Transcript Earthing - kishorekaruppaswamy

ELECTRICAL
EARTHING
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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
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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)
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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
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Benefits (1)
• Fault damage now minimal
– Reduces fire hazard (especially in mines)
• Lower outage times
– Less lost production, less lost revenue
• Touch potentials kept within safe limits
– Protects human life
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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
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Fault in an Ungrounded System:
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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
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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
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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
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Effect of neutral (system) grounding:
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Ground fault current flow:
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Grounding methods 1
• Ungrounded System
– Neutral connection on Generator/ Transformer is
not connected to earth at all
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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
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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
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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
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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
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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
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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)
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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
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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
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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
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Hazards of improper Earthing
• Electrocution
• Burns from arcing
• Electric shock leading to falls
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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
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Typical Earthing System
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Touch, Step, Transferred Voltages
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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
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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
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Touch potentials (Reb =1 )
Person
touches
transformer
tank now live
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Touch potentials (Reb = 10 )
Lower
touch
potential
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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
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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
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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
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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
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A typical ground electrode:
Materials
• Copper
• Copper clad steel
• Galvanized steel
• Copper clad stainless
steel
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A typical chemical electrode:
• Sometimes called a
leach electrode
• Chemical mixtures are
added to lower
resistance of soil
• Needs regular
maintenance
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Ground Potential Rise-Example:
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Ground Potential Rise-Example:
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Voltages in grounding system:
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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
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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
resistance of the grid
in
lower
ground
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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
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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
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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
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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
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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
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Rings of equal potential on earth fault
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Recommended maximum grounding
resistance values
Maximum Grounding Resistance (Ω)
Substation Capacity
Below 4.16kV
Above 4.16kV
Below 1000kVA
5
10
1000kVA ~ 5000kVA
2
5
More than 5000kVA
1
2
15
25
Small Distribution
Transformer Banks
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Grounding Methods - Comparison
Characteristics
Ungrounded
Solid
Grounding
Low
Resistance
Grounding
High
Resistance
Grounding
Personnel Safety
Poor
Better
Good
Best
Immunity to
Transient Over
voltages
Poor
Good
Good
Best
Voltage stress
during ground fault
Poor
Best
Good
Poor
Potential flashover
to ground
Poor
Worst
Good
Best
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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
Worst
Good
Better
Best
Service Reliability
Worst
Good
Better
Best
Continuity of Service
Better
Poor
Poor
Best
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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
Worst
Good
Better
Best
Protection device
coordination
Not possible
Good
Better
Best
Maintenance Cost
Worst
Good
Better
Best
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Any questions ?
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