grounding system philippine
TRANSCRIPT
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Grounding
By ELASSAD Hakam
Microelettrica Scientifica
MS RESISTANCES
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ELECTRICITY
The Electricity is produced & generated
outside cities at high voltage
On Nuclear plant
On diesel ,or steam generateur
On wind or photovoltaique
On hydro plant
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NUCLEAR POWER PLANT
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Diesel Generation plant
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Steam power process
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Steam generator
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Photovoltaic plant
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WIND POWER PLANT
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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.
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Head over lines
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Transmission & distribution
process
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Sub-Station
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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
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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
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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
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Grounding Method
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Insulated or Ungrounded Neutral
Ungrounded system is no longer recommended
No connection between Neutral & Ground
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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
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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 )
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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
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Neutral grounding Reactor dry
type
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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
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Arc suppression coil
(Peterssen coil )
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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
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Neutral Grounding Resistor
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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
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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
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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
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Where Do We Ground?
Single transformer feeding bus
Transformer secondary Y connected
Connect grounding resistor to neutral of transformer
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Where Do We Ground?
Single generator
feeding bus
Generator Y
connected
Connect grounding
resistor to neutral
of generator
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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
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Where Do We Ground?
Double-ended
substation with
normally open tie
Transformers Y
connected
Connect grounding resistor to each
transformer neutral
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NEUTRAL GROUNDING
RESISTOR
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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
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Rated Voltage
NGR is inserted between the Neutral of
Transformer ( secondary ) & Earth ,
The rated voltage of NGR = Phase to
Neutral Voltage = System Voltage /3
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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
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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 ,
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Insulation
As per IEEE 32 Standards the
applied voltage should be = 2.25 times
the line to neutral voltage + 2 kV
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EXTRACT FROM IEEE 32-1990
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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
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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
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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
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IP 55 type tests
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IP55
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Sub station in the desert Egypt
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Components can be installed into Resistor
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
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accessories
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ROUTINE & Type Tests
Routine Tests Visual Inspection
Dimensional Check
Measurement of Insulation Resistance
Measurement of Resistance
High Potential Test (Dielectric test @ 50Hz – 1min)
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Type Tests
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
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Temperature Rise tests & installation of temperature sensor
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The End