Presentation

Report
Medium and Low Voltage Grounding Methods
August 20, 2010
System Grounding
Establish a voltage
relationship between the
system neutral and ground.
•Overvoltage protection for
system component
insulation
•Controlled single-phase-toground fault current
magnitude
Establish a voltage
relationship between
energized phase conductors
and ground.
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August 20, 2010
Equipment Grounding
Establish a consistent
reference plane for all system
components.
•Personnel safety
•Optimum single-line-toground fault current
distribution
•Safe conduction of
lightning discharge
currents
•a.k.a. bonding
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August 20, 2010
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lg
Human Physiological Response:
1 ma – threshold of sensation
6-9 ma “let go” currents
9-25 ma – muscular contraction
60-100 ma – ventricular fibrillation
Body resistance: 5000 ohms or higher
Cable Sheath
Perceptable gradient voltage: 50 volts & above
Harmful gradients: 375 volts and above
Neutral Wire
Water Pipes
Building Steel
Z
Controlling factors
o Magnitude of Potential Gradientsmagnitude of ground fault current
phase-neutral voltage
complexity of return path
o Duration of Potential Gradientsprotective device settings
fuse, relay, & breaker operation
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Ig
Distribution of Ig:
Kaufmann’s work showed that 90-95% of
the fault current will return through the
cable sheath and/or neutral wire
Ig = I C + I N + I P + I S
Example:
Cable Sheath
Neutral Wire
Water Pipes
Building Steel
Z
IC
IN
IP
IS
Ig = 20000 A
IS = 0.05 x 20000 = 1000 A
For Vs > 100 V, Z > 0.1 Ohms
Power cable- ground shield at both ends to
help equalize electric potential along cable
length, but be mindful of magnitude and
duration
Communication cable- do not ground
communication cable at both ends to avoid
circulating current that would act as noise
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Ig
Cable Sheath
Neutral Wire
Water Pipes
Building Steel
Z
IC
IN
IP
IS
Conclusions:
Ineffective grounding at any voltage sets the
stage for personnel injury or death.
Ineffective grounding at higher voltages can set
the stage for potential gradient shocks which
are severe enough to distract personnel in the
workspace
The likelihood of these “distracting gradients” is
insignificant at low voltages
Low voltage systems can be solidly grounded
without undue concern for “distracting”
potential gradients.
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s
g
System Neutral Ground- an Iintentional
electrical connection between the neutral of
Ineffective grounding at any voltage sets the
the power system and ground
stage for personnel injury or death.
Grounded System- a system in which one conductor, usually the neutral, is
intentionally connected to ground.
Effective grounding at higher voltages can set
Ungrounded System- a system in which none
the electrical
conductors
theof
stage
for potential
gradientisshocks which
intentionally connected to ground
are severe enough to distract personnel in the
IC distributedworkspace
Note: There is an inherent
Cable
Sheath
capacitance
between each conductorThe likelihood of these “distracting gradients” is
s
s
and ground. Hence, an
IN“ungrounded insignificant at low voltages
system”
really capacitively grounded
Neutral isWire
Low voltage systems can be solidly grounded
without
unduebetween
concernthe
for neutral
“distracting”
Solidly Grounded
Neutral- a direct electrical
connection
and
Water Pipes
potential gradients.
ground with no added impedance
IS an electrical connection in which a resistor is
ResistanceBuilding
Grounded
NeutralSteel
inserted between neutral and ground
Reactance Grounded Neutral- an electrical connection in which an inductive
reactance
is inserted between neutral and ground
Z
Capacitance Grounded Neutral- an electrical connection in which a capacitor is
inserted between neutral and ground
IP
August 20, 2010
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Effective Grounding- grounding such that the steady-state operating voltage on the
healthy phases of the power system during a single-line-to-ground fault will not
exceed 140% of the open-circuit line-to-neutral RMS voltage.
and
Both must be met
“Since power sources are fewer in number than
loads and are less likely to be disconnected, they
are preferred as grounding points.”
- Industrial Power Systems Handbook
General Electric Company ©
Donald Beeman, editor
August 20, 2010
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Forms of
Neutral
Grounding
-G Fault
Magnitude
Transient
Overvoltages
Arrester
Applications
(% of VL-L)
System
Protection
Selectivity
Comments
Ungrounded
0
Very high
100%
None
Not Recommended
Solidly
 I3
< 140%
80%
Generally
Good
Common at high voltages
and low voltages
Low
Resistance
100 – 1200 A
Not Excessive
100%
Generally
Good
Common at medium
voltages
High
Resistance
2 – 10 A
Not Excessive
100%
Requires
Special
Equipment
Alarm application for
continuity must trip >5kV
Reactive
< I3
> 0.125 I3
Not Excessive
100%
Generally
Good
Special case – rarely need
Resonant
0
Not Excessive
100%
Special
Treatment
Special case – very rarely
need
Capacitive
 I3
High
100%
Generally
Good
Special case – very rarely
need
August 20, 2010
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or
Ungrounded
or
Solidly
Grounded
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Low
Resistance
XL
Reactance
High 
Low 
High
Resistance
Ground
R
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Resonant Neutral
Grounding
A
A
B
B
C
C
Capacitive Neutral
Grounding
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B
A
B
C
400 A
10 sec
R
A
B
400 A
10 sec
C
R
C
B
B
A
A
7970
7970
C
VR=0
C
Effectively
Grounded
VR = 7970 Volts
If V = IR, and you wish to limit
I to 400 Amps, 7970 = 400 R
R = 19.9 
A
Resistor must be insulated at
One terminal for 8000 V and to
pass 400 Amps for 10 seconds
without damage
B
7970
VR
C
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For Low
Resistance
Resistors
ANG80-4
13800
8000
400
20
46
60
76
900
Neutral Grounding Resistor
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Let’s limit ground fault
current to 10 amps
7970:240 V
Grounding
Transformer
Impedance Transformer
Effectively
grounded under
normal conditions
August 20, 2010
If IG is 10 amps, then the power through
the transformer is:
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Power in the secondary is the same:
Transformer spec : 75 kVA or 100 kVA
7970 : 120 V
Resistor spec:
0.723  Rated for
240 VAC operation for 10 seconds
IR
IG
IG
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- Low voltage or 5 kV process
plant distribution
- Delta winding transformers
- Generators
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Scott-T Connection
1
3
Zig-Zag
H3
2
3
H2, 2
-Small, lightweight
-Economical
-Off-the-shelf in common ratings
-Not practical for unusual applications
(i.e. voltage, frequency, current levels)
-Limited to Low-R applications
X2
Y -  Transformer
X3
-Standard transformer
X1
-Applications are readily
H1,1
field-designed
H3,3
-Can be used for any grounding mode
-Grounding resistor can be inside delta
if single phase units are used in Hi-R scheme
-Offers option for redundant backup protection
X2
H2
-Custom designed
X1
-Can fit any application
X3
1

2
H1
-Can be designed to
provide full reactive
limitation with no external impedance
-Can be used for effectively grounded system
Potential Transformers
X2
X2
H2, 2
X3
-Usually required
for metering
-Economical
-Thermal ratings suitable for X1
H1,1
H3,3
-highly restricted schemes only
-Application may not provide desired limitation of
transient overvoltages because the grounded wye
winding is high impedance
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13800 V
13800 V
This wye-delta transformer connection doesn’t limit fault current,
except the winding impedance of the grounding transformer.
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13800 V
13800 V
Low resistance ground fault
limiting – same type calculations as before.
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Legend
AM- Ammeter
CPB- Control Power Breaker
CR- Main Contactor
CT- Current Transformer
HR- Horn Relay
HRX- Auxiliary Relay
MR- Meter Relay
PR- Pulsing Relay
PT- Potential Transformer
R3- Fault Time Delay
R4- Pulsing Adjustment
TR- Timing Relay
UV- Undervoltage
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Placing a Resistor in the transformer secondary will limit the primary
ground fault current.
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B
H2
X4
X2
R= 0.106
A
X1
X6
X5
H1
H0
H3
C
X3
Transformer ratio is 23900 GRY – 120 V delta. This is 13800 V to
ground on the primary, and is the voltage on each winding.
The secondary voltage of 120 V is the voltage across
each winding.
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V=0
120
13800
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Under non-faulted balanced system conditions, the voltage at
the corner delta = 0
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13800
When one phase suffers a bolted fault,
a phase for instance, the voltage vectors
change:
B
C
The resulting voltage at the delta corner rises to 208 V.
For a 0.106  Resistor with 208 V across it, the current through it
is 1963 Amps.
(The resistor is rated 208 V, 1960 Amps, 10 sec.)
Ours is actually a 13800 V L-L system:
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69.3 V
7970
B
C
The resulting voltage across the resistor is 120 V, and the
0.106  resistor has 1,132 Amps through it.
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69.3 V
7970
B
C
This system is a high impedance grounding system that limits
current to  10 Amps.
The resulting voltage across the resistor is 120 V, and the
0.106  resistor has 1,332 Amps through it.
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The relay settings for the job were:
300 Amp pickup on the secondary side, which equates to 2.6 Amp
primary. 50 V setting on the 59 G relay is equal to
Voltage element is
much more
sensitive than
current element
A voltage element, looking at voltage across the resistor, and a
current element, looking at current through the resistor, are used
in conjunction for redundant ground fault detection.
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High Voltage Bus
•Normal central station practice – no
generator breaker
•GSU neutral not required
on generator side
•Saves cost of startup transformer
•Availability of suitable breakers
•GSU neutral grounding required
on generator side
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Low Voltage
(< 1000 volts)
Medium Voltage
(through 15kV)
High Voltage
(> 15kV)
1. Solid Grounding
2. High Resistance
1. Low resistance grounding
2. High resistance grounding
3. Effective grounding
1. Effective grounding
(at the source)

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