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Medium and Low Voltage Grounding Methods
THE ASSET COMPANY
The ASSET Company
August 20, 2010
Establish a voltage relationship between the system neutral and ground.
•Overvoltage protection for system component insulation•Controlled single-phase-to-ground fault current magnitude
Establish a voltage relationship between energized phase conductors and ground.
System Grounding
The ASSET Company
August 20, 2010
Establish a consistent reference plane for all system components.
•Personnel safety•Optimum single-line-to-ground fault current distribution•Safe conduction of lightning discharge currents•a.k.a. bonding
Equipment Grounding
The ASSET Company
August 20, 2010
What’s the big deal?
Cable Sheath
Neutral Wire
Water Pipes
Building Steel
lg
Earth Potential Gradients – a Personnel Grounding Safety IssueHuman Physiological Response:
1 ma – threshold of sensation6-9 ma “let go” currents9-25 ma – muscular contraction60-100 ma – ventricular fibrillation
Body resistance: 5000 ohms or higher
Perceptable gradient voltage: 50 volts & above
Harmful gradients: 375 volts and above
Controlling factors
o Magnitude of Potential Gradients- magnitude of ground fault current phase-neutral voltage complexity of return patho Duration of Potential Gradients- protective device settings fuse, relay, & breaker operation
Z
The ASSET Company
August 20, 2010
What’s the big deal?
Cable Sheath
Neutral Wire
Water Pipes
Building Steel
Ig
Earth Potential Gradients – Equipment and Insulation SurvivabilityDistribution of Ig:
Kaufmann’s work showed that 90-95% of the fault current will return through the cable sheath and/or neutral wire
Ig = IC + IN + IP + IS
Example: Ig = 20000 AIS = 0.05 x 20000 = 1000 AFor 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
IC
IN
IP
IS
Z
The ASSET Company
August 20, 2010
What’s the big deal?
Cable Sheath
Neutral Wire
Water Pipes
Building Steel
Ig
Really Only a Large Issue at System Voltages Higher than 1000 VACConclusions:
Ineffective grounding at any voltage sets the stage for personnel injury or death.
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.
Ineffective grounding at higher voltages can set the stage for potential gradient shocks which are severe enough to distract personnel in the workspaceIC
IN
IP
IS
Z
The ASSET Company
August 20, 2010
Cable Sheath
Neutral Wire
Water Pipes
Building Steel
IgIneffective grounding at any voltage sets the stage for personnel injury or death.
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.
Effective grounding at higher voltages can set the stage for potential gradient shocks which are severe enough to distract personnel in the workspaceIC
IN
IP
IS
Z
Working Definitions (Inexact):
System Neutral Ground- an intentional electrical connection between the neutral of the power system and groundGrounded System- a system in which one conductor, usually the neutral, is intentionally connected to ground.Ungrounded System- a system in which none of the electrical conductors is intentionally connected to ground
Solidly Grounded Neutral- a direct electrical connection between the neutral and ground with no added impedanceResistance Grounded Neutral- an electrical connection in which a resistor is inserted between neutral and groundReactance Grounded Neutral- an electrical connection in which an inductive reactance is inserted between neutral and groundCapacitance Grounded Neutral- an electrical connection in which a capacitor is inserted between neutral and ground
Note: There is an inherent distributed capacitance between each conductor and ground. Hence, an “ungrounded system” is really capacitively grounded
ss s
The ASSET Company
August 20, 2010
Exact Definitions (applies to surge arrester applications)
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
- Industrial Power Systems Handbook General Electric Company ©
Donald Beeman, editor
“Since power sources are fewer in number than loads and are less likely to be disconnected, they are preferred as grounding points.”
Forms of Neutral
Grounding
-G Fault Magnitude
Transient Over-
voltages
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
Medium Voltage System Grounding
The ASSET Company
August 20, 2010
or or
Ungrounded Solidly Grounded
The ASSET Company
August 20, 2010
System Grounding Options
The ASSET Company
August 20, 2010
Solidly Grounded Generator
Low Resistance
High
Reactance
HighResistance Ground
Low
XL R
The ASSET Company
August 20, 2010
Grounding Options
A
B
C
A
B
C
Resonant Neutral Grounding
Capacitive Neutral Grounding
The ASSET Company
August 20, 2010
Two grounding methods you will probably never see.Let’s Dispense with
The ASSET Company
August 20, 2010
Low Resistance Grounding Details
R
AB
C400 A10 sec
VR=0Effectively Grounded
R
AB
C400 A10 sec
VR = 7970 VoltsIf V = IR, and you wish to limitI to 400 Amps, 7970 = 400 RR = 19.9 Resistor must be insulated at One terminal for 8000 V and to pass 400 Amps for 10 seconds without damage
7970
B
C
7970
7970
7970
A
B
C
7970
7970
7970
A
B
C VR
7970
7970
7970
A
B
C
The ASSET Company
August 20, 2010
Standard
Ratings
For Low Resistance
Resistors
ANG80-4 13800 8000 400 20 46 60 76 900
IBM Substation
Neutral Grounding Resistor
The ASSET Company
August 20, 2010
Impedance Transformer
Effectively grounded under
normal conditions
High Resistance Grounding
7970:240 V Grounding
Transformer
Let’s limit ground fault current to 10 amps
The ASSET Company
August 20, 2010
If IG is 10 amps, then the power through the transformer is:
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
IG IG
IR
The ASSET Company
August 20, 2010
System Sources that are Ungrounded?What about…
- Low voltage or 5 kV process plant distribution- Delta winding transformers- Generators
The ASSET Company
August 20, 2010
Neutral Deriving TransformersScott-T Connection
- 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
Y - Transformer
- Standard transformer- Applications are readily
field-designed- Can be used for any grounding mode
- Grounding resistor can be inside deltaif single phase units are used in Hi-R scheme
- Offers option for redundant backup protection
Zig-Zag
Potential Transformers
-Custom designed-Can fit any application-Can be designed to provide full reactive limitation with no external impedance-Can be used for effectively grounded system
-Usually required for metering-Economical -Thermal ratings suitable for -highly restricted schemes only-Application may not provide desired limitation of transient overvoltages because the grounded wye winding is high impedance
H1,1
H2, 2
H3,3
X2
X1
X3
X2
X1
X3
H1,1
H2, 2
H3,3
1
3
2
X2
X3H1
X1
H2
12
H3
X2
3
This wye-delta transformer connection doesn’t limit fault current, except the winding impedance of the grounding transformer.
The ASSET Company
August 20, 2010
13800 V
13800 V
Low resistance ground fault limiting – same type calculations as before.
The ASSET Company
August 20, 2010
13800 V
13800 V
LegendAM- AmmeterCPB- Control Power BreakerCR- Main ContactorCT- Current TransformerHR- Horn RelayHRX- Auxiliary RelayMR- Meter RelayPR- Pulsing RelayPT- Potential TransformerR3- Fault Time DelayR4- Pulsing AdjustmentTR- Timing RelayUV- Undervoltage
The ASSET Company
August 20, 2010
Schematic DiagramHigh Resistance Grounding
Placing a Resistor in the transformer secondary will limit the primary ground fault current.
The ASSET Company
August 20, 2010
With a Secondary Resistor…Neutral Deriving Transformer
The ASSET Company
August 20, 2010
H0
H3
H2
A C
B
H1
R= 0.106X1
X6
X4
X5
X3
X2
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 acrosseach winding.
The ASSET Company
August 20, 2010
Under non-faulted balanced system conditions, the voltage at the corner delta = 0
13800 13800
1380
0
V = 0
120
120
120
The ASSET Company
August 20, 2010
B
When one phase suffers a bolted fault, a phase for instance, the voltage vectors change:
1380
0
13800
C
v
v
208 V
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.)
The ASSET Company
August 20, 2010
120 VB
7970
7970
C
v
v
The resulting voltage across the resistor is 120 V, and the 0.106 resistor has 1,132 Amps through it.
v
v
13800 V
69.3 V
69.3
V
Ours is actually a 13800 V L-L system:
The ASSET Company
August 20, 2010
120 VB
7970
7970
C
v
v
v
v
13800 V
69.3 V
69.3
V
The resulting voltage across the resistor is 120 V, and the 0.106 resistor has 1,332 Amps through it.
This system is a high impedance grounding system that limits current to 10 Amps.
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
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.
Voltage element is much more sensitive than current element
The ASSET Company
August 20, 2010
The ASSET Company
August 20, 2010
Typical Power Plant Grounding
Captive Transformer
High Voltage Bus
Generator Breaker•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
The ASSET Company
August 20, 2010
Low Voltage(< 1000 volts)
Medium Voltage(through 15kV)
High Voltage(> 15kV)
1. Solid Grounding2. High Resistance
1. Low resistance grounding2. High resistance grounding3. Effective grounding
3. Effective grounding(at the source)
System Grounding Summary
Voltage Level Grounding Preferences
