Switchgear_and_Protection_unit_7.pdf

7/27/2019 Switchgear_and_Protection_unit_7.pdf

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Unit-VII

[Neutral Grounding]

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7.1 What is Grounding?

The term grounding is commonly used in the electrical industry to mean both

equipment grounding and system grounding. Equipment grounding means the

connection of a non-current carrying conductive materials such as conduit, cable trays,

junction boxes, enclosures and motor frames to earth ground. System grounding

means the connection of the neutral points of current carrying conductors such as the

neutral point of a circuit, a transformer, rotating machinery, or a system, either solidly or

with a current limiting device to earth ground. Figure 1 illustrates the two types of

grounding.

7.2 What is a Grounded System?

Grounded System a system with at least one conductor or point (usually the

middle wire or neutral point of transformer or generator windings) is intentionally

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7.3 What are the Different Types of System Grounding?

The types of system grounding normally used in industrial and commercial powersystems are:

1) Solid grounding2) Low resistance grounding3) High resistance grounding4) Ungrounded IEEE Std 242-2001 8.2.1

7.4 What is the Purpose of System Grounding?

System grounding, or the intentional connection of a phase or neutral conductor

to earth, is for the purpose of controlling the voltage to earth, or ground, within

redictable limits. It also provides for a flow of current that will allow detection of an

unwanted connection between system conductors and ground [a ground fault].

7.5 What is a Ground Fault?

A Ground Fault is an unwanted connection between the system conductors and ground.

7.6 Why is Ground Faults a Concern?

Ground faults often go unnoticed and can cause problems with plant production

processes. They can also shut down power and damage equipment, which disrupts the

flow of production leading to hours or even days of lost productivity. Undetected ground

faults pose potential health and safety risks to personnel. Ground faults can lead to

safety hazards such as equipment malfunctions, fire and electric shock. Ground faults

cause serious damage to equipment and to your processes. This damage can seriously

affect your bottom line.

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Consequently, the ungrounded system is, in reality, a capacitively grounded system

by virtue of the distributed capacitance. This is shown in Figure 2.

Under normal operating conditions this distributed capacitance causes noproblems. In fact, it is beneficial because it establishes, in effect, a neutral point for the

system, as shown in Figure 3a. As a result, the phase conductors are stressed at only

line-to-neutral voltage above ground. However, problems can arise under ground fault

conditions. A ground fault on one line results in full line-to-line voltage appearing on the

other two phases. Thus, a voltage 1.73 times the normal voltage is present on all

insulation on the ungrounded phase, as shown in Figure 3b. This situation can often

cause failures in motors and transformers, due to insulation breakdown.

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7.8 What Does IEEE Say About Ungrounded Systems? Ungrounded systems employ ground detectors to indicate a ground fault. These

detectors show the existence of a ground on the system and identify the faulted phase,

but do not locate the ground, which could be anywhere on the entire system. If this

ground fault is intermittent or allowed to continue, the system could be subjected to

possible severe over-voltages to ground, which can be as high as six or eight times

phase voltage. This can puncture insulation and result in additional ground faults. Asecond ground fault occurring before the first fault is cleared will result in a phase-

toground- to-phase fault, usually arcing, with a current magnitude large enough to do

damage, but sometimes too small to activate over-current devices in time to prevent or

minimize damage.

Ungrounded systems offer no advantage over high-resistance grounded systems

in terms of continuity of service and have the disadvantages of transient over-voltages,

locating the first fault and burn downs from a second ground fault. IEEE 242-2001 8.2.5

7.9 Why Consider Grounding Your System?

If the ground fault is intermittent (arcing, restriking or vibrating), then severe

overvoltages can occur on an ungrounded system. The intermittent fault can cause the

system voltage to ground to rise to six or eight times the phase-to-phase voltage leading

to a breakdown of insulation on one of the unfaulted phases and the development of a

phase-to-ground-to-phase fault. Over-voltages caused by intermittent faults can beeliminated by grounding the system neutral through an impedance, which is generally a

resistance, which limits the ground current to a value equal to or greater than the

capacitive charging current of the system.

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7.10 What is a Resistance Grounded System?

The intentional connection of the neutral points of transformers, generators androtating machinery to the earth ground network provides a reference point of zero volts.

This protective measure offers many advantages over an ungrounded system,

including:

Reduced magnitude of transient over-voltages

Simplified ground fault location Improved system and equipment fault protection

Reduced maintenance time and expense

Greater safety for personnel

Improved lightning protection

Reduction in frequency of faults

There are two broad categories of resistance grounding: low resistance and high

resistance. In both types of grounding, the resistor is connected between the neutral of

the transformer secondary or generator winding and the earth ground.

7.11 What is a Low Resistance Grounded System?

Low resistance grounding of the neutral limits the ground fault current to a high

level (typically 50 amps or more) in order to operate protective fault clearing relays and

current transformers. These devices are then able to quickly clear the fault, usually

within a few seconds. The importance of this fast response time is that it:

Limits damage to equipment

Prevents additional faults from occurring

Provides safety for personnel

L li th f lt

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7.12 What is a High Resistance Grounded System?

IEEE Standard 142-1991, Recommended Practice for Grounding of Industrialand Commercial Power Systems (Green Book), defines a high resistance grounded

system as follows:

A grounded system with a purposely inserted resistance that limits ground-fault

current can flow for an extended period without exacerbating damage. This level ofcurrent is commonly thought to be 10A or less. High-resistance grounded systems are

designed to meet the criteria RoXco to limit the transient over voltages due to arcing

ground faults. Ro is the per phase zero sequence resistance of the system and Xco is

the distributed per phase capacitive reactance-to-ground of the system.

7.13 Why Consider High Resistance Grounding?

High resistance grounding solves the problem of transient over-voltages, thereby

reducing equipment damage. Over-voltages caused by intermittent (arcing) faults can

be held to phase-to-phase voltage by grounding the system neutral through a resistance

which limits the ground current to a value equal to or greater than the capacitive

charging current of the system. Thus, the fault current can be limited in order to prevent

equipment damage and arc flash hazards. In addition, limiting fault currents to

predetermined maximum values permits the designer to selectively co-ordinate the

operation of protective devices, which minimizes system disruption and allows for quick

location of the fault.

7.14 Why Limit the Current Through Resistance Grounding?

The reason for limiting the current by resistance grounding may be one or more

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2. To reduce mechanical stresses in circuits and apparatus carrying fault currents.

3. To reduce electric-shock hazards to personnel caused by stray ground-fault currents

in the ground return path.

4. To reduce arc blast or flash hazard to personnel who may have accidentally caused

(or who happen to be in close proximity) to the ground fault.

5. To reduce the momentary line-voltage dip occasioned by the occurrence and clearing

of a ground fault.

6. To secure control of transient over-voltages while at the same time avoiding the

shutdown of a faulty circuit on the occurrence of the first ground fault.

7.15 What are the Requirements for Sizing the Resistor?

The line-to-ground capacitance associated with system components determines

the magnitudes of zero-sequence charging current. The resistor must be sized to

ensure that the ground fault current limit is greater than the systems total capacitance-

to-ground charging current. If not, then transient over-voltages can occur. The charging

current of a system can be calculated by summing the zero-sequence capacitance or

determining capacitive reactance of all the cable and equipment connected to the

system.

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GOOD GROUNDINGGOOD GROUNDING

PRACTICESPRACTICES

A Brief Introduction to the BasicsA Brief Introduction to the Basicsr r u r w rr r u r w rSystemsSystems

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n ro uc on o roun ngn ro uc on o roun ng

TABLE OF CONTENTS

.

2.0 Standard Industrial Grounding Methods and Types of Grounding

3.0 Grounding System and Design Considerations

4.0 Open Question and Answer Session

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n ro uc on o roun ngn ro uc on o roun ng The rimar oal of the roundin s stem throu hout an facilit isThe rimar oal of the roundin s stem throu hout an facilit is

SAFETY. Secondary are effective lightning protection, diminishingSAFETY. Secondary are effective lightning protection, diminishingelectromagnetic coupling (EMC), and the protection againstelectromagnetic coupling (EMC), and the protection againstelectromagnetic pulses (EMP).electromagnetic pulses (EMP).

roun ng s mp emente to ensure rap c ear ng o au ts an toroun ng s mp emente to ensure rap c ear ng o au ts an toprevent hazardous voltage, which in turn reduce the risks of fires andprevent hazardous voltage, which in turn reduce the risks of fires and

personnel injuries. Grounding serves the primary functions of referencingpersonnel injuries. Grounding serves the primary functions of referencingthe AC s stems and rovidin a means to ensure fault clearin .the AC s stems and rovidin a means to ensure fault clearin .

99.5% survival threshold99.5% survival threshold

116 mA for one (1) second.116 mA for one (1) second.

367 mA for zero point one (0.1) second.367 mA for zero point one (0.1) second.


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n ro uc on o roun ngn ro uc on o roun ng A fre uentl uoted criteria is the establishment of a one 1 ohmA fre uentl uoted criteria is the establishment of a one 1 ohm

resistance to earth. A large number of equipment manufacturers haveresistance to earth. A large number of equipment manufacturers havethis in their installation guides. The NEC requires only twentythis in their installation guides. The NEC requires only twenty--five (25)five (25)ohms of resistance for made electrodes, while the ANSI/IEEE Standardohms of resistance for made electrodes, while the ANSI/IEEE Standard

resistance of one (1) to five (5) ohms.resistance of one (1) to five (5) ohms.

External changes in the grounding system (environment) may effectExternal changes in the grounding system (environment) may effectthe ultimate functionalit of the entire electrical s stem.the ultimate functionalit of the entire electrical s stem.

Frequency matters in very complex grounding systems. LeakageFrequency matters in very complex grounding systems. Leakagecurrents of equipment do not return to the earth; high frequencycurrents of equipment do not return to the earth; high frequencyleakage currents return to the equipment which generated them, whileleakage currents return to the equipment which generated them, while

power frequency leakage currents return to the derived source.power frequency leakage currents return to the derived source. The impedance of the system is viewed from the perspective of powerThe impedance of the system is viewed from the perspective of power

frequencies and immediate harmonics (i.e., 60Hz and its associatedfrequencies and immediate harmonics (i.e., 60Hz and its associated


..

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n ro uc on o roun ngn ro uc on o roun ng

system wiring practices (i.e. no sharp bends or turns).system wiring practices (i.e. no sharp bends or turns). Grounding systems are not meant to last for ever. The best groundingGrounding systems are not meant to last for ever. The best grounding

quickest.quickest.


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GROUNDING SYSTEMS

There are basically six (6) grounding systems in use. The six (6) systems

are the equipment grounds, static grounds, systems grounds,

ma n enance groun s, e ec ron c groun s an g n ng groun s.

Equipment grounds: An equipment ground is the physical connection to earthof non-current carrying metal parts. This type grounding is done so that all

at or near zero (0) volts with respect to ground. All metal parts must be

interconnected and grounded by a conductor in such away as to ensure a

path of lowest impedance for flow of ground fault current. Typical itemsequ pmen o e groun e are; e ec r ca mo or rames, ou e oxes,

breaker panels, metal conduit, support structures, cable tray, to name a few.

Static grounds: A static ground is a connection made between a piece of

e ui ment and the earth for the ur ose of drainin off static electricit

charges before a flash over potential is reached. This type grounding systemis utilized in dry materials handling, flammable liquid pumps and delivery

equipment, plastic piping, and explosive storage facilities.


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Standard Industrial System Grounding MethodsStandard Industrial System Grounding Methods

Methods of System GroundingMethods of System GroundingCharacteristicsCharacteristics UngroundedUngrounded SolidSolid

GroundGroundLow ResistanceLow Resistance

GroundGroundHigh ResistancesHigh Resistances

GroundGround

Susceptible to Transient overvoltagesSusceptible to Transient overvoltages WORSTWORST GOODGOOD GOODGOOD BESTBEST

Under fault conditions (lineUnder fault conditions (line--toto--ground)ground)increase of voltage stressincrease of voltage stress

POORPOOR BESTBEST GOODGOOD POOR POOR

Arc Fault DamageArc Fault Damage WORSTWORST POOR POOR GOODGOOD BESTBEST

Personnel SafetyPersonnel Safety WORSTWORST POOR POOR GOODGOOD BESTBEST

ReliabilityReliability WORSTWORST GOODGOOD BETTER BETTER BESTBEST

Economics' (Maintenance costs)Economics' (Maintenance costs) WORSTWORST POOR POOR POOR POOR BESTBEST

P ant continues to operates un er sing eP ant continues to operates un er sing elineline--toto--ground faultground fault

FAIRFAIR POOR POOR POOR POOR BESTBEST

Ease of locating ground faults (time)Ease of locating ground faults (time) WORSTWORST GOODGOOD BETTER BETTER BESTBEST

System coordinationSystem coordination NOT POSSIBLENOT POSSIBLE GOODGOOD BETTER BETTER BESTBEST

Upgrade of ground systemUpgrade of ground system WORSTWORST GOODGOOD BETTER BETTER BESTBESTTwo voltage levels on same systemTwo voltage levels on same system NOT POSSIBLENOT POSSIBLE POSSIBLEPOSSIBLE NOT POSSIBLENOT POSSIBLE NOT POSSIBLENOT POSSIBLE

Reduction in number of faultsReduction in number of faults WORSTWORST BETTER BETTER GOODGOOD BESTBEST

Initial fault current Into round s stemInitial fault current Into round s stem BESTBEST WORSTWORST GOODGOOD BETTER BETTER

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Potential flashover to groundPotential flashover to ground POOR POOR WORSTWORST GOODGOOD BESTBEST

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Ungrounded System:Ungrounded System:

between the neutral or any phase and ground. Please note that anbetween the neutral or any phase and ground. Please note that anungrounded system is grounded through the concept ofungrounded system is grounded through the concept of

..system, with balanced loading will be close to ground potential duesystem, with balanced loading will be close to ground potential dueto the capacitance between each phase conductor and ground.to the capacitance between each phase conductor and ground.

Low round fault current.Low round fault current.

Very high voltages to ground potential on unfaulted phases.Very high voltages to ground potential on unfaulted phases.

Sustained faults lead to system lineSustained faults lead to system line--toto--line voltages on unfaultedline voltages on unfaultedline.line.

Insulation failure.Insulation failure.

Failure due to restrike ground faults.Failure due to restrike ground faults.

Continued o eration of facilit .Continued o eration of facilit .

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Solidly Grounded System:Solidly Grounded System:

to ground without an intentional impedance. In contrast to theto ground without an intentional impedance. In contrast to theungrounded system the solidly grounded system will result in aungrounded system the solidly grounded system will result in alarge magnitude of current to flow (Aids in coordination), butlarge magnitude of current to flow (Aids in coordination), butas no ncrease n vo age on un au e p ases.as no ncrease n vo age on un au e p ases.

Low initial cost to install and implement, but stray currents thenLow initial cost to install and implement, but stray currents then..

Common in low voltage distribution systems, such as overheadCommon in low voltage distribution systems, such as overheadlines.lines.

typically feeds to transformer primary with high side fusetypically feeds to transformer primary with high side fuse

protection.protection.Not the preferred grounding scheme for industrial or commercialNot the preferred grounding scheme for industrial or commercialfacilities due to high magnitude fault currents.facilities due to high magnitude fault currents.

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Low Resistance Grounded System:Low Resistance Grounded System:

connected to ground through a small resistance that limits theconnected to ground through a small resistance that limits thefault current. The size of the grounding resistor is selected tofault current. The size of the grounding resistor is selected todetect and clear the faulted circuit..detect and clear the faulted circuit..

The resistor can limit ground currents to a desired level based onThe resistor can limit ground currents to a desired level based on

coordination requirement or relay limitations.coordination requirement or relay limitations...

Low resistance grounding is not recommended for low voltageLow resistance grounding is not recommended for low voltagesystems due to the limited ground fault current. This reduced faultsystems due to the limited ground fault current. This reduced faultcurrent can be insufficient to positively operate fuses and/or seriescurrent can be insufficient to positively operate fuses and/or series

r p un s.r p un s.Ground fault current typically in the 100Ground fault current typically in the 100 600 Amp range.600 Amp range.

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High Resistance Grounded System:High Resistance Grounded System:

connected to ground through a resistive impedance whose resistance isconnected to ground through a resistive impedance whose resistance isselected to allow a ground fault current through the resistor equal to orselected to allow a ground fault current through the resistor equal to orslightly more that the capacitive charging current of the system.slightly more that the capacitive charging current of the system.

The resistor can limit ground currents to a desired level based onThe resistor can limit ground currents to a desired level based oncoordination requirement or relay limitations.coordination requirement or relay limitations.

Limits transient overvoltages during ground faults.Limits transient overvoltages during ground faults.

Ph sicall lar e resistor banks.Ph sicall lar e resistor banks.

Very low ground fault current, typically under 10 Amps.Very low ground fault current, typically under 10 Amps.

Special relaying methods utilized to detect and remove ground faults.Special relaying methods utilized to detect and remove ground faults.

High resistance grounding is typically applied to situations where it isHigh resistance grounding is typically applied to situations where it is

essentia to prevent unp anne outages.essentia to prevent unp anne outages.Recent trend has been to utilize high resistance grounding methods on 600Recent trend has been to utilize high resistance grounding methods on 600volt systems and lower.volt systems and lower.

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GROUNDING SYSTEMSGROUNDING SYSTEMS Several factures should be considered in the initial design of theSeveral factures should be considered in the initial design of thegrounding system.grounding system.

The area available for installation of the grounding system. This could leadThe area available for installation of the grounding system. This could leadto the requirement and utilization of chemical rods, or wells.to the requirement and utilization of chemical rods, or wells.

Water table and seasonal changes to it.Water table and seasonal changes to it.

Soil condition and resistivity, Please see chart of typical results. AlsoSoil condition and resistivity, Please see chart of typical results. Alsoelevation above sea level and hard rocky soil are concerns that would needelevation above sea level and hard rocky soil are concerns that would needto be addressed.to be addressed.

Available fault currents (i.e., three (3) phase, lineAvailable fault currents (i.e., three (3) phase, line--toto--ground, and lineground, and line--toto---- , . ., . .

NEC and ANSI/IEEE requirements. Also include here the requirements ofNEC and ANSI/IEEE requirements. Also include here the requirements ofthe process equipment to be installed.the process equipment to be installed.

per year.per year.

Utility ties and/or service entrance voltage levels.Utility ties and/or service entrance voltage levels.

Utilization of area were round s stem is to be installed i.e. do not installUtilization of area were round s stem is to be installed i.e. do not install

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under paved parking lot).under paved parking lot).

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SOIL RESISTIVITIES(Approximate Ohm-Meters)

Description1,2 Median Min. Max.Topsoil's, loams 26 1 50Inorganic clays of high plasticity 33 10 55Fills-ashes, cinders, brine wastes 38 6 70

Silty or clayey fine sands with slight plasticity 55 30 80Porous limestone, chalk 65 30 100Clayey sands, poorly graded sand-clay mixtures 125 50 200

, ,Clay-sand-gravel mixtures 145 40 250Marls3 155 10 300Decomposed granites, gneisses4, etc. 300 100 500

Clayey gravel, poorly graded gravel 300 200 400Silt sands oorl raded sand-silt mixtures 300 100 500Sands, sandstone 510 20 1,000Gravel, gravel-sand mixtures 800 600 1,000Slates, schists5, gneiss, igneous rocks, shales, granites, basalts 1,500 1,000 2,000Quartzite's, crystalline limestone, marble, crystalline rocks 5,500 1,000 10,000

otes: . ow res st v ty so s are g y n uence y t e presence o mo sture.

2. Low resistivity soils are more corrosive than high resistivity soils.3. Crumbly soil composed mostly of clay with a high limestone content.4. Metamorphic rock formed by recrystallization of granite, separated into bands.5. Metamorphic rock much coarser than gneiss.This chart compiled from data published in:

-

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,British Standard Code of Practice, CP-1013: 1965, EarthingMegger: A Simple Guide to Earth TestingBiddle: Getting Down to Earth

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1. Parity sized grounding conductors.2. Grounding symmetry in all parallel feeders.

3. Zones of equipment with localized transformers to.

4. Limiting the quantity of devices grounded by any

single conductor.5. Utilizing specialty transformers to limit ground

interference.

. .

7. Use different networks throughout the facility asopposed to a single ended data network.

8. Reference grids in all computer, data processing

and information technology rooms..

entrance.

10. Intentional continuity of structural steel.

11. Bonding of all communication cables to structuralsteel.

.

protection.13. Ufer ground treatment per NEC for all main

vertical steel footers.

14. Grounding grid below moisture barrier.

15. Bondin horizontal steel ans to structural steel.

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GROUNDING SYSTEMSGROUNDING SYSTEMS Several factures can degrade initially good grounding systems. TheseSeveral factures can degrade initially good grounding systems. These

factors indicate the importance of continuous periodic testing (Typicallyfactors indicate the importance of continuous periodic testing (Typicallyonce per ca en ar year un ess pro ems ar se . c ange ower n eonce per ca en ar year un ess pro ems ar se . c ange ower n e

water table across the USA would lead to a degrade in the groundingwater table across the USA would lead to a degrade in the groundingsystem. Another consideration in the ground system would be insystem. Another consideration in the ground system would be in

--which do not provide low resistance ground connections. Along withwhich do not provide low resistance ground connections. Along withthe these concerns are the increase load and associated increase inthe these concerns are the increase load and associated increase in

. ,. ,attention should be paid to corroded electrodes. All these could resultattention should be paid to corroded electrodes. All these could resultin the need for a decrease in the grounding resistance.in the need for a decrease in the grounding resistance.

Testin : Periodic testin should be done to assure roundin s stemTestin : Periodic testin should be done to assure roundin s stem

effectiveness.effectiveness.

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