This document discusses various grounding methods for power systems, including solid grounding, ungrounded systems, resistance grounding, and reactance grounding. It provides examples of how each method works and considerations for selecting a grounding approach based on factors like service continuity requirements and the presence of line-to-neutral loads. The document also reviews retrofitting existing systems and includes diagrams demonstrating different grounding configurations.

1. Application Considerations
For
Power System Grounding

2. To Ground
or Not to Ground

3. IS NOT the Question.

4. The real question is ----
What’s the best grounding
method for this application ?

5. Power Systems Grounding is
Probably the most misunderstood
element of any power system
design.

6. There have been two accepted
methods of grounding
Solidly Grounded
Ungrounded

7. Recently, However, we have
alternatives.
Resistance Grounded
Reactance Grounded

8. Principles of Grounding

10. Solid Grounding

11. Ungrounded System
Historically selected for systems where service
continuity is of utmost importance.
However, arcing ground faults force us to
consider a few things!
Multiple ground faults Transient overvoltages
Resonant conditions

12. Let’s look at fiction and fact
THEORETICAL
In theory, the ungrounded system has
absolutely no connection to ground
HOWEVER

13. Reality is a little different
ACTUAL
There is a connection to ground
through the capacitive reactance of
the insulation.

14. This leads to transient
overvoltges
Energy exchange between system
inductance and shunt capacitance to
ground results in some high
voltages with respect to ground
when an arcing ground fault exists.

15. Real Life Case

16. Real Life Case

17. Real Life Case

18. Ungrounded Medium Voltage
Systems
• Voltage escalation during arcing ground
faults will result in catastrophic failures

19. TVSS Failure Due to Arcing
Ground Fault

20. MOTOR STARTER
FAILURE DUE TO
ARCING GROUND
FAULT

21. MOTOR STARTER FAILURE DUE TO ARCING
GROUND FAULT

22. SWGR TAP BOX FAILURE DUE TO ARCING
GROUND FAULT

23. ESP MOTOR FAILURE DUE TO ARCING
GROUND FAULT

24. ESP MOTOR WYE POINT FAILURE
DUE TO ARCING GROUND FAULT

25. Now lets look at resistance
grounding
The ground fault current is limited
to a small magnitude
hundreds of amperes with low
resistance
10 amperes or less at high resistance

26. High Resistance Grounding
Low fault current means no
significant damage at the fault point
This means that the faulted circuit
need not to be tripped off line.
This also means it is likely that the
location of the ground fault is
unknown….
No smoke and fire…..

27. And that is a good thing.
In this way, it performs just like an
ungrounded system...

28. Same Real Life Case

29. What’s the best method of
grounding ?
Lets ask two questions
1. Are there any line - to - neutral loads?
2. How important is service continuity for this
electrical system?
YES…. Solid grounding may be good, but high
resistance is still an option.
No…… Solid grounding is still an option, but
high resistance is easier
Not very… solid grounding is great!
EXTREMELY!!! High resistance is BEST

30. What considerations are
necessary to retrofit a solidly
grounded system to a high
resistance grounded system?
What is the system voltage?
Are there any line-to-neutral loads?
Is there space for a cabinet -
usually 90” high and 18” wide?
If not, that’s ok, there’s always a
wall mount cabinet.

31. What if no neutral point is
available?
No Problem

32. OK , I need to install high
resistance grounding, but we
have some feeders with line-to-
neutral loads… What’s the
procedure?

33. Measure the load on each feeder & determine the
maximum load.
Select a delta-wye isolation transformer sized to
support the load on each feeder with line-to-neutral
loads.
Install the transformers near the distribution panel
supplying the loads
Install the high resistance grounding retrofit
equipment….
That’s all there is to it.

34. Low voltage high resistance
grounding system looks like this.
AM
Control
Circuit
G WR
59N
Grounding
Resistor
Pulser
Resistor
Test
Resistor
G indicates Green Light - Normal Condition
R indicates Red light - Ground Fault Condition
W indicates White Light - Pulser Resistor Operation Condition
480 Volt

35. High resistance grounding looks
like this on a 4160V system.
G indicates Green Light - Normal Condition
R indicates Red light - Ground Fault Condition
W indicates White Light - Pulser Resistor Operation Condition
2.4 KV or 4.16 KV
Grounding
Resistor59N Pulser
Resistor
Control
Circuit
G WR
120 V, 60Hz
Supply
Resistor

36. If it is necessary to create a
neutral at 4160, this works!
G indicates Green Light - Normal Condition
R indicates Red light - Ground Fault Condition
W indicates White Light - Pulser Resistor Operation Condition
2.4KV or 4.16 KV
59N
Resistor
Pulser
Resistor
Control
Circuit
G WR
120 V, 60Hz
Supply

37. The Zigzag transformer will
work also.
Control
Circuit
G WR
Test
Resistor
G indicates Green Light - Normal Condition
R indicates Red light - Ground Fault Condition
W indicates White Light - Pulser Resistor Operation Condition
480 Volt
AM
59N
Grounding
Resistor
Pulser
Resistor
ZIG ZAG
TRANSFORMER

38. A quick review of considerations
for the application of high
resistance ground retrofits.
1.Requirement for continuity of service
2. Capacitive leakage current less than
current through resistor
3. System line-line voltage less than 4160
4. All loads must be balanced 3 phase

39. How about training of personnel?
You BET!

40. Effectively Grounded Systems
• Uses a Reactance for Grounding
• Limit the ground fault current to a value
equal to the three phase fault current
• X0/X1 < 3.0

41. Application Example

42. Application Example

43. Low Resistance Grounding
• Suitable for Medium Voltage Motors and
Generators
• Ground Fault Protection is achieved with zero
sequence CTs
• Limit the ground fault current to acceptable values
100-1200 A
• R0/X0 > 2.0
• X0/X1 < 20

44. Table III:System Characteristics
With Various Grounding Methods
Ungrounded Essentially solid grounding Reactance
grounding
Ground-fault
neutralizer
Resistance Grounding
Solid Low-value reactor High-value reactor Low
resistance
High
resistance
Current for
phase-to-
ground fault
in percent of
three-phase
fault current
Less than 1% Varies, may be
100% or greater
Usually designed
to produce 60 to
100%
5 to 25% Nearly zero fault
current
5 to 20% Less than
1%
Transient
over-voltages
Very high Not excessive Not excessive Very high Not excessive Not excessive Not
excessive
Automatic
segregation
of faulted
zone
No Yes Yes Yes No Yes No
Lightning
arresters
Ungrounded
neutral type
Grounded-neutral
type
Grounded-neutral
type if current is
60% or greater
Ungrounded
neutral type
Ungrounded
neutral type
Ungrounded
neutral type
Unground
ed neutra
type
Remarks Not
recommended
due to over
voltages and
nonsegregation of
fault
Generally used on system (1) 600 volts
and below and (2) over 15kV
Not used due to
excessive over-
voltages
Best suited for
high-voltage over-
head lines where
faults may be self-
healing
Generally used on
industrial systems
of 2.4 to 15kV
Generally
used on
systems
5kV and
below

45. Practicable System Grounding
Selections
• L. Voltage (< 1000V)
– Solid
– H. Resistance
• H. Voltage (>15 KV)
– Solid
• M. Voltage
– Solid
• 3 PH / 4 W
• Aerial Lines
• Unshielded Cables

46. Practicable System Grounding
Selections
• Medium Voltage
– L. Resistance
• Motors / Generators
• Shielded Cables
• No VLN Loads
• Medium Voltage
– H. Resistance
• < 5 KV-No Tripping
• > 5 KV - Tripping
• No VLN Loads

47. Hybrid High Resistance
Grounding HHRG
• A combination of high and low resistance
grounding
• Applicable to medium voltage generators,
motors and transformers

48. Grounding and Ground Fault Protection of
Multiple Generator Installations on Medium-
Voltage Industrial and Commercial Systems
a Protection Committee Working Group Report
Prafulla Pillai, chair
Alan Pierce
Bruce Bailey
Bruce Douglas
Charles Mozina
Clifford Normand
Daniel Love
David Baker
David Shipp
Gerald Dalke
James R. Jones
Jay D. Fischer
Jim Bowen
Lorraine Padden
Louie J. Powell
Neil Nichols
Ralph Young
Norman T. Stringer

49. M.V.-MULTIPLE LOW RESISTANCE
GROUND SOURCES [Author = Powell]

50. Damaged Area – Wedges Removed

51. GENERATOR WINDING FAILURE

52. Stator End-Turns Showing Wedge
Movement

53. CORE DAMAGE

54. TYPICAL GENERATOR GROUND FAULT
[Author = Powell]
400a
400a
13.8kV distribution bus

55. SYSTEM ARC ENERGY
[Author = Powell]
0 100 200 300 400
0
250
500
750
1000
System
1 10
3
0.01 0.1 1 10
0
250
500
750
1000
System
Faultenergy,watt-seconds
Faultenergy,watt-seconds
Current, amperes Time, seconds
• FIG.7A-ENERGY DUE TO “SYSTEM” -
VARIOUS CURRENT MAGNITUDES
• FIG.7B-ARC ENERGY FOR 400A
R.”SYSTEM”

56. GEN. ARC ENERGY
[Author = Powell]
• FIG. 8B-FAULT ENERGY FOR 400A
“GEN CURRENT”
• FIG. 8A-FAULT ENERGY DUE TO
“GEN.” FOR VARIOUS CURRENTS
0 100 200 300 400
0
2500
5000
7500
1 10
4
Generator
0.01 0.1 1 10
0
2500
5000
7500
1 10
4
Generator
Faultenergy,watt-seconds
Faultenergy,watt-seconds
Current, amperes Time, seconds

57. FAULT ENERGY WITH 10A GROUNDING
[Author = Powell]
Generator
Faultenergy,watt-seconds
0.01 0.1 1 10
0
25
50
75
100
Time, seconds

58. LOW RESISTANCE GROUNDED
Advantages;
– Allows for Sensitive Grd Fault Current Available for
Sensitive & Selective Relaying.
– Greatly Minimizes Damage at Fault Point
Disadvantages
– Possibility of Generator Stator Iron Burning
– Hi G. F. Current Avail. With Multiple Sources
– Large Variations of Available Ground Fault Current
Complicates Ground Fault Relaying

59. HIGH RESISTANCE GROUNDED
Low Transient
Overvoltages
Less than 10 amps
Minimal Damage
Don’t Know Where GF is.
Continuous Operation?
System or Generator
Only?

60. GENERATOR SOLUTIONS - HYBRID
• GEN. H. R. GROUNDED AND SYSTEM L. R. GROUNDED
[Author=Shipp]
G 59G
51G
LRG
86
R
HRG
* PHASE RELAYS
*

61. Thank You