500 kv Substation Grounding presentation

Name:

Roll No:

Rabbia Khan

2010-EE-13

Sidra Iqbal

2010-EE-17

Anum Shafqat

2010-EE-49

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INTRODUCTION

Definition: Earthing means a conducting connection by which an electric circuit or  equipment is connected to the earth or some conducting body of  relatively large extent that serves in place of the earth". Earthing means the connection of non-current carrying parts to

ground.

Function: The function of an earthing system for an electrical installation can be split into three broad bands, • To limit the potential of any part of an installation to a pre-determine value with respect to the general mass of earth. • To permit the flow of current to earth. • To ensure that, if a fault occurs, non-current carrying metal work  associated with the equipment does not attain a dangerous potential with respect to the general mass of earth.

Purpose of Earthing: Earthing system has 3 main purposes:

1-Over voltage protection 2-Voltage stabilization 3-Currentt path in order to facilitate the operation of over  current devices.

Reasons: There are a number of good reasons of earthing but primary among them is to ensure personnel safety. A good earthing system will improve the reliability of equipment and reduce the likelihood of  damage as a result of lightning or fault currents.

SAFETY: Main purpose of earthing is “safety”. That is the protection of: 1.

Personnel Safety

2.

Equipment Safety

Personnel Safety: It is for human life safety. Personnel protection from electrical shocks, fire etc.

Equipment Safety: It includes electrical circuit or equipment protection from failure, over current protection, fire, cable failures etc

Grid Station Earthing: The sole purpose of substation grounding/earthing is to protect the equipment from surges and lightning strikes and to protect the operating persons in the substation.

TYPES

CLASSIFICATION OF EARTHING Earthing can be classified into the following categories based on the  purpose for which the part of the equipment connected to the general mass of earth. • System Earthing • Equipment Earthing

SYSTEM EARTHING Earthing associated with current carrying parts of the equipment is called system Earthing. The system security, reliability, performance, voltage stabilization, all relied only on the system Earthing. For example Earthing Neutral of  Transformer, Surge arrester Earthing System Earthing Methods : a. Solid Earthing  b. Resistance Earthing

Solid Earthing This type of grounding system is most commonly used in industrial and commercial power systems, where grounding conductors are connected to earth ground with no intentional added impedance in the circuit.

Resistance Earthing In resistance earthing the resistance is added along the earthing conductor to keep fault currents within limits. Thus protecting the insulation of the conductor. It has 2 types: Low resistance Earthing (voltages below 150V) High resistance Earthing (150-600V)

EQUIPMENT EARTHING Earthing associated with non-current carrying parts of Electrical Equipment are called as Equipment Earthing. Safety of operator, consumer, safety of  their property are mainly based on Equipment Earthing. Eg. Body of the Transformer, Body of Motor 

EARTHING DESIGN

Earthing Design The substation ground grid design is based on the substation layout  plan. The following points serve as guidelines to start a earthing grid design: •The substation should surround the perimeter and take up as much area as possible. It reduces resistance of the earthing grid. •Typically conductors are laid in parallel lines. •Typical substation grid systems may include 4/0 bare copper  conductor buried 0.3-0.5 m (12-18 in) below grade and spaced 3-7 m (10-20 ft) apart in a grid pattern.

 All earth connections are to be made visible as far as possible.

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GROUND RODS (Electrodes) Materials are selected for corrosion resistance: Galvanized steel rods are cheap but have a relatively short service life Solid copper and stainless steel rods have a long service life but are considerably more expensive Copper bonded earth rods are less expensive than solid copper and can be deep driven •

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Comparison of life expectancy

Earthing Conductors: 

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The earthing conductor is commonly called the earthing lead. It  joins the installation earthing terminal to the earth electrode or  to the earth terminal provided by the Electricity Supply Company. It is a vital link in the protective system, so care must be taken to see that its integrity will be preserved at all times. Mostly copper is used for earthing conductors because it

has the highest electrical conductivity of any of the commercial metals. Copper is resistant to corrosion, that is, it will not rust. It is malleable, ductile and has long life.

Earthing Conductors

Types of earthing conductor  

Protective conductor 

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Bonding concuctor 

Circuit Protective Conductor CPC This is a separate conductor installed with each circuit and is present to ensure that some, or all, of the earth fault current will flow back to source along it.

Bonding Conductor  These ensure that exposed conductive parts (such as metal enclosures) remain at approximately the same potential during electrical fault conditions.

Earth resistance: 

Earth Resistance is the resistance offered by the earth electrode to the flow of current in to the ground. The fault current is to be cleared as quickly as possible and this is done by having the earth resistance low. Persons touching any of the non current carrying grounded parts shall not receive a dangerous shock during an earth fault. Each structure, transformer tank, body of equipment, etc, should be connected to earthing mat by their own earth connection.

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Generally lower earth resistance is preferable but for certain applications following earth resistance are satisfactory

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In large power station - 0.5 Ω

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Major Substation above 110KV - 1.0 Ω

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Minor Substations Below 110KV - 2.0 Ω

Monitoring the condition of earth 

For monitoring the healthiness of earth, the condition monitoring equipment used is “EARTH MEGGER ”.

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The megger is a portable instrument used to measure resistance. It is used to measure very high resistance of the order of mega ohms.

Checking and testing 

The Earthing systems are to be inspected regularly.

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Regular checking of joints and broken connections, if any and rectifying the same will prove to be of immense help in maintenance of earth grid and equipment‟s.

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The condition of the electrodes, joints are also to be checked.

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If the electrodes are‟ corroded immediate

steps for replacement are to be taken. 

The earth resistance is to be measured

 periodically.

Earth Electrodes 

A conductor buried in the ground, used to maintain conductors connected to it at ground potential and dissipate current conducted to it into the earth, known as earth electrode; grounding electrode electrode.

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Why must we have earth electrodes?

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The purpose of the earth electrode is to connect to the general mass of earth.The principle of earthing is to consider the general mass of  earth as a reference (zero) potential. Thus, everything connected connected directly to it will be at this zero potential.

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The effectiveness of an earth electrode in making good contact with the general mass of earth depends on factors such as soil type, moisture content, and so on. A permanently-wet situation may  provide good contact with earth, but may also limit limit the life of the electrode since corrosion is likely to be greater 

Earth electrode types Driven Rod Grounding Plates Electrolytic Electrode   

Driven Rod The standard driven rod or copper-clad rod consists of an 8 to 10 foot length of  steel with a 5 to 10-ml coating of copper. copper. This is by far the most common grounding device used in the field today. Driven rods are relatively inexpensive to  purchase, however ease of installation installation is dependent upon the type of soil and terrain where the rod is to be installed. The steel used in the manufacture of a standard driven rod tends to be relatively soft. Purpose of copper on the rod is to  provide corrosion corrosion protection for the steel underneath. 

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Grounding Plates 

Grounding plates are typically thin copper plates buried in direct contact with the earth. The National Electric Code requires that ground plates have at least 2 ft2 of surface area exposed to the surrounding soil. Grounding plates should be buried at least 30 inches below grade level. Non-ferrous materials (copper) need only  be .060 inches thick. Grounding plates are typically typically placed under   poles.

Electrolytic Electrode 

The electrolytic electrode was specifically engineered to eliminate the drawbacks of other grounding electrodes. This active grounding electrode consists of a hollow copper shaft filled with natural earth salts and desiccants whose hygroscopic nature draws moisture from the air. The moisture mixes with the salts to form an electrolytic solution that continuously seeps into the surrounding backfill material, keeping it moist and high in ionic content. The electrolytic electrode is installed into an augured hole and backfilled with a special highly conductive product.

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The electrolytic solution and the special backfill material work together to provide a solid connection  between the electrode and the surrounding soil that is free from the effects of temperature, environment, and corrosion

Multiple Electrodes

Earthing Electrodes A typical earthing electrode (left), consisting of a conductive rod driven into the ground

Soil Resistivity 

Soil resistivity is a measure of how much the soil resists the flow of  electricity.

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It is a critical factor in design of systems that rely on passing current through the Earth's surface. An understanding of the soil resistivity and how it varies with depth in the soil is necessary to design the grounding system in an electrical substation.

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In general there is some value above which the impedance of the earth connection must not rise, and some maximum step voltage which must not be exceeded to avoid endangering people and livestock.

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The soil resistivity value is subject to great variation, due to moisture, temperature and chemical content.

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Typical values are:

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Usual values: from 10 up to 1000 (Ωm)

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Exceptional values: from 1 up to 10000 (Ωm)

SI unit of resistivity 

The SI unit of resistivity is the Ohm-meter (Ωm); in the United States the Ohm-centimeter (Ωcm) is often used instead.

Measurement of soil resistivity 

Wenner method

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Schlumberger method

Wenner method 

The “Wenner ” method is one of the widely used methods for measuring soil resistivity.

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In this method, four test rods are inserted a short distance into the soil in a straight line with equal spacing between the probes. A test current is applied to the outer probes and the resulting potential difference between the inner   probes, is measured.

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The potential difference divided by the test current give an apparent resistance in ohms. The apparent soil resistivity is obtained from the measured resistance.

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Using the Wenner method, the apparent soil resistivity value is:

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where

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ρE = measured apparent soil resistivity (Ωm)

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a = electrode spacing (m)

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 b = depth of the electrodes (m) RW = Wenner resistance measured as “V/I” in Figure (Ω) If b is small compared to a, as is the case of probes penetrating the ground only for a short distance (as normally happens), the previous equation can  be reduced to: ρE = 2πaR 

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Schlumberger method

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In the Schlumberger method the distance between the voltages probe is a and the distances from voltages probe and currents probe are c.

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Using the Schlumberger method, if b is small compared to a and c, and c>2a, the apparent soil resistivity value is:

where ρE = measured apparent soil resistivity (Ωm) a = electrode spacing (m)  b = depth of the electrodes (m) c = electrode spacing (m) RS = Schlumberger resistance measured as “V/I” in Figure (Ω)     

Measurement of soil resistivity

EARTH MAT DESIGN 

Earthing System in a Sub Station comprises of Earth Mat or Grid, EarthElectrode, Earthing Conductor and Earth Connectors.

Earth Mat or grid: 

Bonding all metal parts of the system to be earthed, the earth conductor and the earth electrodes put all together form an Earth Grid.

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Primary requirement of Earthing is to have a low earth resistance. Substation involves many individual Electrodes, which will have fairly high resistance. But if these individual electrodes are inter linked inside the soil, it increases the area in contact with soil and creates number of parallel  paths. Hence the value of the earth resistance in the interlinked state which is called combined earth value which will be much lower than the individual value.

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These Earth Mat and Earth electrode is connected to the equipment structures, neutral points for the purpose of Equipment earthing and neutral  point earthing.It keeps the surface of substation equipment as nearly as absolute earth potential as possible.

EARTH MAT

Potential Hazards

Potential Hazards 

In electrical engineering, earth potential rise (EPR) also called ground potential rise (GPR) occurs when a large current flows to earth. The potential relative to a distant point on the Earth is highest at the point where current enters the ground, and declines with distance from the source. Ground potential rise is a concern in the design of electrical substations because the high  potential may be a hazard to people or equipment.

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The change of voltage over distance (potential gradient) may be so high that a person could be injured due to the voltage developed  between two feet, or between the ground on which the person is standing and a metal object

Electrical Shock Situations: There are three main electrical shock situations that can occur when a person is around a substation. foot-to-foot shock  hand-to-feet shock  hand-to-hand shock or metal-to-metal contact

Step Potential: Step potential is the potential difference between the feet of a person standing on the floor of the substation, with 1m spacing between the feet (one step), through the flow of earth fault current through the grounding system.

Touch Potential: Touch potential is a potential difference between the fingers of raised hand touching the faulted structure and the feet of the person standing on the substation floor. The Touch Potential should be very small.

Point to be noted 

Tolerable touch potential of human body is less than tolerable step potential.

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Hence „Touch potential ‟ is more critical for design while Step potential is usually academic.

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Step potential is independent of the diameter ( cross- section) of the earthing conductor.

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For 400% increase in diameter, reduction in Touch potential is only 35%.

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Thus cross- section has minor influence on Touch and Step  potentials.

Ensuring Proper Grounding The following steps, when put into practice, will ensure a reliable, safe and trouble-free substation grounding system: 1. Size conductors for anticipated faults

2. Use the right connections 3. Ground rod selection 4. Soil preparation 5. Attention to step and touch potentials

1-Size Conductors For Anticipated Faults Conductors must be large enough to handle any anticipated faults without fusing (melting).Failure to use proper fault time in design calculations creates a high risk of melted conductors. For example, a AWG conductor can withstand 42,700A for 0.5 sec before fusing. However, this same conductor can withstand only 13,500A for 5 sec. Two aspects govern the choice of conductor size: the first is the fault current that will flow through the conductor and the second is the time for which it can flow. The IEEE 80 suggests using a time of 3.0 s for the design of  small substations.

2. Use the Right Connections Grounding Connections, Resistance Test and Bonding Test  It is very evident that the connections between conductors and the main grid and between the conductor and ground rods are as important as the conductors themselves in maintaining a permanent low-resistance path to ground.  Connections must maintain the integrity of the conductor and the system as a whole for up to 40 years.  They must be of an appropriate material and mass to:  carry prospective fault currents   be able to resist corrosion  maintain original low resistance  The basic issues here are:  The type of bond used for the connection of the conductor in its run, with the ground grid and with the ground rod.

The temperature limits, which a joint can withstand.

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The most frequently used grounding connections are mechanical  pressure type.

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Pressure-type connections produce a mechanical bond between conductor and connector by means of a tightened bolt-nut or by crimping using hydraulic or mechanical pressure.

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On the other hand, the exothermic process fuses the conductor ends together to form a molecular bond between all strands of the conductor.

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Temperature limits are stated in standards such as IEEE 80 and IEEE 837 for different types of joints based on the joint resistance normally obtainable with each type. Exceeding these temperatures during flow of fault currents may result in damage to the joint and cause the joint resistance to increase, which will result in further  overheating.

3. Ground Rod Selection In MV and HV substations, where the source and load are connected through long overhead lines, it often happens that the ground fault current has no metallic path and has to flow through the groundmass (earth). This means that the ground rods of both source and load side substations have to carry this current to or from the groundmass. 

The ground rod system should be adequate to carry this current and ground resistance of the grounding system assumes importance.

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The length, number and placement of ground rods affect the resistance of  the path to earth. Doubling of ground rod length reduces resistance by a value of 45%, under uniform soil conditions. Usually, soil conditions are not uniform and it is vital to obtain accurate data by measuring ground rod resistance with appropriate instruments.

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For maximum efficiency, grounding rods should be placed no closer  together than the length of the rod. Normally, this is 10 ft (3 m).

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It should be noted that as the number of rods is increased, the reduction of ground resistance is not in inverse proportion. Twenty rods do not result in 1/20th of the resistance of a single rod but only reduce it by a factor of 1/10th.

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For economic reasons, there is a limit to the maximum distance  between rods.

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 Normally, this limit is taken as 6 m. At more than 6 m, the cost of  additional conductor needed to connect the rods.

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4. Soil Preparation

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Soil resistivity is an important consideration in substation grounding system design. The lower the resistivity, the easier it is to get a good ground resistance.

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Areas of high soil resistivity and those with ground frost need special consideration. The highest ground resistivity during the annual weather  cycle should form the basis of the design since the same soil will have much higher resistivity during dry weather when percentage of moisture in the ground becomes very low.

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One approach to take care of this problem is to use deep driven ground rods so that they are in contact with the soil zone deep enough to remain unaffected by surface climate.

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The other approach is to treat the soil around the ground rod with chemical substances that have the capacity to absorb atmospheric/soil moisture.

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Use of chemical rods is one such solution

5. Attention to Step and Touch Potentials 

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Limiting step and touch potential to safe values in a substation is vital to  personnel safety. Step potential is the voltage difference between a  person‟s feet and is caused  by the voltage gradient in the soil. The potential gradient is steepest near the fault location and thereafter reduces gradually. Touch potential represents the same basic hazard, except the potential exists  between the  person‟s hand and his or her feet. This happens when a person standing on the ground touches a structure of the substation, which is conducting the fault current into ground . In both situations, the potential can essentially be greatly reduced by an equipotential wire mesh safety mat installed. Such an equipotential mesh will equalize the voltage along the worker‟s path and between the equipment and his or her feet. With the voltage difference (potential) thus essentially eliminated, the safety of personnel is virtually guaranteed. To ensure continuity across the mesh, all wire crossings are brazed with a 35% silver alloy. Interconnections between sections of mesh should be made so as to provide a permanent low-resistance high-integrity connection.

CONCLUSION

CONCLUSION: For good earthing following considerations must be followed: •Size conductors •Selection of right connector  •Pay attention to ground rod length, number, placement, and spacing •Prepare the soil •Eliminate step and touch potential •Ground the foundation •Ground all disconnects switch handles •Ground all surge arrestors •Pay attention to temporary grounding