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