Application Considerations for Power System Grounding
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Transcript of Application Considerations for Power System Grounding
Application ConsiderationsFor
Power System Grounding
To Groundor Not to Ground
IS NOT the Question.
The real question is ----
What’s the best grounding
method for this application ?
Power Systems Grounding is Probably the most misunderstood
element of any power system design.
There have been two accepted methods of grounding
Solidly Grounded
Ungrounded
Recently, However, we have alternatives.
Resistance Grounded
Reactance Grounded
Principles of Grounding
Solid Grounding
Ungrounded SystemHistorically 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
Let’s look at fiction and factTHEORETICAL
In theory, the ungrounded system has absolutely no connection to ground
HOWEVER
Reality is a little differentACTUAL
There is a connection to ground through the capacitive reactance of the insulation.
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.
Real Life Case
Real Life Case
Real Life Case
Ungrounded Medium Voltage Systems
• Voltage escalation during arcing ground faults will result in catastrophic failures
TVSS Failure Due to ArcingGround Fault
MOTOR STARTER FAILURE DUE TO ARCING GROUND FAULT
MOTOR STARTER FAILURE DUE TO ARCING GROUND FAULT
SWGR TAP BOX FAILURE DUE TO ARCING GROUND FAULT
ESP MOTOR FAILURE DUE TO ARCING GROUND FAULT
ESP MOTOR WYE POINT FAILURE DUE TO ARCING GROUND FAULT
Now lets look at resistance grounding
The ground fault current is limited to a small magnitudehundreds of amperes with low resistance
10 amperes or less at high resistance
High Resistance GroundingLow fault current means no significant damage at the fault pointThis 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…..
And that is a good thing.
In this way, it performs just like an ungrounded system...
Same Real Life Case
What’s the best method of grounding ?
Lets ask two questions1. 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
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.
What if no neutral point is available?No Problem
OK , I need to install high resistance grounding, but we
have some feeders with line-to-neutral loads… What’s the
procedure?
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 loadsInstall the high resistance grounding retrofit equipment….
That’s all there is to it.
Low voltage high resistance grounding system looks like this.
AM ControlCircuit
G WR
59NGroundingResistor
PulserResistor
TestResistor
G indicates Green Light - Normal ConditionR indicates Red light - Ground Fault ConditionW indicates White Light - Pulser Resistor Operation Condition
480 Volt
High resistance grounding looks like this on a 4160V system.
G indicates Green Light - Normal ConditionR indicates Red light - Ground Fault ConditionW indicates White Light - Pulser Resistor Operation Condition
2.4 KV or 4.16 KV
GroundingResistor59N Pulser
Resistor
ControlCircuit
G WR
120 V, 60HzSupply
Resistor
If it is necessary to create a neutral at 4160, this works!
G indicates Green Light - Normal ConditionR indicates Red light - Ground Fault ConditionW indicates White Light - Pulser Resistor Operation Condition
2.4KV or 4.16 KV
59NResistorPulser
Resistor
ControlCircuit
G WR
120 V, 60HzSupply
The Zigzag transformer will work also.
ControlCircuit
G WR
TestResistor
G indicates Green Light - Normal ConditionR indicates Red light - Ground Fault ConditionW indicates White Light - Pulser Resistor Operation Condition
480 Volt
AM
59N GroundingResistor
PulserResistor
ZIG ZAGTRANSFORMER
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
How about training of personnel?
You BET!
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
Application Example
Application Example
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
Table III:System Characteristics With Various Grounding Methods
Ungrounded Essentially solid grounding Reactancegrounding
Ground-faultneutralizer
Resistance Grounding
Solid Low-value reactor High-value reactor Lowresistance
Highresistance
Current forphase-to-ground faultin percent ofthree-phasefault current
Less than 1% Varies, may be100% or greater
Usually designedto produce 60 to100%
5 to 25% Nearly zero faultcurrent
5 to 20% Less than1%
Transientover-voltages
Very high Not excessive Not excessive Very high Not excessive Not excessive Notexcessive
Automaticsegregationof faultedzone
No Yes Yes Yes No Yes No
Lightningarresters
Ungroundedneutral type
Grounded-neutraltype
Grounded-neutraltype if current is60% or greater
Ungroundedneutral type
Ungroundedneutral type
Ungroundedneutral type
Ungrounded neutratype
Remarks Notrecommendeddue to overvoltages andnonsegregation offault
Generally used on system (1) 600 voltsand below and (2) over 15kV
Not used due toexcessive over-voltages
Best suited forhigh-voltage over-head lines wherefaults may be self-healing
Generally used onindustrial systemsof 2.4 to 15kV
Generallyused onsystems5kV andbelow
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
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
Hybrid High Resistance Grounding HHRG
• A combination of high and low resistance grounding
• Applicable to medium voltage generators, motors and transformers
Grounding and Ground Fault Protection of Multiple Generator Installations on Medium-Voltage Industrial and Commercial Systems
a Protection Committee Working Group Report
Prafulla Pillai, chairAlan PierceBruce BaileyBruce DouglasCharles MozinaClifford Normand
Daniel LoveDavid BakerDavid ShippGerald DalkeJames R. JonesJay D. Fischer
Jim BowenLorraine PaddenLouie J. PowellNeil NicholsRalph YoungNorman T. Stringer
M.V.-MULTIPLE LOW RESISTANCE GROUND SOURCES [Author = Powell]
Damaged Area – Wedges Removed
GENERATOR WINDING FAILURE
Stator End-Turns Showing Wedge Movement
CORE DAMAGE
TYPICAL GENERATOR GROUND FAULT [Author = Powell]
400a400a
13.8kV distribution bus
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
Faul
t ene
rgy ,
wat
t- se c
ond s
Fau l
t en e
r gy,
wa t
t -sec
o nds
Current, amperes Time, seconds
• FIG.7A-ENERGY DUE TO “SYSTEM” -VARIOUS CURRENT MAGNITUDES
• FIG.7B-ARC ENERGY FOR 400A R.”SYSTEM”
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 4000
2500
5000
7500
1 104
Generator
0.01 0.1 1 100
2500
5000
7500
1 104
Generator
Faul
t ene
rgy,
wat
t-sec
onds
Faul
t ene
rgy,
wat
t-sec
onds
Current, amperes Time, seconds
FAULT ENERGY WITH 10A GROUNDING [Author = Powell]
Generator
Faul
t ene
rgy,
wat
t-sec
onds
0.01 0.1 1 100
25
50
75
100
Time, seconds
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
HIGH RESISTANCE GROUNDED
Low Transient OvervoltagesLess than 10 ampsMinimal DamageDon’t Know Where GF is.Continuous Operation?System or Generator Only?
GENERATOR SOLUTIONS - HYBRID
• GEN. H. R. GROUNDED AND SYSTEM L. R. GROUNDED [Author=Shipp]
G 59G51G
LRG86
R
HRG
* PHASE RELAYS
*
Thank You