1. ELECTRICAL EARTHING

2. Special Note:
The approach here is based on standard IEEE 80 (Safety in AC substation grounding)
These discussions are for illustration only
Grounding practices are subject to local regulations/codes which will take precedence

3. Objectives of grounding:
Provides an electrical supply system with a reference to the groundmass (system grounding)
Protective grounding of electrical equipment enclosures
Makes them safe to persons who may come into contact with them
Enables the flow of fault current in the event of a failure
Provides a low impedance path for accumulated static charges and surges (lightning protection grounding)
Helps in mitigating the generation and propagation of noise (grounding of shields and signal reference planes)

4. Earthing System
Shall satisfy Safety, Functional requirements of electrical installation
Shall ensure
Protection against indirect contact
Proper functioning of electrical protective devices
Protective and functional requirements are met under expected conditions
Earth fault, earth leakage currents can be carried safely
Adequate strength appropriate to external influences
Adequate value of earthing resistance

5. Benefits (1) Fault damage now minimal Lower outage times
Reduces fire hazard (especially in mines)
Lower outage times
Less lost production, less lost revenue
Touch potentials kept within safe limits
Protects human life

6. Benefits (2) Low Fault Currents reduce possibility of igniting gases
Minimizes explosion hazard
Lower Magnetic or thermal stresses imposed on plant during fault
Transient overvoltages limited
Prevents stressing of insulation, breaker restrikes

7. Fault in an Ungrounded System:

8. System grounding
Provides reference for the entire power system to groundmass
Establishes a path for current to ground during insulation failure
Provides protection against equipment damage due to faults
Provides protection against high voltage transients
Enables detection by circuit protective devices for isolation
Reduces maintenance time and expenditure

9. Fault in Ungrounded system
Single phase ground fault does not result in flow of high fault currents
Voltage to ground of other two phases rises by 73% - Causes insulation stress
Fault current = Vector sum of capacitive currents in the distributed capacitive reactance of other two phases

10. Fault in Ungrounded system
Intermittent first fault causes transient voltages up to 6 to 8 times nominal system voltage
Such high transient voltage can initiate a second fault at weakest insulation point
Uncleared arcing fault can cause extensive damage to system
Early detection of first fault therefore of paramount importance

11. Effect of neutral (system) grounding:

12. Ground fault current flow:

13. Grounding methods 1 Ungrounded System
Neutral connection on Generator/ Transformer is not connected to earth at all

14. Grounding methods 2 Solid grounding
Neutral connection on Generator / Transformer is connected to earth by a solid Conductor
Cost Reductiuons due to avoidance of sensitive relays and grounding device, Grading of insulation towards neutral end.
But Circulation of third harmonic currents between neutrals

15. Grounding methods 3 Resistance grounding
Neutral connection on Generator / Transformer is connected to earth (0V) through a fixed resistance to limit the earth fault current
Mainly used below 33 KV
Full line to line insulation required towards neutral

16. Grounding methods 4 Reactance grounding
Neutral connection on Generator / transformer is connected to earth (0V) through a fixed reactance to limit the earth fault current
Can be cheaper compared to resistance

17. Grounding methods 5 Petersen Coil grounding (arc suppression)
Neutral connection on transformer is connected to earth (0V) through a variable reactance to neutralise the capacitive earth fault current. Results in arc extinction

18. Grounding methods 6 NEC grounding (with and without resistance)
In HV delta systems no earth connection is available. A 3 phase neutral grounding compensator is connected to allow earth fault currents to flow - allowing detection of these faults

19. Protective grounding Protects personnel against shocks
Personnel do not experience dangerous high voltages when contacting enclosure accidentally connected to live parts
Provides a low impedance path for accumulated static charges and surges (lightning protection grounding)
Helps in mitigating the generation and propagation of noise (grounding of shields and signal reference planes)

20. Importance for Earthing
An Electrical equipment is considered dead when
At or about zero potential
Disconnected/ Isolated from live system
Disconnection alone not adequate
Can retain stored charge
Can acquire a static charge
Can accidentally be made alive
Nearby live conductors may induce voltage

21. Importance of Earthing
! Ensure earthing before working on electrical equipment
Earthing
Connect apparatus electrically to general mass of earth in such a manner as will ensure at all times an immediate safe discharge of electrical energy
Connect to earthed metal earth bar or spike with good metallic conductor
Earthing by
Closing of earthing links
Attaching of fixed earthing devices
Affixing of portable earthing straps

22. Importance of Earthing
Ensure before applying earth
Earthing connection is mechanically, electrically in good condition
No broken strands
Clamps should be rigid and without defect
Applied properly in intimate contact with conductors and earth-bar/ spike
Earthing cable tails as short as possible
Connect to earth first when installing earthing, disconnect earth last while removing earthing

23. Hazards of improper Earthing
Electrocution
Burns from arcing
Electric shock leading to falls

24. Bonding
Connecting of various grounding systems and non current carrying parts
To achieve potential equalization between different accessible conducting surfaces
Potential difference between different accessible conducting surfaces, different grounding systems hazardous

25. Typical Earthing System

26. Touch, Step, Transferred Voltages

27. Touch Voltage Permissible Touch Voltage
Voltage at any point of contact with uninsulated metal work
Within 2.5 mtrs from ground surface and
Any point on ground surface within horizontal distance of 1.25 mtrs from vertical projection of point of contact with uninsulated metal work

28. Step Voltage
Difference in surface potential experienced by a person bridging a distance of 1 mtr with his feet apart, without contacting any other earthed object
Shall not exceed twice the value of Touch voltage

29. Touch potentials (Reb =1 )
Person touches transformer tank now live

30. Touch potentials (Reb = 10 )
Lower touch potential

31. Transferred Potential
Make Allowances for
Transferred potential during design, installation
Voltage drop in conductors (where voltage rise in earthing system is transferred by metal work to remote location)
Otherwise regard Transferred potential as earthing system voltage rise

32. Transferred Earth Potential Rise
Earthing system shall be designed to prevent
Transfer of earth grid potentials to a remote earth
Transfer of a remote earth potential into a station
Breakdown of cable over-sheaths due to voltage differences

33. Substation grounding practices-1:
Grounding design approach depends upon:
Voltage class of the system
Type of installation (Utility or consumer)
LV Systems:
Usually solidly grounded
Metallic contact between ground of consumer and system neutral
Special cases such as SELV power supplies may be of ungrounded type

34. Substation grounding practices-2:
MV Systems:
Self contained systems (E.g., Turbo generators): High resistance grounding
Industrial systems: Low resistance
Critical systems: Tuned grounding OR Ungrounded (for small systems)
Utilities: Solid grounding
HV/EHV Systems: Solid grounding

35. A typical ground electrode:
Materials
Copper
Copper clad steel
Galvanized steel
Copper clad stainless steel

36. A typical chemical electrode:
Sometimes called a leach electrode
Chemical mixtures are added to lower resistance of soil
Needs regular maintenance

37. Ground Potential Rise-Example:

38. Ground Potential Rise-Example:

39. Voltages in grounding system:

40. Area of ground grid:
HV (outdoor) substations use a buried ground grid for dissipation of fault current to the soil
Lower grid resistance to earth is preferable
It reduces the ground potential rise with ref. to remote earth
Also reduces mesh voltage. Lower mesh voltage is safer because it lowers touch/step voltages
Grid of larger area has lower ground resistance and results in lower grid voltage and mesh voltage
Further reduction by using vertical electrodes welded to the grid

41. Soil resistivity:
Soil resistivity should be based on an average of measurements done in the substation area
Simple designs assume uniform soil
Non uniform soil involves complex design steps and requires computer programs
Lower resistivity results in lower ground resistance of the grid

42. Other issues to be taken note of:
Soil layers may differ in resistivity
Two layer soil model to be considered if variations are found to be high at different depths
Transferred potential due to buried services going to/ from the substation (water mains, cable metallic sheath)
Introduce non-conducting sections where feasible to avoid transfer of Ground potential

43. Special attention to be paid to:
Operating handles of equipment:
Most fatalities occur due to high touch potential at these points
Should be made safe by special arrangements
Substation fence:
Decision regarding keeping isolated vs connecting to substation ground
Surge arrestors:
Short, direct and low impedance connections
GIS extensions

44. Effective substation grounding - 1:
Size ground conductors adequately
Use proper bonding and jointing in ground conductors. Poor joints cause high temperatures during a fault to ground
Select appropriate ground electrode system
Pay attention to soil resistivity. Carry out soil improvement if resistivity is too high. Use chemical electrodes if situation warrants

45. Effective substation grounding - 2:
Pay attention to step and touch potentials
Building foundations can also be used as grounding electrodes
Integrate fence of outdoor substations as far as possible. Avoid transferred potential from services entering or leaving a substation
Pay special attention to operating handles

46. Effective substation grounding - 3:
Cable trays to be properly grounded
Surge arrestors to be grounded using short low impedance connections
Carry out temporary grounding for safety when personnel have to work on parts which are normally live

47. Rings of equal potential on earth fault

48. Recommended maximum grounding resistance values
Substation Capacity
Maximum Grounding Resistance (Ω)
Below 4.16kV
Above 4.16kV
Below 1000kVA 5 10
1000kVA ~ 5000kVA 2
More than 5000kVA 1
Small Distribution Transformer Banks 15 25

49. Grounding Methods - Comparison
Characteristics
Ungrounded
Solid Grounding
Low Resistance Grounding
High Resistance Grounding
Personnel Safety
Poor
Better
Good
Best
Immunity to Transient Over voltages
Voltage stress during ground fault
Potential flashover to ground
Worst

50. Grounding Methods - Comparison
Characteristics
Ungrounded
Solid Grounding
Low Resistance Grounding
High Resistance Grounding
Protection against Arc Fault damage
Worst
Poor
Better
Best
Ease of providing Ground fault protection
Good
Service Reliability
Continuity of Service

51. Grounding Methods - Comparison
Characteristics
Ungrounded
Solid Grounding
Low Resistance Grounding
High Resistance Grounding
Reduction in Fault frequency
Worst
Better
Good
Best
Ease of Fault Location
Protection device coordination
Not possible
Maintenance Cost

52. Any questions ?