Grounding
By ELASSAD Hakam
Microelettrica Scientifica
MS RESISTANCES
ELECTRICITY

The Electricity is produced & generated
outside cities at high voltage
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On Nuclear plant
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On diesel ,or steam generateur
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On wind or photovoltaique
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On hydro plant
NUCLEAR POWER PLANT
Diesel Generation plant
Steam power process
Steam generator
Photovoltaic plant
WIND POWER PLANT
Transmission & Distribution

Electricity should be transported & distributed
 The transportation is made through Cables (
High voltage head over lines Transmission )
 63 , 132,220 , 330 , 400 or 750 kV the voltage
vary from countries , distance , total power to be
transported Etc.
Head over lines
Transmission & distribution
process
Sub-Station
Types of Neutral Grounding in Power
Distribution

In industrial high voltage network , ground
or earth is the reference point from which
voltages are measured , a common return
path for electrical current or direct physical
connection to the earth
Method of grounding (Earthing)

To determine the Earthing mode a
compromise between three requirements
– Damp over voltages
– Limit damage & disturbances cause by
the earth fault
– Provide simple selective protection
devices
Criteria to Choose

Voltage level: The insulation level of material must
be in accordance with induced over voltage at the
time of Short circuit
 Insulation coordination : Ground fault current will
induce locally over voltage which must be
compatible with the insulation of components
 Limitation of Fault current : To reduce the
electrodynamics stress on material , limit the induced
voltage on telecommunication lines
Grounding Method
Insulated or Ungrounded Neutral

Ungrounded system is no longer recommended
 No connection between Neutral & Ground
Solidly earthed or direct Earthling

The Neutral of Transformer or
Generator is directly connected
to ground by Bus Bar
 No limitation of Fault current
 Is usually used in low voltage
600 V and lower
Impedance fault current Limitation

There is three types
 By neutral grounding
Resistor
 By Neutral grounding
reactor
 By tuned reactor (
Petersen coil or
ground fault
neutralizer )
Through a Neutral Earthing Reactor

The Neutral of Transformer is connected to earth
through a Reactor
 The ground fault current should be at least 60% of
the three phase fault current
 Reactor grounding is not considered as an
alternative to the resistor grounding
 This solution is rarely used in industry or bear
cities due to induced magnetic field
Neutral grounding Reactor dry
type
PETERSEN COIL

An Earthing reactor connected between the
neutral of a system and earth and having
relatively high value of reactance
 The reactive current to earth under fault
conditions balances the capacitance current
to earth so that the earth current at the fault
is limited to practically zero
Arc suppression coil
(Peterssen coil )
Through a Neutral Earthing Resistor

The Neutral of Transformer is connected to
earth through a Resistor
 The fault current is limited to chosen value
 I f = (System Voltage /3)/R

There is two types of resistors
LOW RESISTANCE & HIGH RESISTANCE
Neutral Grounding Resistor
Low Resistance Grounding

Connected to earth ground through low resistance
 Limits ground fault current to a 20 % of 3 phase fault
current (10-3000 A)
 Resistor is connected between neutral of power source
High Resistance Grounded

Connected to earth ground through a high
resistance
 Limits ground fault current to a few amperes (1-10
A is common)
 Commonly used on low voltage systems in
industrial plants
 Used by utilities for large generators
Where Do We Ground?

Single source feeding
bus
 Source  connected
Derive neutral with
Y- or zig-zag
transformer
Connect grounding
resistor to this
derived neutral
Where Do We Ground?

Single transformer
feeding bus
 Transformer
secondary Y
connected
Connect grounding
resistor to neutral
of transformer
Where Do We Ground?

Single generator
feeding bus
 Generator Y
connected
Connect grounding
resistor to neutral
of generator
Where Do We Ground?

Multiple sources
feeding bus
 Sources Y or 
connected
Derive neutral with Y or zig-zag
transformer
Connect grounding
resistor to this
derived neutral
Where Do We Ground?

Double-ended
substation with
normally open tie
 Transformers Y
connected
Connect grounding resistor to each
transformer neutral
NEUTRAL GROUNDING
RESISTOR
Neutral Earthing Resistor

NGR is not a protection components
 NGR is installed to reduce the fault current before clearing
the fault
 Phase - Neutral Voltage = V = System voltage / 3
V=U/1.732
 Rated fault Current = V/R ( A )
 Rated time (sec)
 Required Temperature Rise °K
NOTA : IEEE-32-1990 is the only existing standards to
design the resistor
Rated Voltage

NGR is inserted between the Neutral of
Transformer ( secondary ) & Earth ,
 The rated voltage of NGR = Phase to
Neutral Voltage = System Voltage /3
Rated Fault Current





I fault= (U/3)/ R
Phase to Neutral Voltage / resistance value
In High Resistance , The fault current =1 to 10 A
In Low Resistance : The fault current = 10 to
3000A
In case of NO indication we limit the fault current
to the rated current of transformer
Rated Time & temperature Rise

Rated time = 10 sec , 30 sec or 1 minute /hour
 The time rating indicate the time that the NGR can
operate under fault conditions without damage &
without exceeding the specified temperature rise
 385 °C for continuous rating
 760°C for less than 10 minutes
As per I EEE 32 Standards ,
Insulation

As per IEEE 32 Standards the

applied voltage
should be = 2.25 times
the line to neutral voltage + 2 kV
EXTRACT FROM IEEE 32-1990
Resistances material

Different kind of resistance material
 1: Cast iron ( not used any more)
 2: Liquid resistor ( not used any more)
 Stainless Steel ( AISI 304 , 430,310,32/20 or
inconel 601 ) NiCr alloy
Temperature Coefficient of resistance material
Temperature coefficient = 
 indicate the variation of resistivity of resistance
material with temperature
 Resistance material used for NGR should have a
high variation of resistivity to limit the fault
current & keep the voltage constant
 The Stainless Steel AISI 430 is mostly used for
NGR

Protection Degree

The NGR is a passive component , resistor
will absorb the energy developed by the
current during rated time and should
evacuate it
 The Best Protection Degree for NGR = IP23
IP 55 type tests
IP55
Sub station in the desert Egypt
Components can be installed into Resistor
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RECOMMANDED
Current & voltage transformer
On or off load disconnectors ( contactors)
Surge Arrestors
Space heater
Lamp
Doors
NOT RECOMMANDED
Electronic devices like relays or others monitors
( temperature into resistors will reach very high level & cause
damage to electronic
accessories
ROUTINE & Type Tests

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Routine Tests
Visual Inspection
Dimensional Check
Measurement of Insulation Resistance
Measurement of Resistance
High Potential Test (Dielectric test @ 50Hz – 1min)
Type Tests
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As per IEEE-32 Impulse tests are not required for Resistor
Heat run tests = Temperature Rise tests , application of constant voltage during
rated time
measurement of current & resistance value at 0 sec up to end rated time
Measurement of temperature rise by application of tempearture sensor
Calculation of temperature rise tests as per following formula
R2=R1*(1+αΔθ)
α = Temperature coefficient of resistance material
Δθ = Temperature Rise
Temperature Rise tests & installation of temperature sensor
The End