Earthing System
•Introduction:
Contents
•Equipment earthing and neutral point earthing. • Methods & Importance of neutral earthing
• Concepts, Objectives & Classification of Earthing • General Considerations Electric Shock, Touch & Step Potential
•
Soil Resistivity Resist ivity,, Conduction
•
Fault Levels And Max. Earth Fault Current • Earth Potential Rise (E.P.R) And Interference With Teleco Telecommunicatio mmunication n Circuits. •
•Introduction:
Contents
•Equipment earthing and neutral point earthing. • Methods & Importance of neutral earthing
• Concepts, Objectives & Classification of Earthing • General Considerations Electric Shock, Touch & Step Potential
•
Soil Resistivity Resist ivity,, Conduction
•
Fault Levels And Max. Earth Fault Current • Earth Potential Rise (E.P.R) And Interference With Teleco Telecommunicatio mmunication n Circuits. •
INTRODUCTION
Earth - The conductive mass of the earth, whose electric potential at any point is conventionally taken as zero. Earth Electrode -A conductor or group of conductors in intimate contact with and providing an electrical connection to earth. Earth Electrode Resistance - The resistance of an earth electrode to earth.
Earth Leakage Current - A current which flows to earth or to extraneous conductive parts in a circuit which is electrically sound. Earthing Conductor - A protective conductor connecting the main earthing terminal to an earth electrode or to other means of earthing.
Main Earthing Terminal - The terminal or bar (which is the equipotential bonding conductor) provided for the connection of protective conductors and the conductors of functional earthing. Neutral Conductor - A conductor connected to the neutral point of a system and capable of contributing to the transmission of electrical energy. Potential Gradient ( At a Point ) – The potential difference per unit length measured in the direction in which it is maximum.
Touch Voltage - The potential difference between a grounded metallic structure and a point on the earth’s surface separated by a distance equal to the normal maximum horizontal reach, approximately one metre. Step Voltage - The potential difference between two points on the earth’s surface, separated by distance of one pace, that will be assumed to be one metre in the direction of maximum
•Earthing shall generally be carried out in accordance with the requirements of Indian Electricity Rules 1956, as amended from time to time and the relevant regulations of the Electricity Supply Authority concerned. •All medium voltage equipment are earthed by two separate and distinct connections with earth.
IEEE:80 – The IEEE guide for safety in AC substation grounding •IEEE:142 - Grounding of Industrial & commercial power systems •IS:3043 - Code of practice for Earthing •Indian Electricity Act.
Importance of Earthing in Power System 50 % Failure of equipments attributed to Earthing. 40,000 Lightening storms/day
or
100 Lightening storms/second 98 % of the faults in the system are due to SLG Faults 1.5 % of the faults are due to Line to Line Faults 0.5 % of the faults are due to 3 Phase Faults
Importance of System Earthing • Purpose • To minimize potential transient over voltages, to comply
with personnel safety requirements and to assist in rapid detection & isolation of fault areas • IMPARTS ON SHORT AND LONG TERM LIFE OF ELECTRICAL EQUIPMENTS • AT THE LOW COST OF IMPLEMENTATION THERE IS NO MEASURE THAT IS MORE COST EFFECTIVE • Importance • E/F protection is based on method of neutral grounding • System voltage during E/F depends on neutral grounding • Provided basically for discrimination of protection, against
arcing grounds, unbalanced voltage w.r.t. earth, protection
POPULAR ( MIS ) CONCEPTS ABOUT EARTHING
•
EARTH IS A GOOD CONDUCTOR
•
GROUND POTENTIAL IS ALWAYS ZERO
•
PROTO TYPE EARTHING DESIGN IS SUFFICIENT
•
EARTHING IS JUST BURYING CONDUCTOR
•
EARTHING IS ONLY FOR ACHIVEING LOW RESISTANCE VALUE
•
USE OF COPPER FOR EARTHING WILL GIVE LOW RESISTANCE
REASONS WHY EARTHING PROBLEMS ARE COMPLEX •
EARTH
IS A POOR
CONDUCTOR
•
NON HOMOGENEOUS
•
CONDUCTORS BURIED IN SOIL HAVE COMPLICATED SHAPE
•
ACTIVE ONLY DURING FAULT CONDITIONS
•
MOST OF THE ANALYSIS OF EARTHING IS BY EMPIRICAL FORMULAE
What is Earthing? •
•
Earthing means an electrical connection done through a metal link between body of any electrical appliance, or neutral point, as the case may be, to general mass of earth (deeper ground soil) to provide safe passage to fault current to enable to operate protective devices and provide safety to personnel and equipments The metal link is normally of MS flat, CI flat, GI wire which should be penetrated to the ground earth grid
Objectives of Earthing :
Avoid potential rise of parts of equipments other than the live parts.
Safe passage to earth for the fault current.
Suppress dangerous potential gradients on the earth surface.
To retain system voltages within permissible limits under fault conditions.
To facilitate using of Graded insulation in power
..Objectives of Earthing •
For safety of equipments
•
Safety of Operating personnel
•
Avoid Fire Hazards
•
Safety of telecommunication equipments
Classification of Earthing •
System or neutral earthing to ensure system security and protection, it is a connection to ground from one of the currentcarrying conductors of an electrical power system (connection between LV neutral of a power Transformer winding and earth)
•
Equipment earthing (Safety grounding) deals with earthing of non-current carrying parts of equipment to ensure safety to personnel and protection against lightning, it is a connection to ground from one or more of the noncurrentcarrying metal parts of a wiring system or equipment connected to the system (connecting body of equipments like motor body, Transformer tank, Switch gear box, operating rods of air break switches, LV breaker body, HV breaker body, Feeder breaker bodies etc. to earth)
Types of Gr ounding
Un-grounded System • A system of conductors in which there is no intentional connection to ground • Early electrical systems were almost universally operated ungrounded • On small systems an insulation failure on one phase did not cause an outage • The failure could probably be found and repaired at a convenient time without a forced outage
Ungrounded neutral system: Normal Condition
Ungrounded neutral system: Fault Condition
Ungrounded neutral system: Fault Condition
Ungrounded neutral system: Fault Condition
It can be seen from the analysis that: 1) In an ungrounded neutral system, under a single line to ground fault the voltage to earth of the two healthy phases rises from their normal phase to neutral voltage to full line voltage. This may result in insulation breakdown. 2) The capacitive current through the two healthy phases increases to 5 times the normal value. Capacitive fault current flows to earth in excess of 4 A will cause arcing ground 3) A capacitive fault current Ir- flows to the earth.
Advantages of Neutral grounding •
•
Persistent arcing grounds are eliminated. System can be protected against E/F .
Methods of Neutral grounding •
Solid grounding
•
Resistance grounding
•
Reactance grounding
•
Resonant grounding
DIFFERENT METHODS OF EARTHING
SOLID EARTHING
Here neutral is directly connected to earth electrode/mat.
GENERALLY FOR VOLTAGES BELOW 2.2 KV AND ABOVE 33 KV, SOLID EARTHING IS USED. BELOW 2.2 KV, CIRCUIT IMPEDANCE IS SUFFICIENTLY HIGH LIMITTING THE FAULT CURRENT. CURRENT. ABOVE 33 KV, KV, COST OF INSULA INS ULATION TION IS VERY HIGH. THEREFORE, GRADED INSULATION IS USED.
RESISTANCE EARTHING
Here a resistance or an impedance, in general a potential transformer transformer or a single phase distribution transformer transformer is connected between the neutral and the earth electrode/mat.
FOR SYSTEMS OF 2.2 KV TO 33 KV, EARTHING THROUGH RESISTANCE OR REACTANCE IS USED AS INSULATION MATERIAL COST TO TOTAL EQUIPMENT COST IS NOT MUCH.
This is generally applicable to Synchronous generator generator earthing.
REACTANCE EARTHING
WHILE IN RESISTANCE RESISTANCE EARTHING, THE EARTH FAULT FAULT CURRENT IS LIMITED TO FULL LOAD CURRENT OF THE LARGEST GENERATOR GENERATOR OR TRANSFORMER, IN REACTANCE REACTANCE
Effective Earthing A system is called called effectively effectively earthed if XO/X1 < 3 is true & R0/R1 < 1 is true X0 : Zero Zero sequence reactance X1: Positive sequence reactance R0 : Zero sequence sequen ce resistance •
•
Under a phase fault condition the voltage of healthy phase should not rise more than 80% of healthy Line to line voltage. Magnitude of earth fault current is more than 3phase fault current.
Solid grounding
Salient Features of Solid grounding a) When a fault to earth occurs on any phase of the system, the voltage to earth of the faulty phase become zero, but the healthy phase in general, remain at their normal value. As such lightning arresters rated for phase voltage can be insulated for phase voltage. Thus saving in cost. b) The flow of heavy fault current. Ir will completely nullify the effect of the capacitive current ICF and so no arcing ground phenomena will occur. c) The flow of heavy fault current permits the use of discriminative protection gear. d) Used for low voltages up to 600 V and high voltages above 33
Disadvantages of solidly grounded systems High fault currents interfere with communication
circuit. Danger to personnel in the vicinity of fault is high. Heavy fault currents may cause considerable damage to
equipments.
Resistance Grounding
salient features of the resistance grounding 1. It minimizes the hazard of arcing grounds. 2. It permits to use discriminative protective gear. 3. To limit E/F current, a resistance or reactance is introduced between neutral and earth. A resistance grounded system will have low E/F current when compared to solid grounding system and hence will have less influence on neighboring communication circuits.
4. By reducing the value of R, possible to eliminate arcing grounds and if value of R is high, system approaches to ungrounded neutral system 5. Resistance grounding normally adopted for system having system voltage between 3.3 kV to 33 kV
Reactance Earthing System • To limit E/F current, a resistance or reactance is introduced between neutral and earth • Provide additional reactance to system reactance, thereby neutralizes the capacitive currents, hence where high charging
currents are involved reactance grounding is preferred • Synchronous motors and synchronous capacitors are provided with reactance grounding • A system is having reactance grounding if XO/X1 > 3
B Y R
Reactance grounding
Resonant Grounding System • An arc-suppression coil is an iron cored reactor mounted in the neutral earthing circuit and capable of being tuned to resonate
with the capacitance of the system when a line becomes earthed, it makes arcing earth fault self-extinguishing • Also referred as Peterson coil or ground fault neutralizer • For balanced condition L= 1/(w 2C)
B Y R
Fault Analysis •
The fault current and fault voltage at different parts of the network will be affected by the following –
Type of fault
–
Position of the fault
–
Configuration of the network
–
Neutral earthing
Fault Analysis •
•
•
•
The most dangerous phenomena is normally the high current that occurs at a short circuit Open circuit faults not cause high Overcurrent or high overvoltages and therefore normally not dangerous to network, but cause heating in rotating machines, due to the “negative sequence current” that will flow in the system. Machines equipped with negative sequence current protection, needs no fault calculation
The magnitude of the fault current is dependent on type of fault that occurs. At earth faults the size of the fault current is depending on the earthing resistance or reactance (if applicable) and on the resistance in fault. The fault resistance for a phase fault is much smaller than that for an earth fault Three phase faults normally gives the highest short circuit
Symmetrical Components •
Introduced by Fortescue in 1916
•
Developed in a book by Wagner and Evans
•
Very efficient for hand-calculations
•
Forms the base for computer programs
•
An unbalanced system of n related phases could be replaced by a system of n balanced phases which were named the symmetrical components of the original phases
Symmetrical Components
Symmetrical Components •
•
•
Positive-sequence components, consist of three phasors of equal magnitude, spaced 120° apart, and rotating in the same direction as the phasors in the power system under consideration, i.e. the positive direction Negative-sequence components, consist of three phasors of equal magnitude, spaced 120 ° apart, rotating in the same direction as the positive-sequence phasors but in the reverse sequence Zero-sequence components, which consist of three phasors equal in magnitude and in phase with each other, rotating in the same direction as the positive
Symmetrical Components
Symmetrical Components
Computation of Fault Current
Computation of Fault Current
Computation of Fault Current
SUBSTATION EARTHING
MAIN INTENTION OF EARTHING IS TO LIMIT THE TRANSIENT OVER VOLTAGE CAUSED BY RESTRICTING GROUND FAULTS, TO THE LEVEL THAT THE EQUIPMENT CAN BE DESIGNED TO WITHSTAND ABOUT 250 % OF THE RATED VOLTAGE. FOR SAFETY TO MAINTENANCE PERSONNEL AND TO LIMIT THE DAMAGE OF THE EQUIPMENT, IT IS ABSOLUTELY MUST OF FAST CLEARING THE GROUND FAULT.
PROPER SYSTEM EARTHING WILL GIVE A HIGH DEGREE OF PROTECTION AGAINST STEEP WAVE FRONT SURGES ENTERING THE SUB STATION AND PASSING TO EARTH THROUGH ITS GROUNDING SYSTEM.
UNDER FAULT CONDITIONS, THE FLOW OF CURRENT TO EARTH WILL RESULT IN GRADIENTS WITHIN AND AROUND THE STATION. UNLESS THE EARTHING SYSTEM IS DESIGNED CAREFULLY, THE MAXIMUM GRADIENT ALONG THE SURFACE MAY BE GREAT ENOUGH TO ENDANGER A MAN
PRELIMINARY DESIGN OF GROUNDING SYSTEM
A CONTINUOUS EARTHING CONDUCTOR IS PLACED AROUND THE PERIMETER OF THE SUB STATION TO ENCLOSE AS MUCH GROUND AS POSSIBLE TO AVOID CURRENT CONCENTRATION AND HENCE HIGH GRADIENTS AT GROUND CONDUCTOR ENDS. WITHIN THE GRID, CONDUCTORS ARE LAID IN PARALLEL LINES AND AT UNIFORM SPACING.
THE MATERIAL OF THE GROUND ELECTRODES SHOULD HAVE HIGH CONDUCTIVITY AND LOW UNDERGROUND CORROSION. STEEL IS USED NORMALLY IN INDIA FOR EARTHING.
ALUMINIUM IS NOT MUCH IN USE AS CORRODED ALUMINIUM IS ALMOST NON- CONDUCTIVE. COPPER IS COSTLY. HENCE MILD STEEL ELECTRODES WITH ADEQUATE CROSS SECTION ARE PREFERABLE.
IT IS A GOOD PRACTICE TO HAVE AN OVER DESIGNED EARTHING SYSTEM AS THERE ARE A NUMBER OF UNKNOWN FACTORS AND THE SAFETY OF THE OPERATING PERSONNEL IS ALWAYS INVOLVED.
EARTHING INSTALLATION
EARTHMAT IS USUALLY DESIGNED WITH THE FOLLOWING SIZES OF MS RODS. 40 MM DIA. 400 KV SUB STATIONS 40 MM / 32 MM DIA. 220 KV SUB STATIONS 110 KV SUBSTATIONS 32 MM / 25 MM DIA.
CONDUCTOR ABOVE GROUND LEVEL FOR EARTHING EQUIPMENT, STRUCTURES, COLUMNS AND OTHER AUXILIARY STRUCTURES SHALL BE GALVANISED FLATS. ROD ELECTRODES SHALL BE OF MILD STEEL OF SAME DIAMETER AS EARTH CONDUCTORS AND OF LENGTH AS REQUIRED IN THE DESIGN.
NEUTRAL POINTS OF SYSTEMS OF DIFFERENT VOLTAGES, METALLIC ENCLOSURES, FRAMES OF ALL CURRENT CARRYING EQUIPMENTS AND ALL METAL WORKS ASSOCIATED WITH THE CURRENT CARRYING SYSTEM SHALL BE CONNECTED TO THE SINGLE EARTHING SYSTEM. STEEL STRUCTURES, COLLUMNS ETC. SHALL BE CONNECTED TO THE NEAREST EARTHING GRID BY TWO EARTHING LEADS.
Earthmat Layout
STATUTORY PROVISION OF EARTHING
EARTHING SHALL BE CARRIED OUT AS PER INDIAN ELECTRICITY RULES AND AS PER IS 3043- 1987 ALL MEDIUM VOLTAGE EQUIPMENT SHALL BE EARTHED BY TWO SEPARATE AND DISTINCT CONNECTIONS TO EARTH. TO THE EXTENT POSSIBLE, ALL EARTH CONNECTIONS SHALL BE VISIBLE FOR INSPECTION. THE VALUE OF ANY EARTH SYSTEM RESISTANCE SHALL BE SUCH AS TO CONFORM WITH THE DEGREE OF EARTH PROTECTION DESIRED. PREFERABLY, NO CUT OUT, LINK OR SWITCH SHALL BE PROVIDED IN THE EARTHING SYSTEM. HOWEVER, THIS DOES NOT INCLUDE THE CASE OF A SWITCH FOR USE IN CONTROLLING A GENERATOR, A TRANSFORMER OR A LINK FOR TEST PURPOSES. ONLY GOOD QUALITY MATERIALS SHALL BE USED IN THE EARTHING SYSTEM.
C0MMON TYPES OF EARTH ELECTRODES
WHILE EARTH GRIDS ARE USED IN MAJOR SUB STATIONS, DIFFERENT TYPES OF EARTH ELECTRODES ARE USED FOR EARTHING HV AND LV INSTALLATIONS. IN 11 KV AND 33 KV SUB STATIONS, PLATE EARTHING PROVES SUFFICIENT. PLATE ELECTRODES PLATE ELECTRODES ARE OF MAXIMUM SIZE 1.2 M X 1.2 M. IF MORE AREA IS REQUIRED, INSTEAD OF INCREASING THE SIZE, TWO NUMBER OF PLATES ARE USED IN PARALLEL. CAST IRON PLATES OF 12 MM THICK ARE MOST SUITABLE. PIPE OR ROD ELECTRODES PIPES MAY BE OF CAST IRON NOT LESS THAN 100 MM DIAMETER, 2.5 M TO 3 M LONG AND 13 MM THICK. STRIP OR CONDUCTOR ELECTRODES STRIP ELECTRODES ARE USED IN HIGH RESISTIVITY SOIL. WHERE ROUND CONDUCTORS ARE USED AS EARTH ELECTRODES, THEIR AREA OF CROSS SECTION SHALL NOT BE LESS THAN THE SIZES RECOMMENDED FOR STRIP ELECTRODES.
EFFECT OF MOISTURE ON EARTH RESISTIVITY •
•
•
•
THE NORMAL MOISTURE CONTENT OF SOIL RANGES FROM 10 PERCENT IN DRY SEASON TO 35 PERCENT IN WET SEASON, AVERAGE BEING 16 TO 18 PERCENT. IF THE MOISTURE CONTENT IS 20 PERCENT OR ABOVE, THE RESISTIVITY IS NOT AFFECTED. BUT WHEN THIS MOISTURE IS REDUCED, SOIL RESISTIVITY ABRUPTLY INCREASES. ABUNDANCE OF PURE WATER WILL NOT REDUCE THE SOIL RESISTIVITY. NATURAL ELEMENTS AND SOLUBLE INGREDIENTS INCREASES THE CONDUCTIVITY. ARTIFICIAL TREATMENT OF SOIL THE RESISTIVITY OF THE SOIL IMMEDIATELY SURROUNDING THE EARTH ELECTRODE CAN BE REDUCED BY
EFFECT OF MOISTURE ON EARTH RESISTIVITY •
•
ADDING SUBSTANCES LIKE SODIUM CHLORIDE, CALCIUM CHLORIDE,SODIUM CARBONATE, COPPER SULPHATE, SALT AND SOFT COKE AND SALT AND CHARCOAL IN SUITABLE PROPORTIONS DURING DRY SEASONS, EARTH PITS MAY BE REGULARLY WATERED AND KEPT WET TO KEEP THE EARTH RESISTIVITY LOW. GRAVEL OR CRUSHED ROCK COVERING IS ALSO HELPFUL TO RETARD THE EVAPORATION OF MOISTURE FROM EARTH.
COMPONENTS OF EARTH PIT •
1 Conducting Electrode
•
2 Contact Point of the electrode and Soil
•
3 Soil 1
2
3
TYPES OF ELECTRODE 1. Plate electrode 2. Mesh electrode 3. Cast Iron Pipe electrode 4. G.I. Pipe electrode 5. Rod electrode 6. Strip electrode 7. Chemical electrode
DISPERSION FROM ELECTRODE
As electrode offers less resistance to The flow of current compared to soil, It is better to have one of the dimensions Stretched in a given area of dispersion. Hence rod/ pipe grounding may be preferred compared to plate
PLATE ELECTRODE
Sizes of Plate electrode are 1.2m X 1.2m, 0.9m X 0.9m and 0.6m X 0.6 m. Minimum size of plate should be 0.6m X 0.6m The Resistance practically achieved is proportional to the Linear dimension to the electrode. Resistance achievable by different electrode sizes are, 1.2m X 1.2m = soil resistivity / 2.75 0.9m X 0.9m = soil resistivity / 2.20 0.6m X 0.6m = soil resistivity / 1.375 Careful Plate electrode corrodes fast hence recommended thickness Cast Iron = 12.00 mm
INSTALLATION OF PLATE ELECTRODE
600 mm
1500mm min
600 mm
1500mm min
Rod or Pipe electrode •
•
•
•
The size of cast iron pipe used is 100mm dia, with a 13mm thickness and 3m length. The size of GI pipe used is 38/50mm dia and 3m length The size of solid rod used is 13, 16, 19mm having length of 1.2m, in set of 2 or 3 The choice to the electrode is based on Economy of driving it in & Space available
Rod or Pipe Electrode Reduction of soil resistivity 13mm
16mm
25mm
100mm
0.91
0.88
0.81
0.59
0.78
0.76
0.70
0.51
2.0 m
0.51
0.49
0.46
0.35
2.4 m
0.44
0.42
0.39
0.30
3.0 m
0.36
0.35
0.33
0.25
3.6 m
0.31
0.30
0.25
0.22
1.0 m 1.2 m
Comparative Analysis Reduction of soil resistivity 1 0.9 0.8 0.7 0.6
13mm
0.5
16mm
0.4
25mm
0.3
100mm
0.2 0.1 0 1.0m
1.2m
2.0m
2.4m
3.0m
3.6m
Parallel Electrodes •
•
•
When number of rods are connected in parallel, the resultant is the reciprocal of the rods connected. Parallel electrodes should be outside the resistance area of each other. Mutual separation shall more than the depth of the driven electrode 13mm rod 1.2 m
1.2 m min
1 rod R= 27.0 2 rod R = 13.5
Care taken during parallel electrode installatio
Strip Earthing •
•
•
The sizes of strip electrode generally used are 25 X 3, 50 X 6, 75 X 6 flats and 70 sqmm round bare cables. Where the sub stratum is very hard and going deep does not help in lowering of resistance, strip electrode is an effective solution A ready guide for using a strip electrode is presented to you.
600mm
HARD ROCK
Comparative Analysis Reduction of soil resistivity
0.4 0.35 0.3 0.25
25 X 3
0.2
50 X 6 75 X 6
0.15
70 sq mm
0.1 0.05 0 3m
6m
10m
20m
50m
Material of Electrode •
•
•
•
•
Most corrosive but accepted electrode material are cast iron, wrought iron, mild steel etc. Z-90 grade GI has much better life compared to bare material Most preferred material of electrode is copper In many cases molecularly bonded copper over steel is being effectively used.In case of molecularly bonded copper 250 micron thickness of copper is needed over steel. In case the installation is protected by cathodic protection, the material used for grounding should have the same galvanic voltage as that of the cathodically protected installation. Such material may be selected referring the galvanic series. Please take care that Copper
Contact between the Earth and Electrode 1 •
•
•
The earth electrode should be thrusted into the ground and not loosely driven. Compaction of earth is essential for pre-bored pits In case of a heavy short circuit, the moisture from the neighboring soil may evaporate due to heat. Resulting in infinite
2
3
•Use of conductive cement 100mm around the electrode can be very useful
Resistivity Values
Change in Resistance with Salt and Moisture •The maximum amount of ionic substance needed is about 5% by weight
•What we really need is about 20% moisture by weight
Change in resistance with Temperature
•This table clearly shows that we need to be careful of as the water in the soil freezes and introduces very high Temperature coefficient. •Hence the Electrode needs to be buried 2m below the surface
Why is artificial Treatment needed •
To maintain the ionic level of the soil for long
•
To maintain the moisture content
•
To have inner compaction
•
To constantly diffuse into neighboring soil and increase the resistance area of the electrode
TEREC + Miracle Compound for maintenance free Earthing
Horizontal Setting up
Vertical setting up with tubular electrode
Vertical setting with copper / steel rods
AVAILABLE IN INDIA
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More than 1000,000 million installations worldwide Repeatedly used by Defense, Airports, Research Organizations, Space centers and petrochemical refineries and Power houses It is the latest patent in the world of Earthing DRDO, BEL, RBI, BPCL, NPCL, HCL, Hi-tech IT units are already in our list of Indian Clients
EARTHING OF 11 KV AND LT LINES
FOR HT LINES, 25 MM GI PIPES OF LENGTH 1.8 TO 3 M SHALL BE USED AS EARTH ELECTRODES. COIL EARTHING FOR LT LINE MAY BE PROVIDED WITH NO. 6 OR NO. 8 GI WIRE OF LENGTH 10 TO 25 M CLOSELY WOUND INTO A COIL OF DIAMETER 5 CM TO 10 CM, AT A DEPTH OF 1.5 M FROM GROUND LEVEL. RUNNING A SEPARATE EARTH WIRE IS NOT IN VOGUE FOR LT LINES .HT LINES ARE NOT PROVIDED WITH EITHER NEUTRAL WIRE OR EARTH WIRE. EARTHED NEUTRAL IS USED IN LT LINES. NEUTRAL IS EARTHED WITH TWO SEPARATE AND DISTINCT EARTH ELECTRODES ( PIPES ) AT THE TRANSFORMER POINT, BOTH IN HT AND LT. ALL SPECIAL STRUCTURES OR POLES CARRYING TRANSFORMERS, SWITCHES, FUSES ETC. SHALL BE EARTHED. ALL SUPPORTS CARRYING GUARD WIRES SHALL BE EARTHED. THE OHMIC RESISTANCE OF THE EARTHING OF HT AND LT SHALL BE BELOW 10 OHMS.
CORROSION IN EARTHING SYSTEM
•
•
CORROSION RESULTS IN EARTHING SYSTEM DUE TO MECHANICAL, CHEMICAL OR ELECTROCHEMICAL CAUSES. EARTHING SYSTEM DEGRADES IN THE FOLLOWING WAYS. THE CONTACT RESISTANCE OF EARTHING MATERIAL WITH GROUND INCREASES DUE TO A FILM OF CORROSION PRODUCT.
•
THE SURFACE AREA IS REDUCED DUE TO LOSS OF METAL.
•
THE CONTACT POINTS DEGRADE LEADING TO LESS EFFECTIVE EARTHING.
R8 R6
R2 R7 R1
R4 R3
R5
MEASUREMENT OF SOIL RESISTIVITY METHODOLGY ADOPTED MEASUREMENTS ARE MADE ALONG A NO OF RADIALS AT DIFFERENT LOCATIONS IN THE STATION SUCH THAT THE WHOLE AREA IN WHICH EARTHING ELECTRODES / MAT IS LAID IS COVERED •
SPACING BETWEEN THE PROBES WHICH ARE HAMMERED INTO THE SOIL BE VARIED RADIALLY FOR TAKING DIFFERENT READINGS •
TYPICALLY IF THE STATION IS 100 TO 150 MTRS THE SOIL RESISTIVITY READINGS MAY BE TAKEN FOR A PROBE SPACINGS OF 1 , 2, 5, 10 , 15, 25 AND 50 MTRS •
A FEW DROPS OF WATER MAY BE POURED IN THE NEIGHBOURHOOD OF PROBES TO GET GOOD CONDUCTIVE CONNECTION BETWEEN PROBE AND THE SOIL SURROUND IT. •
THE BURIED METALLIC PIPES IN THE NEIGHBOURHOOD
MEASUREMENT OF SOIL RESISTIVITY TWO COMMONLY USED SOIL MODELS ARE UNIFORM SOIL AND TWO LAYER SOIL MODEL AS MAGNITUDE OF SPACING B/W PROBES IS INCREASED FROM SMALL VALUE TO HIGHER VALUE THE MEASURED SOIL RESISTIVITY REFLECTS THE EFFECT OF SOIL AT DIFFERENT DEPTHS UNIFORM MODEL IS CHOOSEN IF THE MEASURED SOIL RESISTIVITY VALUES VARY WITHIN 30 % OF AVERAGE VALUE CASE-1 RESISTIVITY OF UPPER LAYER MORE THAN LOWER LAYERS Rg ( Uniform layer value ) < Rg obtained ( Etouch & Estep) ( Uniform layer value ) < (Etouch & Estep) obtained CASE-2 RESISTIVITY OF UPPER LAYER LESS THAN LOWER LAYERS
MEASUREMENT OF SOIL RESISTIVITY FACTORS DETERMINING SOIL RESISTIVITY •
MOISTURE
•
DISSOLVED SALTS
•
TEMPERATURE
•
GRAIN SIZE AND DISTRIBUTION
•
SEASONAL VARIATION
•
CURRENT MAGNITUDE
VARIATIONS IN RESISTIVITY DUE TO MOISTURE,TEMPERATURE,SALT
SEASONAL VARIATIONS To account for the seasonal variations , the average Soil resistivity is multiplied by the factor as shown below, which is termed as the apparent resistivity. Season of measurement Multiplication factor
Summer
1.0
Winter
1.15
Rainy
1 3
Effect of Salt Moisture and Temperature on Soil Resistivity
The The objectives objectives of of earthing earthing system system ::
Safety to operating personnel by limiting step & touch potential
To provide a sufficiently low-resistance path to ground to
minimize rise in ground potential with respect to remote ground for proper functioning of the protective devices of the substation Healthiness of the power equipments by providing ground
connection for transformers, reactors and capacitors To provide path for lightning rods, arresters and similar
devices To provide a means of discharging and de-energizing
equipment in order to proceed with maintenance on the equipment
objectives ofwhich earthinginfluence system :- the earthing TheThe Parameters system : Magnitude and Time of fault current Earthing Conductor Material Earth Electrode Resistivity of Soil Resistivity of Surface Insulation Material (gravel) Design Methodology
The objectives of earthing systemCurrent::Magnitude & Time of Fault Earthmat of substation shall be suitable for the expected
maximum current (including expected increase in future expansion) Time of current clearing shall be such that it covers the time of back-up protection Shock duration (0.5 sec) shall be such that the human body can tolerate the intended current passing through the body. Maximum fault current can be obtained from Power System Studies. Future expansion shall also be considered. The worst fault current is the equipment short Ckt current rating which is normally higher than power system fault current
Earthing Conductor :-
Size of earthing conductor shall be suitable for the worst
fault current with 1 sec as fault clearing time
Normally for conductor sizing, the equipment short Ckt
current rating is considered.
Earthing Conductor size:For MS Rod conductor & corrosion allowance of 0.12mm /yr for 40 years
1.0 2.0 3.0
Magnitude of Fault Current Duration of fault Current (Sec) Minimum Area of the earth conductor (sq.mm)
31.5kA 1 406
40kA 1 515.5
50kA 1 664.4
63kA 1 811.9
4.0
Minimum Diameter of the earthing conductor (mm)
22.7
25.6
28.6
32.2
5.0
Diameter of the conductor with corrosion allowance (mm)
32.3
35.2
38.2
41.8
Safe Current for human body Current Range
Effects on Humans
1 mA
Threshold of perception Let go currents Pain full, hard to let go Muscular contractions Ventricular fibrillation
1-6 mA 9-25 mA 25-60 mA 60-100 mA
Maximum Body Current: Ik
=
0,11 6
√t
for t = .03s to 3s
Potential Rises during fault
Touch Potential & Step Potential – Tolerable
0,11 6
√t
Step potential: The potential difference shunted by a human body between two accessible points on the ground separated by a distance of one pace assumed to be equal to one meter
Touch voltage circuit
Touch potential:- The potential
difference between a point on the ground and a point on an object likely to carry fault current (e.g., frame of equipment) which can be
Mesh potential: The maximum touch potential within a mesh of the grid.
For Safe Design, (i)
Attainable Touch Potential shall be less than Tolerable Touch Potential
(ii) Attainable Step Potential shall be less than Tolerable Step Potential (iii) Earth Potential Rise (EPR) shall remain within permissible limit (iv) For most transmission and other large substations, the
ground resistance is usually about 1 Ω or less. In smaller distribution substations, the usually acceptable range is
from 1 Ω to 5 Ω, depending on the local conditions.
For Safe Design,
In any switch yard, chances of exposure to ‘Touch potential’ is higher than that to ‘step potential’. ii. Resistance offered by the feet of a person against ‘Touch potential’ is much less compared to that against ‘Step potential’. iii. Hence ‘Touch potential ’ is more critical for design while i.
Step potential is usually academic. iv. Step potential is independent of the diameter ( crosssection) of the earthing conductor. v. For 400% increase in diameter, reduction in Touch potential is only 35%. vi. Thus cross- section has minor influence on Touch and Step potentials. vii. Length of earthing conductor has significant effect on Touch and Step potentials.
For Safe Design, (i)
Attainable Touch Potential shall be less than Tolerable Touch Potential
(ii) Attainable Step Potential shall be less than Tolerable Step Potential (iii) Earth Potential Rise (EPR) shall remain within permissible limit (iv) For most transmission and other large substations, the
ground resistance is usually about 1 Ω or less. In smaller distribution substations, the usually acceptable range is
from 1 Ω to 5 Ω, depending on the local conditions.
For Safe Design,
In any switch yard, chances of exposure to ‘Touch potential’ is higher than that to ‘step potential’. ii. Resistance offered by the feet of a person against ‘Touch potential’ is much less compared to that against ‘Step potential’. iii. Hence ‘Touch potential ’ is more critical for design while i.
Step potential is usually academic. iv. Step potential is independent of the diameter ( crosssection) of the earthing conductor. v. For 400% increase in diameter, reduction in Touch potential is only 35%. vi. Thus cross- section has minor influence on Touch and Step potentials. vii. Length of earthing conductor has significant effect on Touch and Step potentials.
Soil Resistivity
Soil Resistivity varies with type of soil, temperature, moisture contents and climatic condition Measurement of soil resistivity shall preferably be done in dry season Maintaining accuracy in soil resistivity measurement is difficult Analysis of soil resistivity with the type of soil may be necessary In case of +30% variation, two layer soil modeling helps in correct modeling and optimal design Measurement of soil resistivity in eight directions with
A non-accurate soil resistivity will lead to unsafe earthing
Wenner’s four electrode method is better
Tolerable Touch & Step Potentials
Gravel resistivity is generally considered as 3000 ohm.m in the design though the range of gravel resistivity is 1000 – 10000 ohm.m Considering of higher gravel resistivity (>3000 ohm.m) o hm.m) means withstanding of higher touch & step voltages As the gravel resistivity also changes with environmental condition, a lower value of gravel will lead to risk of limit of step and touch potentials Hence, measures such as integration of gravel with a P.C.C layer under the gravel may be applied. Requirement of gravel for future equipment area shall examined w.r.t the requirement of step voltage. More shock duration means less the withstanding voltage
Area of Gravel spreading
Equipment Area has to be graveled as the design is done for the same. Requirement of gravel in future area of the substation with no equipment shall be seen from possible rise in step potential. The step voltages (tolerable & attainable) shall be calculated (which is different with consideration of gravel) without gravel, gravel resistivity is equal to soil resistivity if empirical formula are applied. Gravel shall also to be laid 2m away from fencing to ensure that if a person touches the fence, he should stand on the gravel. Even spreading of gravel may be reviewed if the design is safe and have enough confidence.
Earthing in difficult situations The earthing resistance can be improve by any one or more of the following methods. 1.
Increase the area of the earth mat.
2.
Provide deep earth electrodes.
3. Provide auxiliary earth mat in a near by place where the resistivity is low and connect it to the main earth mat. 4. Treating the earthmat and the electrode with suitab suitable le chemic chemicals als.. Depending upon the situation any one or more of the above methods can be used to reduce the earth
Satellite Earthmat EARTH POTENTIAL RISE ( E.P.R )
If = Fault current In = Neutral current If = In Im = Total current flowing in the Earthmat
SATELLITE EARTHMAT
Ig = Part of the fault current Entering the Main Earthmat through the Earth. Ig = Ig1+Ig2+Ig3.........+Ign
Is In TRANS.
Igw = Part of the fault current Entering the Main Earthmat through the Overhead Ground wirw. Is = Part of the fault current Entering the Main Earthmat through the Satellite Earthmat.
Is
MAIN EARTHMAT GROUND WIRE
Igw
FAULT-2
Im
Ig
FAULT-1
If Ig5
Ig4
Ig3
Ig2
Ig1
In = Im = If = Ig+Igr+Is Ig = Ig1+Ig2+Ig3.........+Ign
ONLY THE CURRENT Ig CONTRIBUTES TO THE E.P.R AND NOT THE TOTAL CURRENT FLOWING IN THE EARTHMAT (Im) OR NEUTRAL (In)
The diversion of fault current through the main earth mat.
References 1. IEEE guide for AC Substation Grounding ( IEEE 80) 2.
IEEE guide for Measuring earth Resistivity, Ground impedance, and earth surface potentials for a ground system(IEEE 81)
3. IEE recommended practice for grounding industrial and commercial power systems ( IEEE 142) 4. IEEE Guide for generating station grounding(IEEE 665) 5. Indian standard specifications ( 3043 Earthing)
