ts= 240 ms Then tolerable Ib= 0.116/√.240= 236milli-amps.

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Earthing of EHV sub-stations- challenges• With advent of transmission voltage of 765KV &

ambitious programme of addition of Generation capacities at national level (78700 MW by 2012) fault levels are going up.

• With higher X/R ratios of lines, D. C. component in fault current takes longer time for decay. Results in higher temp. rise of earthing system.

• Breaker re-closures increase duration of shock currents needing more efficient earthing system.

• Switching & lightning surges need good earthing. 54

Earthing system of EHV sub-stations- care to be taken up

• Measurement of average resistivity of new s/s by taking numerous readings spread all over the area.

• Projected fault level at the s/s in next 10 years to be considered while deciding earthing system.

• Allowance for corrosion of electrode(15% for 12 years) in moderately corrosive soil to be kept.

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Monitoring & maintenance of Earthing system

• Continuity of under-ground earth conductors to be checked.• Continual observation, tightening, painting of

over-ground connections. • Measurement of earth resistance at least once a

year (I.E. Rules, section 67).• Treatment by Bentonite where necessary.• Maintaining Stone metal cover 100 mm. Removal

of grass, providing earth mat at operating points. 56

EARTHING OF EHV TRANSMISSION LINE

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System of earthing• Ground-wire strung on tower peaks & bonded to

tower body.• Earthing at tower bases by ‘Pipe’ type or

‘Counter-poise type’.• Surge Arresters provided at wide creek crossings.• Electrode line with special earthing station for

HVDC line for ‘Single pole earth return ‘mode.• Earth resistance below 10 ohms to avoid back –

flash. 58

THANK YOU!

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