grounding

Note: The source of the technical material in this volume is the ProfessionalEngineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for SaudiAramco and is intended for the exclusive use of Saudi Aramco’semployees. Any material contained in this document which is notalready in the public domain may not be copied, reproduced, sold, given,or disclosed to third parties, or otherwise used in whole, or in part,without the written permission of the Vice President, EngineeringServices, Saudi Aramco.

Chapter : Electrical For additional information on this subject, contactFile Reference: EEX20501 W.A. Roussel on 874-1320

Engineering EncyclopediaSaudi Aramco DeskTop Standards

Design And ApplicationOf System Grounding

Engineering Encyclopedia Electrical

Design and Application of System Grounding

Saudi Aramco DeskTop Standards

CONTENTS PAGE

Locating System Grounding Information.......................................................... 1

Basis For Installation Of System Grounds In Saudi AramcoElectrical Systems.......................................................................................... 12

Determining The Appropriate Method Of System Grounding ForSaudi Aramco Electrical Systems .................................................................. 21

Designing A Substation/Plant Ground Grid For Saudi AramcoElectrical Installations..................................................................................... 38

Work Aid 1: Saudi Aramco And Industry Standards ForLocating System Grounding Information .................................. 55

Work Aid 2: Table Of Saudi Aramco System GroundingMethods .................................................................................... 56

Work Aid 3: Procedures And References For DesigningSubstation/Plant Ground Grids ................................................. 57

Glossary ......................................................................................................... 69



Saudi Aramco DeskTop Standards 1

LOCATING SYSTEM GROUNDING INFORMATION

An Engineer should consult the following Saudi Aramco documents and the followingindustry codes and standards for information on system grounding:

_ Saudi Aramco Design Practices (SADP's)_ Saudi Aramco Engineering Standards (SAES's)_ Institute of Electrical and Electronics Engineers Standards (IEEE)_ National Electrical Code (NEC)

Saudi Aramco Design Practices

The purpose of the SADP's is to provide the background information that is needed toexplain, amplify, and apply the mandatory requirements of the SAES's. An Engineer shouldreference a SADP when he needs tutorial or background information on the design andapplication of system grounding. The information in the SADP's is not mandatory, andwritten approval is not needed to deviate from the SADP's. Statements in the SADP's that arein capital letters are mandatory because they are taken from the SAES's. The SADP thatcontains information on the design and application of system grounding is SADP-P-111.




Saudi Aramco Design Practices (Cont'd)

SADP-P-111

The title of SADP-P-111 is "Grounding." This SADP includes two parts. Part one providesthe rationale for technical requirements in SAES-P-111 that are not obvious. This rationale isbased on Saudi Aramco experience.

Part two contains fourteen chapters of tutorial information that explains the principles andapplication of grounding to meet the requirements of Saudi Aramco installations. AnEngineer can use this information to clarify the technical requirements that are given inSAES-P-111. Figure 1 shows the table of contents for SADP-P-111. Only Chapters One,Two, Three, Four, Five, Seven, Ten, Eleven, and Thirteen contain information that isapplicable to system grounding. Each of the sections that follow describes the scope of one ofthese chapters.

Chapter One describes the general grounding requirements for Saudi Aramcoinstallations. This chapter also contains a list of all the references that were used towrite SADP-P-111. The latest edition of the references that are listed are for use ininterpreting and/or in modifying the text in SADP-P-111.

Chapter Two contains the definitions of the technical terms that are related to groundsand grounding.

Chapter Three provides guidance on the selection and installation of groundingconductors for high and low voltage systems.

Chapter Four provides guidance on the design of grounding electrodes. This chapterdiscusses the design of all types of grounding electrodes from single rods to extensiveburied grids.

Chapter Five provides guidance on the design and application of the various methodsthat are available to ground Saudi Aramco electrical systems. This chapter includesboth generator transmission system and distribution system grounding.

Chapter Seven provides guidance on the measures that Saudi Aramco uses to combatthe corrosion that is associated with grounding.





SADP-P-111 Table of ContentsFigure 1





Chapter Ten provides guidance on the measurement of soil resistivity for the purpose ofground grid design. The information in this chapter is for use with the information inChapter Four.

Chapter Eleven defines ground potential rise (GPR) and other electric power parametersthat have an effect on communication systems.

Chapter Thirteen provides guidance on the measurement of a ground grid's resistanceafter the ground grid has been installed.

Saudi Aramco Engineering Standards

The SAES's contain the minimum mandatory requirements for the design and installation ofelectrical equipment and systems. Engineers cannot deviate from the requirements of theSAES's without written approval from the Saudi Aramco Chief Engineer (Dhahran).User/specifier requirements that exceed the minimum requirement of the SAES's need nowaiver approval, even though the requirements are different.

The following SAES's apply to system grounding:

_ SAES-P-100_ SAES-P-111_ SAES-P-119

SAES-P-100

This SAES states the minimum mandatory requirements for the design and installation ofelectrical power systems. This standard is intended to assist Design Engineers in areas thatare not specifically referenced in another Saudi Aramco standard. The only sub-section ofthis SAES that applies to system grounding is sub-section 4.4. Sub-section 4.4 contains atable that lists the system grounding methods that should be used according to the voltagelevel of the system that is being grounded.




Saudi Aramco Engineering Standards (Cont'd)

SAES-P-111

This SAES states the minimum mandatory requirements for the grounding of electricalequipment and for the design and installation of grounding and lightning protection systems.The following sub-sections apply to system grounding:

_ Sub-section Four_ Sub-section Five

Sub-section Four, titled "System Design," states the minimum mandatory requirementsfor the design of a ground system.

Sub-section Five, titled "Materials," states the minimum mandatory requirements for thematerials that are used in the design and installation of a ground system.

SAES-P-119

This SAES states the minimum mandatory requirements for the design and installation ofonshore power substations. The only sub-section of this SAES that applies to systemgrounding is sub-section six. Sub-section six states the minimum mandatory requirements forterminating surge arrestors and overhead ground wires to the system ground grid.

Institute of Electrical and Electronic Engineers Standards (IEEE)

IEEE Standards provide information on how to design, test, measure, and specify electricalsystems. The information in the IEEE Standards represents the consensus opinion of a groupof subject matter experts. The requirements and procedures that are given in IEEE Standardsare useful in the design and application of grounding systems. The following IEEE Standardsapply to system grounding:

_ IEEE 80_ IEEE 81_ IEEE 142_ IEEE 367




Institute of Electrical and Electronic Engineers Standards (IEEE) (Cont'd)

IEEE 80

IEEE 80 is titled "IEEE Guide for Safety in AC Substation Grounding." The intent of IEEE80 is to provide guidance and information to ensure that safe grounding practices are appliedin AC substation designs. Figure 2 shows the table of contents of IEEE 80. The sections thatfollow provide a brief description of the scope of each chapter.

Chapter One describes the purpose and scope of the standard.

Chapter Two reviews the objectives of safe grounding system design and the potentialdangers that must be considered during grounding system design.

Chapter Three discusses the effects of passage of an electric current through the vitalparts of a human body. The effects are discussed in terms of the electric current'sfrequency, amplitude, and duration.

Chapter Four discusses how to determine the limits to the amount of electrical currentthat can pass through the human body.

Chapter Five discusses calculations involving the resistance of the human body whenthe body becomes an accidental ground circuit.

Chapter Six discusses the four voltages that must be considered in the design of aground system to prevent electrical shocks:

_ Step voltage_ Touch voltage_ Mesh voltage_ Transferred voltage

Chapter Seven discusses the principal design considerations for a grounding system.

Chapter Eight discusses grounding requirements for gas-insulated substations.

Chapter Nine discusses the requirements for grounding conductor materials and sizes.





IEEE 80 Table of ContentsFigure 2





Chapter Ten discusses the soil characteristics that relate to system grounding.

Chapter Eleven discusses the different soil structures and how to select a soil model foruse in ground system design.

Chapter Twelve discusses how to evaluate the ground resistance of a ground system.

Chapter Thirteen discusses how to calculate the maximum ground grid current.

Chapter Fourteen discusses grounding system design criteria and provides a procedurefor use in ground grid design.

Chapter Fifteen reviews the hazards that can result during ground fault conditions thatare due to the transfer of potential between the ground-grid area and the points that areoutside the ground grid area.

Chapter Sixteen discusses the grounding of equipment that requires special attention:

_ Operating handles_ Fences_ Cable sheathes_ Surge arrestors

Chapter Seventeen describes the different methods for construction of a ground grid.

Chapter Eighteen discusses methods for performance of field measurements on aninstalled grounding system.

Chapter Nineteen describes methods for use of scale models in the design of agrounding system.





IEEE 81

IEEE 81 titled "IEEE Guide for Measuring Earth Resistivity, Ground Impedance, and EarthSurface Potentials of a Ground System," discusses present techniques for performance of themeasurements. The discussion includes the types of instruments that are available and thepossible sources of error. The following specific testing methods are covered in IEEE 81:

_ The measurement of the resistance and the impedance to earth ofelectrodes. The electrodes can be small rods, plates, or large groundingsystems.

_ Ground potential surveys that include the measurement of step-and-touch voltages and potential contour surveys.

_ Scale-model tests for a laboratory determination of the groundresistance and the potential gradients for an idealized design.

_ The measurement of earth resistivity.

IEEE 142

IEEE 142 is titled "IEEE Recommended Practice for Grounding of Industrial and CommercialPower Systems." Also called the "Green Book," IEEE 142 contains four sections ofinformation, two of which apply to system grounding:

_ Section One_ Section Two

Section One discusses the problems that are associated with system grounding and theadvantages and disadvantages of grounded versus ungrounded systems. This sectionalso provides information on how to ground an electrical system, where to ground anelectrical system, and how to select equipment for the grounding of neutral circuits.

Section Four discusses the problems of obtaining a low-resistance connection to theearth. The discussion includes the use of ground rods, ground grids, and buried pipes.





IEEE 367

IEEE 367 is titled "IEEE Recommended Practice for Determining the Electric Power StationGround Potential Rise and Induced Voltage from a Power Fault." IEEE 367 providesguidance on how to calculate the values of ground potential rise and longitudinally inducedvoltages that interfere with wire-line telecommunications facilities. IEEE 367 also providesguidance on how to reduce the worst case values of ground potential rise and longitudinallyinduced voltages for use in wire-line telecommunications protection design.

National Electrical Code

The NEC is published by the National Fire Protection Association. The intent of the NEC isthe practical safeguarding of persons and property from the hazards that can arise from theuse of electricity. The NEC is updated every three years through proposals that are submittedby the public. The proposals must be reviewed and approved by a series of committee's andcouncils before the public proposal can become part of the standard.

The NEC has two articles that contain information on grounding:

_ Article 100_ Article 250

Article 100

Article 100 contains the definitions of terms that are used in the NEC and that are essential tothe proper application of the NEC. Article 100 defines the term "ground" as a conductingconnection (whether intentional or accidental) between an electrical circuit or equipment andthe earth, or to some conducting body that serves in place of the earth.




National Electrical Code (Cont'd)

Article 250

Article 250 provides information on the requirements for the grounding and bonding ofelectrical installations. Article 250 is divided into the following 12 parts, A through M,excluding I:

_ Part A, General Requirement_ Part B, Circuit and System Grounding_ Part C, Location of System Grounding Connections_ Part D, Enclosure Grounding_ Part E, Equipment Grounding_ Part F, Methods of Grounding_ Part G, Bonding_ Part H, Grounding Electrode System_ Part J, Grounding Conductors_ Part K, Grounding Conductor Connections_ Part L, Instrument Transformers, Relays, Etc._ Part M, Grounding of Systems and Circuits of 1 kV and Over (High

Voltage)




BASIS FOR INSTALLATION OF SYSTEM GROUNDS IN SAUDI ARAMCOELECTRICAL SYSTEMS

The basis for installation of system grounds is to ensure stable and safe operation of anelectrical system. This section discusses the following topics pertinent to stable and safeoperation:

_ Electrical Shock Avoidance_ Fire Protection_ Ground Fault Protection_ Electric Noise Control_ Lightning Protection_ Static Control

Electrical Shock Avoidance

A ground fault in an electrical system can expose a person to a hazardous potential gradientnear the fault. Anyone who touches "electrically-live equipment" is exposed to a potentialgradient that causes current to flow. The human body operates through the use of low levelelectrical impulses. A shock current from an electrical power system will overpower theselow level electrical impulses.

Currents that overpower the impulses that control the voluntary muscles, such as arms andlegs, can also affect the involuntary muscle systems of the heart and lungs. The heart andlungs are usually involved with fatalities due to electrical shock. The heart system becomesuncoordinated, going into what is known as fibrillation, in which the pumping action is lost.Once this fibrillation starts, normal operation is rarely resumed even when the shock current isremoved. Without blood circulation, human tissues start to die very quickly, particularly inthe brain. Even when fibrillation is ended by massage or countershock, irreparable damagehas been done if the fibrillation period exceeds one or two minutes.

Stoppage of breathing has the same general effect. Even if blood circulates, the lack ofoxygen reaching human tissues causes rapid degradation. Resuscitation, when startedimmediately, does get oxygen into the blood, even before the victim starts breathing naturally.

The effects of an electric current passing through the vital parts of a human body depend onthe duration, the magnitude, and the frequency of the current. Humans are very vulnerable tothe effects of electric current at frequencies of 50 or 60 Hz. Currents of approximately 0.1 Acan be lethal. Authorities generally agree that the human body can tolerate more current atlow AC frequencies (e.g., 25 Hz or less). Authorities also agree that the body can tolerate fivetimes more DC current than AC current. Even higher currents can be tolerated at frequenciesof 3000 - 10,000 Hz.




Electrical Shock Avoidance (Cont'd)

A current of 1 mA is generally recognized as the threshold of perception. The threshold ofperception is the current magnitude at which a person is able to detect a slight tinglingsensation (in his hands or fingertips) that is caused by the passing current.

Currents of 1 to 6 mA (often termed let-go currents) are unpleasant to sustain, but thesecurrents generally do not impair the ability of a person who is holding an energized object tocontrol his muscles and release the object.

Currents can be painful in the 9 to 15 mA range. People who receive shocks of thismagnitude also experience great difficulty in releasing the energized object that is causing theshock. For still higher currents, muscular contractions can make breathing difficult. Theseeffects are not permanent and disappear when the current is interrupted unless the contractionis very severe and breathing is stopped (not for seconds, but for minutes). Such cases oftenrespond to resuscitation.

In the range of 60 to 100 mA, ventricular fibrillation, stoppage of the heart, or inhibition ofrespiration might occur and cause injury or death. A person who is trained incardiopulmonary resuscitation should administer CPR until the victim can be treated at amedical facility.

Design guides emphasize the importance of the fibrillation threshold. If shock currents can bekept below this value through a carefully designed grounding system, injury or death can beavoided. The probability of electric shock is greatly reduced through fast fault clearing times,in contrast to situations in which the fault currents persist for several minutes, or possiblyhours. Both tests and experience show that the chance of severe injury or death is greatlyreduced if the duration of current flow through the body is brief. The allowed current valuecan be based on the clearing time of primary protective devices or that of the back-upprotection.

Studies have determined that 99.5% of all healthy persons (50 kg or more) can tolerate acurrent through the heart region that is defined by the following formula:

where: IB = body current in amperests = duration of current in seconds

Electrical Shock Avoidance (Cont'd)

The following publications contain further information on the hazards of electrical shock:

_ IEEE Standard, 80 "IEEE Guide for Safety in AC SubstationGrounding," Chapters 2, 3, 4, and 5.




_ IEEE Standard 399, "IEEE Recommended Practice for Power SystemAnalysis," Chapter 12.




_ ISA Monograph 110, "Electrical Safety Practices," Chapter on "HumanElectrical Safety."

Pages 28 and 29 of the publication "Grounding and Shielding in Facilities," by Morrison andLewis, discuss how shock hazards are developed in electrical equipment and show theimportance of grounding conductors in preventing electrical accidents.

Fire Protection

When a fault occurs in an electrical system, heat is generated at the fault point. If the systemis properly grounded, a conductor provides a low-impedance return path for the fault current.This low-impedance path results in a high-fault current that trips the circuit protective device.

Improper system grounding or poor ground connections can result in reduced fault currentsthat might not trip the circuit breaker or fuse. Heating is a function of time. More heat isgenerated for long-duration faults than for short-duration faults. In addition, electrical faultstend to get worse until the faults burn themselves out. As the two previous statementsindicate, when faults are removed in a timely manner, the chances of fire are greatly reduced.The hazard of an electrical fire can be eliminated or reduced through adequate grounding, inaccordance with relevant codes and standards, such as those issued by the IEEE and NEC.The IEEE Standards generally deal with the grounding of large electrical substations ordistribution systems. The NEC grounding regulations are primarily concerned with lowervoltage equipment that are installed within buildings and plants and that are accessible tountrained personnel.

Read grounding for fire protection in the supplemental text "Grounding and Shielding inFacilities," page 28. This section provides information on the generation of heat due to faults.

Ground Fault Protection

Good system grounding, coupled with a low impedance ground return path, will result in acurrent flow during fault conditions that will activate a ground fault protection device and thatwill isolate the damaged circuit. There are two forms of ground fault protection: onedesigned to protect people and the other designed to protect equipment.

Devices that protect people operate on currents of 5 mA. The rating of 5 mA is far toosensitive to be applied to normal industrial systems. In industrial systems, the protection(safety) for people is provided through use of the ground grid and ground system. Groundfault protection for many power systems of 480V and less is provided by circuit breakers andfuses. Low voltage circuit breakers and fuses trip on current values that exceed their ampereratings. However, the NEC defines ground-fault protection as a system that is intended toprotect equipment from damaging line-to-ground fault currents by causing a disconnectingmeans to open all ungrounded conductors of the faulted circuit. This protection is provided atcurrent levels that are less than the levels that are required to protect conductors from damage




through the operation of a supply circuit overcurrent device. The NEC has specific ruleswhere ground fault protection is required.




For voltage levels above 480V, circuit breakers and fuses are used to isolate circuits forground faults.

Circuit breakers can be controlled through ground fault or overcurrent relays. Ground faultrelays operate on low ground fault currents. The type of relay that is used depends on howthe system is grounded and on the location of the circuit breaker in the system.

Fuses are devices in which tripping time depends on the magnitude of the ground faultcurrent. If the system has a high grounding resistance or is otherwise poorly grounded, a fusemight not operate.

For more information on ground fault protection, the Engineer should consult the followingstandards:

_ IEEE Std. 242 - 1986 Protection and Coordination of Industrial andCommercial Power Systems, Chapter 7, Ground-Fault Protection.

_ National Electrical Code - 1990- Article 250.




Electrical Noise Control

Electrical noise is a disturbance in an electrical system or in an instrumentation and controlsystem that interferes with the transmission of the signal containing the information.

Noise adversely affects both analog and digital electronic equipment systems that are below30V. Analog systems that operate in the mA ranges and digital signals are most affected byelectrical noise. The ability of a control system to perform its designed function is directlydependent upon the quality of the signal of the measured variables. This quality is dependantupon the elimination or attenuation of noise that can deteriorate the actual transducer signal.Electrical noise can be a severe problem in industries such as steel, power, and petroleum,where power consumption is high and complex electrical networks exist.

Electrical noise can be reduced in a number of ways. The proper selection of cables andinstallation is important. Careful evaluation of grounding methods help to ensure that noisegeneration is eliminated. Conformance with the requirements of the National Electrical Codewill also help to eliminate noise generation.

Read the supplemental text "Grounding and Shielding," chapters 6 and 7. These chaptersprovide detailed information on electromagnetic interference (EMI) in different situations andpresent solutions to noise problems.

Lightning Protection

Lightning is the discharge of high-potential cells (usually negative) within clouds to the earth.The discharge current increases from zero to a maximum in 1 to 10 _s and then declines tohalf the peak value in 20 to 1000 _s. This discharge current can be repeated one or moretimes, over the same path, in rapid succession. The average peak stroke is about 20,000A --although some peak strokes are as great as 270,000A.

Lightning can strike a facility in two ways. One way is a direct strike on a building or onanother elevated structure; such a strike causes fires or physical damage. The other way isthrough a lightning strike on an overhead power line. The overvoltage surge can betransmitted through the power line to the facility substation and through the transformers to allthe electrical equipment in the facility.

It would be necessary to completely enclose a building in metal to provide the building with100% protection from lightning. This extent of protection is not practical. Design techniquesfor lightning protection are based on the building size, occupancy, location, and other suchfactors to provide a practical level of safety at a reasonable cost.




Lightning Protection (Cont'd)

Read the supplemental text "Grounding and Shielding in Facilities," pages 46 and 85. Thesepages provide general information on lightning protection for buildings, including thefollowing:

_ Where lightning terminals are required._ Installation of lightning terminals._ Installation of ground cables.

The electrical path for lightning should be straight to ground. Preferably, this path should notinclude bends. If bends in the path are required, the bend should have a large radius. Whenpossible, multiple air terminals should be interconnected vertically and horizontally.

High voltage lightning surges on transmission lines are eliminated or greatly reduced throughthe use of devices that are known as lightning or surge arresters. Surge arresters act asinsulators during normal system operation. During a high voltage surge, however, thesedevices directly shunt the current to ground without developing dangerous voltages. Thesedevices should be connected directly to the system ground. An overhead grounding wire thatruns above the phase wires and is grounded at frequent intervals also is used to protectequipment from lighting strokes.

For more information on lightning protection, the Engineer should refer to the followingstandards:

_ IEEE Std. 142-1982-IEEE Recommended Practice for Grounding ofIndustrial and Commercial Power Systems, Chapter 3, Static andLightning Protection Grounding.

_ NFPA 78 - 1989 - Lightning Protection Code.

The following Saudi Aramco Engineering Standards and Saudi Aramco Design Practicesapply to lightning protection:

_ SAES-P-111 Chapter 9, Lightning Protection_ SAES-P-119 Chapter 6, Substation Yard_ SADP-P-111 Chapter 9, Lightning Protection of Building and

Structures




Static Control

Static electricity is the accumulation of electrostatic charges on the surfaces of non-conductors or on conducting bodies that are insulated from their surroundings. The followingare the common ways that static electricity is generated in industry:

_ Belts that are made of rubber, leather, or other insulating materials thatare running at moderate or high speeds generate considerable quantitiesof static electricity. The generation occurs where the belt separates fromthe pulley. The charges will occur on the pulley (regardless of whetherit is conducting or nonconducting) and the belt.

_ When a tank truck that is insulated from the ground by dry rubber tiresis being filled with liquid, a charge develops on the surface. Thissurface charge will attract a charge of the opposite polarity on theinterior of the metal tank wall. The exterior of the tank will have a freecharge of the same polarity as the surface charge of the liquid. Thischarge is capable of producing a spark to ground.

_ The human body in a low-humidity area can accumulate static chargesof several thousand volts in different ways. Contact of shoes with floorcoverings can develop a charge. Also, proximity to machinery thatgenerates static electricity can also result in a charge being developed.

Static electricity can be controlled and eliminated in industrial processes. A common methodof control is to allow the static charge to bleed off through bonding or grounding. Unlikegeneral system grounding, a low resistance to ground is not necessary to dissipate staticcharges.

For information on static control, the Engineer should consult the following Saudi Aramcoand industry codes and standards:

_ SADP-P-111 Chapter 14 - Safeguard Against Static Electricity,Lightning and Stray Currents.

_ IEEE Std. 142 - 1982 IEEE Recommended Practice for Grounding ofIndustrial and Commercial Power Systems, Chapter 3 - Static andLighting Protection Grounding.

_ NFPA 77 - 1988 Static Electricity.

_ NFPA - Electrical Installations in Hazardous Locations.




DETERMINING THE APPROPRIATE METHOD OF SYSTEM GROUNDING FORSAUDI ARAMCO ELECTRICAL SYSTEMS

Electrical Engineers must be able to determine the appropriate method of system groundingbefore ground grids can be designed for Saudi Aramco electrical systems.

Work Aid 2 provides a table showing the grounding methods that should be used for differentcombinations of voltages, phases, and loads. The sections that follow provide information onthe following topics:

_ Solid Grounding_ Grounding Transformers_ Impedance/Resistance Grounding_ Reactance Grounding_ Ungrounded Systems_ Comparison of Methods (Advantages/Disadvantages)

Solid Grounding

A solidly-grounded system is a system of conductors in which one conductor or point isgrounded. Figure 3 shows a 115 kV transmission system, a 69 kV transmission system, a480V bus and a 240/120 V bus that are all solidly grounded. Solid grounding indicates thatno impedance is intentionally inserted between the electrical system and the earth groundpoint. The connection point is normally the middle wire or the neutral point of a transformeror generator winding. Solid grounding provides the highest level of ground fault current andthe lowest level of transient overvoltages.

All Saudi Aramco systems that are rated 600V and below should be solidly grounded. SaudiAramco transmission and distribution systems of 34.5 kV, 69 kV, 115 kV, and the receivingpoint of 230 kV systems should also be solidly grounded. The reasons for solid grounding athigher voltages are as follows:

_ Rotating equipment is seldom rated above 15 kV.

_ It is not necessary to limit ground fault current to protect motors athigher voltages.

_ Voltages above 15 kV are usually outdoors.

_ Hazards to buildings and personnel are reduced at high voltages.




Solid Grounding (Cont'd)

Solidly Grounded SystemsFigure 3




Grounding Transformers

Saudi Aramco 2400V distribution systems receive power from transformers with ungroundeddelta secondary windings. These ungrounded systems are subject to overvoltage conditionsand unstable phase voltage conditions due to lack of a grounded neutral.

Saudi Aramco Engineering Standards and current engineering practices require that allelectrical systems be grounded for reasons of safety and reliability. The existing ungroundeddelta secondary winding could be corner grounded, but corner grounding places the other twophases at line-to-line voltage, which is undesirable. The preferred method for provision of aneutral point for grounding in an existing ungrounded delta system is to use a groundingtransformer. A grounding transformer is a transformer that is installed in a system for the solepurpose of providing a neutral point for grounding the system.

Saudi Aramco uses two types of grounding transformers:

_ Distribution Transformers_ Zig Zag Transformers

Both types of grounding transformers provide a suitable system ground connection, althoughthe zig zag transformer is more economical and should be selected over the distributiontransformer for most installations. The only time a distribution transformer is normally usedis when a distribution transformer is readily available and a zig zag transformer is not readilyavailable.

Distribution Transformers

A three-phase distribution transformer with wye-delta connections or three single-phasedistribution transformers that are connected in a wye-delta configuration can be used toprovide a system ground on an existing ungrounded system. Figure 4 shows a three-phasedistribution transformer that is connected to provide a system ground.

The distribution bus that is shown in Figure 4 receives power from the ungrounded delta-connected secondary of the system power supply transformer. The system ground for thedistribution bus is obtained through connection of the distribution type grounding transformer.When a phase-to-ground fault occurs on the distribution bus, a complete path for the ground-fault current (shown by the arrows) exists:

_ Ground-fault current will flow through the phase-to-ground fault fromthe power supply end and the load end of the distribution bus.




Grounding Transformers (Cont'd)

Three-Phase Distribution Transformer Connected to Provide a System GroundFigure 4





_ The ground-fault current will then flow through the earth and into thesystem ground connection.

_ The ground-fault current will flow through the grounding resistor andinto the neutral of the wye connected primary of the distribution typegrounding transformer.

_ The ground-fault current will flow out of the wye-connected primary ofthe distribution type grounding transformer and back to the distributionbus to complete the path for the ground-fault current.

_ When the ground-fault current flows through the wye-connectedprimary of the distribution type grounding transformer, current will alsoflow in the delta-connected secondary of the distribution type groundingtransformer.

When a distribution transformer with wye-delta connections is used to provide a systemground, the secondary winding must be a closed-delta connection to allow zero sequencecurrents to flow. An open-delta secondary connection would also reflect an extremely highimpedance into the primary winding, and the resulting high primary winding impedancewould severely limit the amount of ground-fault current that could flow.

Zig Zag Transformers

Zig Zag is defined as a line or course that turns sharply in one direction and later turns sharplyin another direction. A zig zag transformer has two phase windings on each leg of thetransformer core. The internal connection of the transformer is shown in Figure 5. Theimpedance of the zig zag transformer to a balanced three-phase voltage is high. The zig zagtransformer has a neutral lead that is connected to ground and three other leads (line leads)that are connected to the bus. When there is no fault on the system, only a small magnetizingcurrent flows in the transformer winding. However, the transformer impedance to zero-sequence voltages is low so that the transformer allows high ground fault currents to flow.The transformer divides the ground fault current into three equal current components. Theseequal currents are in phase with each other and flow in the three windings of the zig zagtransformer. The method of winding, shown in Figure 5, is such that when these three equalcurrents flow, the current in one section of the winding on each leg of the core is in a directionthat is opposite to the current flow in the other section of the winding on that leg of the core.The result is that the ground-fault current is equally divided in the three lines. This divisionaccounts for the low impedance of the transformer to ground currents.





Zig Zag TransformerFigure 5




The distribution bus in Figure 6 receives power from the system power supply transformer.The system ground is obtained through connection of a zig zag transformer, as shown inFigure 6. When a phase-to-ground fault occurs, zero sequence currents will flow. Zerosequence currents are in phase and have same magnitude. The zero sequence currents willflow through the grounding resistor and into the zig zag transformer, where the currentsdivide equally in the three legs of the transformer. Each leg has two windings that are woundin the reverse direction. The two windings cancel each other's magnetic flux. Thiscancellation results in a low impedance for zero sequence currents.

Under normal operating conditions, the windings of each leg are 120o out of phase. Becausethis phase relationship results in a large transformer impedance, only a small magnetizingcurrent flows through the zig zag transformer when there is no ground fault in the system.





Zig Zag Transformer Connected to Provide a System GroundFigure 6




Impedance/Resistance Grounding

The terms "impedance grounding" and "resistance grounding" have similar meaning. Theelectrical characteristic that is known as resistance is actually a component of the electricalcharacteristic that is known as impedance. SADP-P-111 defines both impedance groundingand resistance grounding. In an impedance grounded system, the neutral line is connected toground through an impedance. SAES-P-111 lists grounding impedances as a resistor, areactor, or a distribution transformer. In a resistance grounded system, the neutral line isconnected to ground through an impedance, and the principal element of that impedance isresistance.

IEEE-Std. 142-1982 and SADP-P-111 give the "resistance grounded system" the samedefinition. IEEE Std. 142-1982 does not define the term "impedance grounded." IEEE Std.142-1982 uses the terms "resistance grounded" and "reactance grounded" to refer to the twomethods of grounding an electrical system through an impedance. When the Saudi AramcoStandards refer to impedance grounding, these standards are actually referring to resistancegrounding because resistors are the preferred type of grounding impedance for Saudi Aramcoelectrical systems. In cases where Saudi Aramco does not use resistors as the groundingimpedance, the type of grounding impedance that is required will be specified by a term otherthan "impedance grounded" (e.g., reactance grounded).

In an ideal electrical system, impedance grounding would be used for all voltages above 600Vbecause the short circuit capability of the system increases as the system voltage increases.The short circuit capability of a system refers to the ability of the system to damage itselfunder fault conditions (grounds) due to the excessive current that flows through the systemunder ground-fault conditions.

When a ground-fault occurs in a system, the ground fault current must flow from the powerline through ground and must return through the grounded neutral line. The magnitude of theground-fault current can be significantly reduced through placement of an impedance in serieswith the neutral line. The specific reasons for limiting the amount of ground-fault current thatcan flow in a system can include one or more of the following:

_ To reduce the burning and melting effects in failed electrical equipmentsuch as transformers, cables, and rotating equipment

_ To reduce the mechanical stresses in circuits and apparatus that arecarrying fault currents




Impedance/Resistance Grounding (Cont'd)

_ To reduce the electrical shock hazards to personnel that are caused byground-fault currents in the ground return path

_ To reduce the arc blast or flash hazard to personnel who may havecaused or who are close to the ground-fault

_ To reduce the momentary line voltage drop caused when ground-faultsoccur and when ground-faults are subsequently cleared

_ With high resistance grounding to gain control of transient overvoltagesand to avoid the shutdown of a faulty circuit on the occurrence of thefirst ground fault.

Saudi Aramco follows the current engineering practice of using impedance grounding only inmedium voltage electrical systems (1001V - 15,000V). The main reason impedancegrounding is not used above 15,000V is that the required resistors are so large that the costsare prohibitive.

Two classes of impedance grounding are available:

_ high resistance_ low resistance

The two classes of impedance grounding differ in the magnitude of ground-fault current thatis permitted to flow. The sections that follow describe each class in more detail.

High Resistance Impedance Grounding

Saudi Aramco does not have any normal applications for high resistance impedancegrounding. High resistance impedance grounding uses grounding resistors that limit theground-fault current to 10A or less. High resistance impedance grounding has very fewpractical applications because of this low level (10A) of ground-fault current. 10A of ground-fault current is not enough to reliably operate protective devices. High resistance impedancegrounding can only be used in applications in which power supply continuity is critical and inwhich the system can tolerate a ground-fault for the anticipated period of time that isnecessary to locate and clear the ground-fault.





Low Resistance Impedance Grounding

Low resistance impedance grounding uses grounding resistors that limit the ground-faultcurrent to 25A or more. Low resistance impedance grounding is the preferred method ofimpedance grounding because low resistance impedance grounding limits the ground-faultcurrent to safe levels and provide sufficient ground-fault current to reliably operate protectivedevices.

In the following systems, Saudi Aramco uses low resistance impedance grounding withresistors that are sized to limit the ground-fault current to 400 amps for ten seconds:

_ Distribution systems serving 4160 volt, three-phase loads_ Distribution systems serving 13,800 volt, three-phase industrial loads

In 13,800 volt residential distribution systems, Saudi Aramco uses low resistance impedancegrounding with resistors that are sized to limit the ground-fault current to 1000 amps for tenseconds.

Sizing Impedance Grounding Resistors

Three electrical ratings are required to select the correct size of grounding resistor:

_ Grounding Resistor Voltage Raging_ Grounding Resistor Current Rating_ Grounding Resistor Time Rating

Grounding Resistor Voltage Rating - The grounding resistor voltage rating is equal to thephase-to-neutral voltage of the system. The phase-to-neutral voltage is also called thephase-to-ground voltage. The phase-to-neutral voltage of the system is calculatedthrough division of the phase-to-phase voltage by the . The phase-to-phase voltage isalso called the line-to-line or system voltage. For example, the phase-to-neutralvoltage of a 13,800 volts distribution system would be calculated as follows:

This example shows that the required grounding resistor voltage rating for a 13,800volt distribution system is 7967 + 10% voltage variation = 8764 volts.





Grounding Resistor Current Rating - The grounding resistor current rating is equal to thevalue of ground-fault current that will flow in the system. The designer of the systemmust choose the grounding resistor current rating on the basis of a compromise of thefollowing two requirements:

_ The ground-fault current must be low enough to minimize the damageresulting from a ground-fault.

_ The ground-result current must be high enough to reliably operateprotective relays.

The generally accepted engineering standard is that ground relays should operate on10% of the maximum current allowed by the grounding resistor. Most distributionsystems use grounding resistors with current ratings of 50 amps and higher because ofthis 10% standard. With 50 amps or higher ground-fault currents, there are manyreadily available relays and CT's that will reliably operate on 5 amps of current (10%of 50 amps). Saudi Aramco uses grounding resistor current ratings of 400 amps.

Grounding Resistor Time Rating - The standard grounding resistor time ratings are asfollows:

_ Ten seconds_ Nine minute_ Ten minutes_ Extended time

The grounding resistor time rating indicates the amount of time that a groundingresistor can operate under ground-fault conditions without exceeding the allowabletemperature rise above 50oC. The allowable temperature rises above 50oC are asfollows:

_ 60oC temperature rise for ten second time ratings._ 60oC temperature rise for one minute time ratings._ 10oC temperature rise for ten minute ratings._ 10oC temperature rise for extended time ratings._ 85oC temperature rise for steady state conditions.





Grounding resistors with ten second time ratings should be specified for systems thathave protective relays to isolate the circuit under ground-fault conditions. The tensecond time rating is standard for Saudi Aramco electrical systems. Some Engineersmay specify a one minute or ten minute time rating for an extra margin of safety;however, grounding resistors with higher time ratings are more expensive. Theextended time rating is normally only specified for distribution systems that mustcontinue to operate with a ground-fault for more than ten minutes. An example wouldbe a distribution system that supplies power to a facility that cannot be shutdown inmid-process without a significant cost.

Reactance Grounding

A reactance grounded system is a system in which the neutral line is connected to groundthrough an impedance and in which the principal element of that impedance is reactance. Thereactance is provided through the use of a grounding reactor.

Grounding reactors are less expensive than grounding resistors for low impedance, highcurrent applications. Saudi Aramco uses reactance grounding in place of solid grounding forsystems in which the ground-fault current could exceed the three-phase fault current by 25%.A possible application might arise on a system when a solid ground is indicated, but theground fault currents could exceed the three-phase fault levels and the circuit breaker shortcircuit capacities.

Ungrounded Systems

In an ungrounded system, the generator or transformer neutral does not have a physicalconnection to ground. In reality, all systems are grounded to some degree because of thenatural capacitance between the system elements and ground through the insulation system.

Ungrounded electrical systems were originally designed on the basis of the assumption that anungrounded system would provide greater service continuity than a grounded system. Greaterservice continuity is achieved because grounding of any one phase of an ungrounded systemwill not cause the protective devices to operate; a complete path for the ground fault currentwill not exist. The only change in the system will be an increase in the voltage of theungrounded phases.

Recent experience has shown that in many systems greater service continuity is obtained withgrounded-neutral systems than with ungrounded neutral systems. Because of this recentexperience, current engineering practice is to install grounded rather than ungroundedelectrical systems.




Ungrounded Systems (Cont'd)

Saudi Aramco still has some 2400V ungrounded electrical systems; however, Saudi Aramcodoes not permit the installation of new ungrounded systems. Saudi Aramco also does notpermit the extension of existing ungrounded electrical systems without the approval of theConsulting Services Department, Dhahran.

Comparison of Methods (Advantages/Disadvantages)

The advantages and disadvantages of each method of system grounding change with thevoltage level of the system in which the grounding method is used. The following sectionslist the advantages and disadvantages of the different methods of system grounding for thefollowing three ranges of system voltages:

_ 80 Volts and Below_ 1.4 kV to 13.8 kV_ above 13.8 kV

480 Volts and Below

Solidly grounded systems in this voltage range provide the following advantages:

_ Transient overvoltages will not be excessive.

_ The faulted zone of the system can be automatically segregated.

_ Initial costs for installation are lowest.

_ Sufficient ground-fault currents are provided at remote locations tooperate protective devices when the ground network is installedproperly.

Solidly grounded systems in this voltage range have the following disadvantage:

_ The grounding network for the system must provide a very lowimpedance return path for the ground-fault currents in order for theprotective devices to operate properly.

There are no stated advantages and disadvantages for low resistance grounding in 480 voltsystems. Low resistance grounding is not specifically covered by the National ElectricalCode and should not be considered as an adequate grounding system.




Comparison of Methods (Advantages/Disadvantages) (Cont'd)

High resistance grounding in this voltage range is permitted when all of the followingconditions are met:

_ The conditions of maintenance and supervision ensure that onlyqualified people will service the installation.

_ Continuity of power is required.

_ Ground detectors are installed on the system.

_ Fine-to-neutral loads are not served.

High resistance grounding in this voltage range provides the following advantages:

_ Because one ground fault will not isolate power to electrical equipment,operation will not be interrupted.

_ Transient overvoltages will not be excessive.

High resistance grounding in this voltage range has the following disadvantages:

_ A dangerous potential will exist on the faulted equipment until theground fault is cleared.

_ Most circuit breakers do not sense a sufficient amount of ground currentto function.

_ Ground fault can be difficult to locate.

_ ine-to-neutral loads cannot be used.

_ lectrical coordination is lost.

_ Additional expense of ground fault detection is required.

_ Ground fault on one phase increases the voltage of the other phases toline voltage-to-ground.





2.4 kV to 13.8 kV

Solidly grounded systems in this voltage range provide the following advantages:

_ Transient voltages are limited.

_ Solidly grounded systems are less expensive than are resistance orreactance grounded systems.

Solidly grounded systems in this voltage range have the following disadvantages:

_ Ground fault currents may exceed three-phase fault currents._ Ground fault currents at this level are dangerous to equipment and

people.

Low resistance grounded systems in this voltage range provide the following advantages:

_ Phase-to-ground fault currents are greatly reduced._ Haulted zones are automatically tripped._ Transient overvoltages are not excessive.

Low resistance grounded systems in this voltage range have the following disadvantages:

_ These systems are more expensive than solid grounding._ Relaying is required.

High resistance grounded systems in this voltage range provide the following advantages:

_ Phase-to-ground fault current is reduced to a low level._ Power is not disconnected for ground faults._ Transient overvoltages are not excessive.

High resistance grounded systems in this voltage range have the following disadvantages:

_ These systems are more expensive than solidly grounded systems._ Ground detection equipment is required._ Fault levels are too low for normal relaying._ Ground faults on the system are dangerous to people and equipment.





Above 13.8 kV

Solidly grounded systems in this voltage range provide the following advantage:

_ Transient voltages are limited._ These systems are the least expensive.

Solidly grounded systems in this voltage range have the following disadvantages:

_ High fault currents can be produced and these fault currents represent ahazard to personnel if these voltages are carried inside buildings.

Resistance grounded systems in this voltage range provide the following advantage:

_ Tansient voltages are limited_ Phase-to-ground fault currents are reduced

Resistance grounded systems in this voltage range have the following disadvantages:

_ Resistors are too expensive at this level

Low reactance grounded systems in this voltage range provide the following advantages:

_ These systems reduce ground fault currents to produce 26-100% of thethree-phase fault value.

_ Transient overvoltages are limited.

_ Faults can be automatically isolated.

Low reactance grounded systems in this voltage range have the following disadvantage:

_ These systems are more expensive than solid grounding.




DESIGNING A SUBSTATION/PLANT GROUND GRID FOR SAUDI ARAMCOELECTRICAL INSTALLATIONS

An Engineer must be familiar with the following topics in order to design substation/plantground grids:

_ Ground Grid Concepts_ Surface Soil Resistivity_ Ground Potential Rise_ None of Influence_ Transfer Potential_ Step-and-Touch Potential_ Grid Depth and Number of Ground Rods_ Wire Sizing_ Fault Times

Ground Grid Concepts

As explained in SAES-P-111, grounding systems perform the following main functions:

_ To safeguard a person from electric shock by ensuring that, under faultconditions, all surfaces with which a person is in simultaneous contact,including those of metallic equipment and the ground, remain at saferelative potentials.

_ To safeguard electrical equipment by grounding power systems toensure that, under fault conditions, both voltages and currents are withinpredictable limits and that the protective devices will operate reliablyand with appropriate discrimination.

_ To provide a path to ground from lightning arrestors that might operatedue to direct lightning strikes, to lightning induced surges, or toswitching surges.

_ To reduce the possibility of static electricity discharge that wouldpresent a fire risk in hazardous areas.

A grounding system consists of the grounding conductors that connect all items to begrounded and of a grounding electrode or grounding electrodes. The use of multiplegrounding electrodes is known as a ground grid, and the ground grid forms the medium ofcontact with the earth. The ground grid can consist of buried conductors in a cross, of a grid,or of another formation.




Ground Grid Concepts (Cont'd)

The purposes of the ground grid are to provide a low resistance path to earth for fault currentsand to limit the rise of ground potentials that could generate surface gradients that are unsafefor human contact.

The following factors influence the design of a grounding system:

_ The maximum prospective ground-fault current that can pass betweenthe fault location and the system neutral point or points and the durationof the ground-fault current flow. The size of the ground fault currentgoverns the grounding conductor size.

_ The proportion of the ground fault current that will pass between thegrounding system and the body of earth and the duration of the currentflow. This factors govern the electrode design.

_ Site soil resistivity.

_ The degree of exposure to mechanical damage and corrosion. Thisfactor will influence the choice of materials and the manner ofinstallation.

Surface Soil Resistivity

The grid resistance and the voltage gradient within a substation are directly dependent on thesoil resistivity. The surface soil resistivity is the resistivity of the upper layer of the soil. Thisresistivity is important because it helps limit the body current through addition of resistance tothe equivalent body resistance. That is, if the upper layer of the soil is high in resistivity, theamount of current through the body of a person in contact with an energized component isreduced. A thin layer of crushed rock on the surface can raise the surface soil resistivity andcan result in higher permissible step and touch voltages.

Because the effective length of the grid conductor is inversely proportional to the permissiblebody contact voltages, an increase in surface soil resistance allows a greater body contactvoltage; consequently, a shorter grid length can be used for the same area. The result isgreater grid spacing.




Ground Potential Rise (GPR)

Ground potential rise (GPR) is an AC potential difference between remote earth (reference ofzero potential) and local ground. The magnitude of the maximum expected GPR determinesthe type of protection that is required for communication equipment as follows:

_ Locations at which the maximum expected GPR (or voltage stressmagnitude) is less than 300V are classified as low risk sites. Theamount of protection that is required for circuits at these sites is minimaland depends upon the reliability needs.

_ Locations at which the maximum expected GPR (or voltage stressmagnitude) is between 300V and 1500V are classified as moderatehazards. Protection must be applied to all circuits. The acceptableprotection methods should be determined by SAES-T-887.

_ Locations at which the maximum expected GPR is above 1500V areconsidered severe hazard sites. Protection methods will includeisolating or neutralizing transformers as determined by SAES-T-887.

Ground potential rise (GPR) is essentially the product of the following:

_ The total ground grid resistance to a remote earthing point (outside thezone of influence of the GPR at the power system fundamentalfrequency).

_ The total net fault current that flows through the ground grid.

GPR can be expressed by the following equation:

GPR = IG x RG

where: IG = portion of the total fault current that flows through thegrid to remote earth.

RG = ground grid resistance to remote ground.




Ground Potential Rise (GPR) (Cont'd)

The total prospective ground-fault current at a location should be determined through asystem fault current study. The sub-transient symmetrical R.M.S. value of the ground-faultcurrent "Ig" should be used. Frequently, a fault level analysis or relaying study will havebeen performed for the location and will give the required information. For all power plants,it should be assumed that the maximum ground-fault current is equal to the breaker'ssymmetrical interrupting capability.

The Electrical Engineer should determine the proportion of this current that is passingbetween the electrode and earth; he should then apply a factor for future system growth. Thisfactor is a matter of judgement and is based on the following indicators:

_ Nearness of the calculated three-phase symmetrical fault level for thelocation to the circuit breaker interrupting capacity. If these values areclose, the location is at a point of high fault level and the chance offuture increases are limited.

_ The probability of development in the area (especially powergeneration). A remote location on the fringe of an established oil fieldcan increase little in fault level. A location in an area of futuredevelopment can increase greatly.

The computer program MALT, developed for Saudi Aramco installations, can analyze theeffects of buried grounding electrodes. MALT should be utilized to review the groundingelectrode design for all major industrial facilities (e.g., desalting facilities, seawater injectionplants, gas plants, etc.). MALT can be used to determine the following:

_ The resistance to remote earth of grounding electrodes (grid) in a onelayer soil model (regardless of shape, depth of burial, and size ofconductors).

_ The resistance to remote earth of electrodes (grid) in a two-layer soilmodel (regardless of shape, depth of burial and size of conductors).

_ The GPR of a given site.

Zone of Influence

The elevated potential of the industrial site grounding electrode during a ground fault resultsin a rise in potential of the earth inside and outside of the plant boundaries. The potentialgradient that is outside of the site decreases as the distance outward increases. Thisdecreasing potential gradient is known as the zone of influence.




Zone of Influence (Cont'd)

A potential contour survey can locate the hazardous potential gradients near groundedelectrical structures for each fault type and location. The voltage drop to points surroundingthe structure are measured from a known reference point and is plotted on a map of thelocation. A potential contour map then can be drawn through connection of the points ofequal potential with continuous lines. If the contour lines have equal voltage differencesbetween them, greater hazards are indicated by closer lines. Actual gradients that are due toground fault currents are obtained through multiplication of test current gradients by the ratioof the fault current to the test current. A typical contour map of a substation grid is shown inFigure 7. The area that is contained by the perimeter (B) in Figure 7 is termed the zone ofinfluence of the GPR. The permissible magnitude of the voltage that is along the perimeter(B) is by choice or design and is often limited by an agreement among the authoritiesconcerned to a maximum of 300V.

The most accurate measurements of potential gradients are made through use of the volt-ammeter or current injection method. A known current, usually between 1 and 100 A andbetween 55 and 70 Hz, is injected into a remote ground test electrode through use of aninsulated conductor. A current that is greater than 50 A (personnel and equipment safetyconsiderations are to be observed) is preferred where the ground impedance is less than 1_.Where electronic measuring instruments are used (for example, a digital frequency selectivevoltmeter), a test current much less than 50 A is satisfactory.

This procedure would not apply where insulated overhead ground wires are employed andwhere calculations would be required. A remotely located ground test electrode is necessaryto prevent gradient distortion from the mutual impedance of inadequately spaced groundelectrodes. The distance between the ground under test and the remote current electrode canvary from less than 300 meters for a small ground grid or an isolated station to a kilometer ormore for larger installations and for installations in densely populated areas. Measurementsof the potential should be made with a very high impedance meter that is connected betweenthe ground grid and a test probe, which is driven into the earth along the profile lines radial tothe power station. Unless suitable means are employed to mask out the residual groundcurrent and the other interference, the test current must be of sufficient magnitude to do themasking. External power frequency and harmonic components are removed through use offiltering. At the same time, in order to avoid variations in voltage gradients during a series ofmeasurements, care must be taken to prevent heating and drying of the soil that is in contactwith the ground grid or the test electrode. Low-current test methods will produceapproximate results.

Economics and the necessary or the desired accuracy that is required will dictate the use ofthese methods or other methods and the number of measurements to be made.





Boundary of the GPR Zone of InfluenceFigure 7





When more than one overhead or underground cable is connected to a substation, potentialgradients in and around the substation can be quite different for faults that are on differentlines or cables. Similarly, faults at different locations in large substations also can result indifferences in potential gradients in and around the power station. Potential gradients in andaround a large substation should be determined for two or more fault conditions.

Underground metallic structures, metallic structures on the surface of the earth, metallicfences, and overhead ground wires that are near a substation, whether connected to the groundgrid or not, will usually have a significant effect on the potential gradients and should beconsidered in potential gradient measurements. These structures include neutral conductors,metallic cable sheaths, metallic water and gas lines, and railroad rails.

When a potential gradient study cannot be economically justified, potential gradients can becalculated from ground resistance and soil resistivity measurements. The accuracy of suchcalculations will depend on the accuracy of the measurements and on the unknownabnormalities of the earth around and below the ground grid. The adequacy of suchcalculations then can be verified with relatively few potential gradient measurements.

Depending upon the magnitude of a GPR, the following effects can arise outside a substationor adjacent to a power line grounding electrode or transmission line tower (within the zone ofinfluence of the GPR):

_ The potential can be transferred through a metal part, bonded with orcoupled resistively to the plant grounding electrode(s), to remotelocations.

_ The touch voltage between a part that is grounded to the plantgrounding electrode and a local ground (for example, a high-voltageinterface ground) can be excessive.

_ Reversed touch voltage (or voltage stress) between the local ground anda part having a lower or even zero potential (for example, a telephonecable protection interface) can become excessive.





Paragraph 4.1.6 in SAES-P-111 describes a situation in which a new grounding system isconnected to an existing grounding system. The resultant zone of influence is that of thecomposite system. SAES-P-111 paragraph 4.1.6 reads as follows:

"Where a new grounding system is connected to or located within the zone ofinfluence of an existing grounding system, the two grounding systems shall beinterconnected by a minimum of two conductors per grid. The designer of the newgrounding system shall be responsible to review the overall grounding system.Recommendations for upgrading the existing system(s), if required, shall be made tothe Operations Department of the respective area."

Transfer Potential

Transfer is defined as the relocation of a hazardous potential from a ground-grid area to pointsthat are outside of the ground-grid area. A serious hazard may result during a fault from thetransfer of potentials between the ground-grid areas and outside points. This transfer ofpotentials is done by conductors (such as communication and signal circuits, low-voltageneutral wires, conduit, pipes, rails, and metallic fences). The danger usually is from contactsof the touch type. The importance of the problem results from the very high magnitude ofpotential difference that is often possible. Induced voltages on unshielded communicationcircuits, static wires, and pipes, can result in transferred potentials exceeding the sum of theGPR's of both the faulted substation and the source substation. Rails entering the station,when connected either intentionally or otherwise to the ground grid, can theoretically create ahazard at a remote point by transferring the grid potential rise during a fault. Similarly, if therails are grounded remotely, a hazard can be introduced into the station area.

Hazards are possible where the neutrals of low-voltage feeders or secondary circuits that servepoints that are outside of the station area are connected to the station ground. When thepotential of the station ground grid rises as the result of ground-fault current flow, all or alarge part of this potential rise can appear at remote points as a dangerous voltage betweenthis grounded neutral wire and the adjacent earth. Where other connections to earth are alsoprovided, the flow of fault current through these connections can, under unfavorableconditions, create gradient hazards at points that are remote from the station. Each installationshould be reviewed for transfer potential hazards, and corrective action should be taken asrequired.




Transfer Potential (Cont'd)

For communication circuits, schemes have been developed that involve protective devicesand insulating and neutralizing transformers to safeguard personnel and terminalcommunication equipment. The introduction of fiber optics to isolate the substationcommunications terminal from the remote terminal can eliminate the transfer of highpotentials. Fiber optics should be considered when potentials cannot be easily controlled bymore conventional means.

Rail hazards can be removed by removable track sections where the rails leave the ground-grid area, or through installation of several insulating joints in the rails that are leaving thegrid area. A second set of insulating joints that are beyond the first set would protect againstthe shunting of a single set by a metal car or the soil itself and would also reduce the remotehazard of potential differences across a joint itself. The insulating joints must be capable ofwithstanding the potential difference between remote earth and the potential transferred to thejoint. Adequate creepage distance should be ensured to offset any pollution or contaminationproblems.

Step and Touch Potential

Step potential (voltage) is the difference in surface potential that is experienced by a personthat is bridging a distance of one meter with his feet, without contacting any other groundedobject.

Touch potential (voltage) is the potential difference between the ground potential rise and thesurface potential at the point where a person is standing with his hands in contact with agrounded structure. The maximum touch voltage is to be found within a mesh of a groundgrid.

Death can occur from step and touch potentials, depending on the magnitude and the durationof the fault. Body conditions that can reduce resistance, such as wet hands or shoes, also canincrease the probability of death or injury.




Step and Touch Potential (Cont'd)

As given by IEEE Std. 80, the following formulas are used to determine the allowable step-and-touch potentials:

_ The maximum driving voltage of any accidental circuit should notexceed the following limits:

- For step voltage the limit is for a man weighing 50 kilograms is:

Similarly, the touch voltage limit is:

where:Cs (hs1 K) = 1 for no protective surface layer.

_s = the resistivity of the surface material in ohm - M =150 (assumed measured value) or 3000 ohm-m forcrushed rock.

ts = duration of shock current in seconds - (1 secondfor Saudi Aramco installations).

Grid Spacing

If either the step voltage limit or the touch voltage limit are exceeded, a revision of the griddesign is required. The revision may include smaller conductor spacings, and addingadditional ground rods. Details of the potential revision can be found in IEEE Std. 80,Section 14.7.

Even if the step voltage limit or touch voltage limits are met, additional grid conductors andground rods can be required if the grid design does not include conductors that are near theequipment to be grounded (such as surge arrestors and transformers).





Another method to improve step and touch potentials is addition of crushed rock on thesurface of the soil. Crushed rock will change the K factor in the CS (hS1K) portion of thestep voltage limit and touch voltage limit formula as follows:

where: _s = crushed rock resistivity in ohm-m

_ = earth resistivity in ohm-m

The reduction factor Cs is also changed as a function of the change in K and the change in thethickness of the layer of crushed rock (hs), as shown in Figure 8.

Reduction Factor Cs as a Function of Reflection Factor K andCrushed Rock Layer Thickness hs

Figure 8





Example: Find the ETouch 50 for the following conditions:

_s = 2000 (using crushed rock)ts = 1hs = 0.1_ = 150

for this example, K is determined as follows:

From Figure 8, Cs = 0.58

Therefore:= 318V

Grid Depth and Number of Ground Rods

Saudi Aramco Engineering Standard SAES-P-111 states that ground grids are to be buried toa minimum depth of 460 mm (18 in.). This method effectively reduces step and touchvoltages on the earth's surface.

The total length of the grid reduces the step and touch voltages and the grid resistance. Thegrid length includes both conductor length and ground rod length.

The physical conditions at a substation dictate the number and the length of the ground rodsvs. the conductor grid length. The ground rods are normally installed at the perimeter of thegrid to moderate the increase of the surface gradient that is near the peripheral meshes.Ground rods should also be installed at major equipment and especially at lightning arresters.Rods that penetrate the lower resistivity soil are far more effective in dissipating fault currentswhen a two-or-multilayer soil is encountered and when the upper soil layer has higherresistivity than the lower soil layers. Ground rods that are in proximity are far less effective atdissipating fault currents than individual ground rods that are well spaced.

Wire Sizing

In AWG, the numbers are regressive: that is, a larger number denotes a smaller wire. Eachwire size (in AWG) often is represented in circular mils. One circular mil (cm) is the area of awire with a diameter of 0.001 inches. The cm measure is simply the diameter in mils squared.




Under fault conditions, all of the heat is assumed to be retained in the conductor, because littletime is available to dissipate the heat. The fusing temperature of the conductor, thetemperature limit of the connections, and the physical strength of conductors are evaluated todetermine a conductor size. The conductor size must relate to the current/time rating of theneutral grounding device or the devices, subject to a minimum size of 780 sq. mm. (No. 2/0AWG) for mechanical robustness. Figures 9 and 10 show the minimum size for conductors tobe used to ground Saudi Aramco systems.

Solidly Grounded Systems over 600V




Figure 9




Wire Sizing (Cont'd)

Impedance Grounded Systems over 600VFigure 10

Fault Times

Fault time is the duration of time that the fault current flows before being interrupted. Therequired wire size (Figures 9 and 10) may not be adequate for assumed long fault times. Iflong fault times are assumed, larger wire sizes may have to be used. The required wire size,based on fault times, can be calculated to determine whether the required wire size is withinthe suggested size in Figures 9 and 10.




Fault Times (Cont'd)

The following formula is used to relate the conductor's size to the assumed faulttime/temperature limits:

where: I = RMS current in kA.A = conductor cross-section in cmils.TM = maximum allowable temperature in oC.Ta = Ambient temperature in oC.ar = thermal coefficient of resistivity at reference temperature

Ta in oC._r = the resistivity of the ground conductor at reference

temperature Ta in __-cm.Ko = I/ao, or (I/dr) - Tr.tc = time of current flow in sec.TCAP = thermal capacity factor, in J/cm3/oC.

For standard annealed soft copper wire:

ar @ 20oC = 0.00393

_r @ 20oC = 1.7241

TCAP (J/cm3/0o) = 3.422

Tm(0oC) = 1083

Ko (1/ao @ 0oC) = 234

I = KA




Fault Times (Cont'd)

As an example, assume a ground fault of 22,000A that lasts for a duration of five seconds.

By calculation, the required wire size for a 22 kA fault that is assumed to last five seconds is530 cm. Because the calculated value exceeds the values that are given in Figure 9, thecalculated value must be used.




WORK AID 1: SAUDI ARAMCO AND INDUSTRY STANDARDS FORLOCATING SYSTEM GROUNDING INFORMATION

This Work Aid is designed to help the Participants in performing Exercise 1.

Saudi Aramco Design Practices

_ SADP-P-111 : Grounding

Saudi Aramco Engineering Standards

_ SAES-P-100 : Basic Power System Design Criteria_ SAES-P-111 : Grounding_ SAES-P-119 : Substations

IEEE Standards

_ IEEE 80 : IEEE Guide for Safety in AC Substation Grounding

_ IEEE 81 : IEEE Guide for Measuring Earth Resistivity, GroundImpedance, and Earth Surface Potentials of a Ground System

_ IEEE 142 : IEEE Grounding of Industrial and Commercial PowerSystems

_ IEEE 367 : IEEE Recommended Practice for Determining the ElectricPower Station Ground Potential Rise and Induced Voltage from a PowerFault

National Electrical Code




WORK AID 2: TABLE OF SAUDI ARAMCO SYSTEM GROUNDING METHODS

This Work Aid is designed to assist the Participants in performing Exercise 2. This Work Aidshows the grounding method that should be used for different combinations of voltage,phases, and loads.

System Voltage Phase Grounding Method Comments120/240V 1 Solidly Grounded208/120V 3 Solidly Grounded

480V 3 Solidly Grounded4160V 3 Low Resistance Grounded

(400A, 10 Sec, Resistor)13,800V 3 Low Resistance Grounded

(400A, 10 Sec, Resistor)Industrial Load

13,800V 3 Low Resistance Grounded(1000A, 10 Sec. Resistor)

Residential Distr.System

69,000V 3 Solidly Grounded115,000V 3 Solidly Grounded230,000V 3 Solidly Grounded




WORK AID 3: PROCEDURES AND REFERENCES FOR DESIGNINGSUBSTATION/PLANT GROUND GRIDS

This Work Aid is designed to help the Participants in performing Exercise 3. Exercise 3requires the Participants to design a substation/plant ground grid for a hypotheticalinstallation. The Participant must complete the following steps that are covered in thedesignated Work Aid to design a substation/plant ground grid.

_ Determine need for Ground Grid Protection - Work Aid 3A

_ Determine Step Potential - Work Aid 3B

_ Determine Touch Potential - Work Aid 3B

_ Determine Required Grid Spacing - Work Aid 3B

_ Determine Number of Grounding Rods - Work Aid 3C

_ Determine Ground Wire Sizes - Work Aid 3D

_ Adjust Ground Wire Size for Fault Time - Work Aid 3E




WORK AID 3 (Cont'd)

Work Aid 3A: Checklist for Evaluating the Need for Ground Potential Rise (GPR)Protection

This Work Aid is designed to help the Participant in performing Exercise 3A.

To evaluate the need for ground potential rise protection, perform the following steps:

_ Calculate Rg; the impedance to remote earth of the grounding electrode.

where: _ = Earth resistivity of substation in ohms-m.A = Area occupied by ground grid in m2.h = Depth of the ground grid in m.L = Buried length of conductors in m not including

ground rods.

_ Calculate Ig; fault current flowing to the grounding electrode.

Ig = Ground fault current current division factor

_ Calculate GPR; Ground potential rise

GPR = Rg � Ig

_ If the GPRis less than 300V, the area is classified as a low risk site. Noprotection is required for communication equipment.

_ If the GPR is between 300V to 1500V, the area is classified as amoderate hazard site. Protection must be applied to all communicationequipment circuits.

_ If the GPR is above 1500V, the area is classified as severe hazard site.Protection must be applied to all communication equipment circuits.




WORK AID 3 (Cont'd)

Work Aid 3B: Formulas and Procedures for Determining Step and TouchPotentials and Grid Spacing

This Work Aid is designed to help the Participant in performing Exercise 3B.

_ To calculate the step voltage limit for a man weighing 50 kg, use theformula:

where: Cs (hsiK) = factor for surface soil_s = resistivity of surface soil materialts = duration of shock current inseconds

_ To calculate the touch voltage limit for a man weighing 50 kg use theformula:

_ Compare the resultant ESTEP 50 limit and the ETOUCH 50 limit to thecalculated GPR. If both the ESTEP 50 limit and the ETOUCH 50 limitare above the GPR, no further actions are required, the ground griddesign is complete. If either (or both) of the ESTEP 50 limit or theETOUCH 50 limits are below the GPR, further design improvements ofthe ground grid are required.

_ If the ground grid design uses the normal soil as the surface soil,consider addition of a layer of crushed rock to the surface, which willincrease the resistivity of the surface.

_ If crushed rock is added recalculate ESTEP 50 and ETOUCH 50.

_ A further reduction in ESTEP 50 and ETOUCH 50 can be accomplishedthrough an increase in the length of the ground grid conductor andthrough an increase in the number of ground rods. (Note: An increasein the length of the ground grid conductor and the number of groundgrids can only be accomplished through change to the grid spacingbecause the overall dimensions of the grid have already beenestablished). The following equation will give an estimation of therequired length of the ground grid conductor to obtain the maximumvoltage below the ESTEP 50 and ETOUCH 50 limits:




WORK AID 3 (Cont'd)

here: L = Total length of grounding grid including length of grid conductor and ground rods in m.

Ki = 0.656 + 0.172n.

n = Number of parallel conductors in one direction; also equals for equally spaced rectangular grids where

NA is the number of conductors running in one direction, and NB is the number of conductors

running in the other direction._ = Soil resistivity in ohm-m.

IG = Maximum grid current that flows between the groundgrid and the surrounding earth.

ts = Duration of shock current in seconds.

Cs(hs1K) = Factor for surface soil.

_s = Resistivity of the surface soil.

Km = Mesh factor determined by formula below:

where: D = spacing between parallel conductors in m.

h = depth of ground grid in m.

d = diameter of ground grid conductor.

Kh =

WORK AID 3 (Cont'd)

Kii = 1 if ground grid has rods along perimeter.

or

Kii = for grids with no ground rods or only a few ground rods and not on perimeter.




_ When the required length of ground grid (L) is known, determine howmuch of L will be Lc (total grid conductor length) and how much willbe LR (total ground rod length). L can be expressed by the equation:

L = Lc - LR

_ For the new value of LC and LR, determine the maximum meshvoltage. The maximum mesh voltage should be less than (or equal to)the ETOUCH 50 limit. The maximum mesh voltage (Em) is calculatedthrough use of the following equation:

_ Use the new value of L to calculate the maximum step voltage. Themaximum step voltage should be less than (or equal to) the ESTEP 50limit. The maximum step voltage (Es) is calculated through use of thefollowing equation:

_ If Em and Es are equal to or less than the ETOUCH 50 and ESTEP 50limits, the length of LC and LR are acceptable.




WORK AID 3 (Cont'd)

Work Aid 3C: Formulas, Procedures, and References for Determining Number ofGround Rods

This Work Aid is designed to help the Participant perform Exercise 3C.

_ To calculate the total resistance for a group of ground rods, theresistance of one rod must be calculated first.

_ Through use of SADP-P-111, the resistance of one rod can bedetermined from Figure 13.

_ Ratio the value taken off of Figure 13 to the actual soil resistivitythrough use of the following equation:

_ Find the group ratio for the spacing of the ground rods for Figure 14.

_ To find the resistance for the group of ground rods, take the valuecalculated as the actual value for a single rod, and the group ratio; usethe following equation to calculate:




WORK AID 3 (Cont'd)

Resistance of a Single RodFigure 13




WORK AID 3 (Cont'd)

Ground RatioFigure 14




WORK AID 3 (Cont'd)

Work Aid 3D: Table of Wire Sizes and Ampacity

Use Work Aid 3D to complete Exercise 3D.

The size of ground grid conductors is determined by the magnitude of the fault current andthe time of flow, based on the maximum allowable temperature rise.

Figures 15 and 16 show wire sizes vs. short-time ampacity for Saudi Aramco Systems.

Solidly Grounded Systems over 600VFigure 15




WORK AID 3 (Cont'd)

Impedance Grounded Systems over 600VFigure 16




WORK AID 3 (Cont'd)

Work Aid 3E: Procedures and References for Adjusting Ground Wire Sizes forFault Times

Use Work Aid 3E to complete Exercise 3E.

_ After the size of the ground wire has been determined, the ability of theground wire to handle the expected short-time current withoutexceeding the temperature limit must be verified. The temperature limitof 450oC is for use when the ground grid has brazed connections. The250oC limit is for use when the connections are bolted.

_ Use Figure 17 to find the minimum allowed circular mils for theexpected fault current, given the time duration.

_ Multiply the value from Figure 17 by the expected ground fault current.




WORK AID 3 (Cont'd)

Nomogram for Conductor SizingFigure 17




GLOSSARY

circuit breaker A device that is designed to open and close a circuit bynonautomatic means and to open the circuit automatically on apredetermined overcurrent without damage to itself whenproperly applied within its rating.

electric potential The potential difference between the point and someequipotential surface, usually the surface of the earth, which isarbitrarily chosen as having zero potential (remote earth).

electrical noise The disturbance in an electrical system that interferes with thenormal transmission of signals carrying information.

equipment ground A ground connection to non-current carrying metal parts of awiring installation or of electric equipment, or both.

fault time The duration for which a fault current flows prior to beinginterrupted.

ground A conducting connection, whether intentional or accidental, bywhich an electric circuit or equipment is connected to the earth,or to some conducting body, of relatively large extent and thatserves in place of the earth.

ground bus A bus to which the grounds from individual pieces of equipmentare connected and that, in turn, is connected to ground at one ormore points.

ground circuit A circuit in which one conductor or point (usually the neutralconductor or neutral point of transformer or generator windings)is intentionally grounded, either solidly or through a groundingdevice.

ground conductor A conductor or system that is intentionally grounded, eithersolidly or through a current-limiting device.

ground current Current that is flowing in the earth or in a grounding connection.




ground grid A system of grounding electrodes consisting of interconnectedbare cables that are buried in the earth to provide a commonground for electric devices and metallic structures.

ground potential An AC potential difference between remote earth and local rise(GPR) ground.

grounded Connected to earth or to some extended conducting body thatserves in place of the earth, whether the connection is intentionalor accidental.

grounding conductor The conductor that is used to establish a ground and that(ground conductor) connects an equipment, device, wiringsystem, or another conductor (usually the neutral conductor)with the grounding electrode or electrodes.

grounding electrode A conductor that is used to establish a ground (for instance,(ground electrode) ground grids, ground rods, or groundwells).

grounding transformer A transformer that is primarily intended to provide a neutralpoint for grounding purposes.

impedance grounded Grounded through impedance.

neutral ground An intentional ground applied to the neutral conductor or neutralpoint of a circuit, transformer, machine, apparatus, or system.

reactance grounded Grounded through impedance, the principle element of which isreactance.

resistance grounded Grounded through impedance, the principle element of which isresistance.

resistivity (material) A factor such that the conduction-current density is equal to theelectric field in the material divided by the resistivity.

service ground A ground connection to a service equipment or a serviceconductor or both.




solidly grounded Grounded through an adequate grounded connection in which(directly grounded) no impedance has been insertedintentionally.

static electricity The accumulation of electrostatic charges on the surfaces ofconducting and non-conducting bodies that are insulated fromtheir surroundings.

step potential The potential difference between two points on the earth'ssurface, separated by a distance of one pace, that will beassumed to be one meter, in the direction of maximum potentialgradient.




surface soil The resistance of the upper layer of soil in a ground-grid area.resistivity

transfer potential The relocation of a hazardous potential from a ground-grid areato outside points.

touch potential The potential difference between a grounded metallic structureand a point on the earth's surface separated by a distance equal tothe normal maximum horizontal reach (approximately onemeter).

ungrounded A system, circuit, or apparatus without an intentional connectionto ground except through potential indicating or measuringdevices or other very high impedance devices.

voltage to ground The voltage between any live conductor of a circuit and theearth.

zig zag transformers A special grounding transformer that has two-phase windings oneach core leg.

zone of influence The potential gradient outside of the site.

grounding

Documents

10oc temperature

national electrical

zig zag transformer

commercial

electronic

ieee recommended

work aid 3e

work aid 3d