Chapter One Introduction

CHAPTER ONE INTRODUCTION 1.1 General Concepts The practice of grounding of electrical systems is almost as old as the development and widespread use of electric power itself, Poor grounding not only contributes to unnecessary downtime, but a lack of good grounding is also dangerous and increases the risk of equipment failure. Without an effective grounding system, we could be exposed to the risk of electric shock, not to mention instrumentation errors, harmonic distortion issues, power factor problems and a host of possible intermittent dilemmas. If fault currents have no path to the ground through a properly designed and maintained grounding system, they will find unintended paths that could include people. However, good grounding isn‟t only for safety; it is also used to prevent damage to industrial plants and equipment. A good grounding system will improve the reliability of equipment and reduce the likelihood of damage due to lightning or fault currents. Billions are lost each year in the workplace due to electrical fires. This does not account for related litigation costs and loss of personal and corporate productivity.(" Practical Grounding, Bonding, Shielding and Surge Protection") 1.2 Problem Statement This research aims to study the effect of earthing resistance on the service voltage of distribution transformers. 1.3 Objectives This research aims to study the effect of earthing resistance on the service voltage of distribution transformers, decrease of the earthing resistance of distribution transformers, and also balance of distribution of loads to face transformers faults.

1.4 Methodology Earth resistance readings obtained by using a device named (Earth/Ground tester 1625). Voltage readings of earth to neutral, line to neutral, line to earth, and loads obtained by using a device named Clampmeter 1.5 Project layout This Project consists of five chapters, Chapter one deals with an introduction that consists of, general concepts, problem statement, objective and project organization. .Chapter two consist of an introduction of distribution transformers, its classification, its connection, its component, grounding, earthing, terms used in electrical earthing, its function, ungrounded systems, its types, grounding resistance, and point of grounding. Chapter three consist of an introduction, grounding electrode, soil resistance, measurement of soil resistivity, resistance of a single rod electrode, Current-Carrying Capacity of an Electrode, Use of Multiple Ground Rods in Parallel, Measurement of Ground Resistance of an Electrode, Concrete-Encased Electrodes, Corrosion Problems in Electrical Grounding Systems, Maintenance of Grounding System and Chemical Electrodes. Chapter Four includes, Introduction, Results and Analysis.Chapter five include conclusions and recommendation.

CHAPTER TWO DISTRIBUTION TRANSFORMERS AND GROUNDING

2.1 Introduction Distribution transformer is a static device consisting of a winding, or two or more coupled windings, with or without a magnetic core, for inducing mutual coupling between circuits. They are exclusively used in electric power systems to distribute power by electromagnetic induction between circuits at the same frequency, usually with changed values of voltage and current. Distribution transformers is one of the important electrical isolation transformers which provide the final output voltage to the end users. as the name specifies it distributes the power to costumers depends on their need. The basic function of distribution transformers is to step down the line voltage (33KV–11KV–6.6KV) to the consumer low voltage (415V) for supplying it to them. Because normally the transmission line voltages will be high to increase the power transfer without losses but the electrical devices which the consumer uses will operate for low voltage. So the line voltage will be stepped down to consumer level and distribute it to them efficiently. Distribution transformers are used extensively by traditional electric utility companies, power plants, and industrial plants. As mentioned above, they perform a very simple function and they can have many applications. Transformers are used in every power plant, all grid substations, buildings, in the industry, the underground installations, wind turbines, on platforms, marine vessels, under the sea, etc. Due to peculiarities of all these applications, many different types of distribution transformers have been developed in the course of history.(" Practical Troubleshooting Electrical Equipment")

2.2 Classification To simplify the overview of many distribution transformer types, it is useful to have some kind of systematic classification. However, this is not easy to do because there are many ways of doing it. The distribution transformer types could be classified according to their power rating, voltage, current, weight, type of cooling etc., but such approach would have a limited applicability. 2.2.1 classification due to number of phases in A- Single a phase transformers. B- Three-phase transformers. In a three-phase system, the single-phase units are used in a bank of three transformers linked together. A single three-phase transformer costs approximately 15% less and occupies less space than one unit of three single-phase transformers within the same tank. However, due to limitations during the manufacturing and mainly transportation, particularly of large units, the transformers sometimes must be produced as single-phase transformers. Another reason for using a single-phase unit rather than a three-phase unit is the possibility of having a fourth identical unit as a spare. Despite its simplicity and clarity, this type of classification does not overly help in classification of the whole distribution transformers family.("Practical Troubleshooting Electrical Equipment") 2.2.2 According to basic technology, design and manufacturing

There are two main technologies for designing and manufacturing the distribution transformers: A- Core type B- Shell type In a shell-formed transformer, the primary and secondary windings are quite “flat”

and are positioned on one leg surrounded by the core. In a core-formed transformer, cylindrical windings are like “coils” and cover the core legs. However, this classification is also limited in the large portfolio of either of those two distribution transformer types. 2.2.3 According to the insulating/cooling fluid in A- Liquid-filled transformers B- Gas-filled transformers (mainly with SF6) C- Dry-type transformers 2.2.4 According to location A. INDOOR TRANSFORMERS – Is one which because of construction much be protected from weather. Usually dry type or non-flammable oil immersed type. Shown in appendix(a). B. OUTDOOR TRANSFORMERS – Is of weather resistant construction suitable for service without the additional protection from weather. Usually of the mineral oil immersed type. Shown in appendix(b). 2.3 Connection of Distribution Transformers Modern electrical systems are almost exclusively three-phase systems, notwithstanding the many miles of distribution circuits that are configured a single- phase taps off of these systems. In addition, there still exist remnants of two-phase systems (typically in mining operations) that were fairly common years ago. When two polyphase systems have different voltages and/or phase angles, these systems can be interconnected using transformers having various possible types of connection.AS any one of these connections can be accomplished either with a bank of single phase transformers or by a single polyphase transformer. As we shall see in this chapter, it is in fact possible to interconnect two poly-phase systems having a different number of phases using special transformer connections. Single-phase two-winding transformer is nothing more than a primary and a 5

secondary winding wound around the same magnetic core. Single-phase two- winding transformers can be used in either single-phase circuits or poly-Phase circuits. A polyphase two-winding transformer contains a number of sets of primary and secondary windings. Each set wound around a separate magnetic core leg. A three-phase two-winding transformer has three sets of primary and secondary windings, and a two-phase two-winding transformer has two sets of primary and secondary windings. 2.3.1The Y-Y connection in three-phase systems The most obvious way of transforming voltages and currents in a three-phase electrical system is to operate each phase as a separate single-phase system. This requires a four-wire system comprised of three phase wires plus a common neutral wire that is shared among the three phases. This is commonly referred to as the Y- Y connection, as illustrated in Figure 2.1.The left-hand part of Figure2.1 shows the physical winding connections as three separate two-winding transformers. Both the primary and secondary windings of each of these transformers are connected between one phase, labeled A, B, and C, and the neutral, labeled N. The right-hand part of Figure2.1 shows the winding connections as a vector diagram. The direction of the phase rotation is assumed to be A-B-C expressed in a counterclockwise direction.

Figure 2.1 : Y-Y transformer connection and vector diagram. Figure2.2 depicts the situation where the primary neutral is returned to the voltage source in a four-wire three-phase circuit. Each of the magnetizing currents labeled iA, iB, and iC contain the 60Hz fundamental current and all of the odd harmonic currents necessary to support sinusoidal induced voltages.

Figure 2.2:Y-Y Connection with the primary neutral brought out. The zero-sequence magnetizing currents combine to form the neutral current iN, which returns these odd harmonics to the voltage source. Assuming that the primary voltage is sinusoidal, the induced voltages EA, EB, and EC (in both the primary and secondary) are sinusoidal as well. This situation changes dramatically if the neutrals of both sets of the primary and secondary windings are open- circuited, as shown in Figure 2.3:

Figure 2.3 :Voltage at the primary neutral of a Y-Y connection with the primary and secondary neutrals isolated.

2.3.2 Advantages of The Y-Y connection Although care must be exercised when using the Y-Y connection, this connection has certain inherent and important advantages over other three-phase transformer connections: 1- The primary and secondary circuits are in phase; i.e., there are no phase angle displacements introduced by the Y-Y connection. This is advantage when transformers are used to interconnect systems of different voltages in a cascading manner. For example, suppose there are four systems operating at 500, 230, 138, and 69kV that need to be interconnected. Substations are using Y-Y transformer connections to interconnect any two of these voltages. The 500kV system can be tied with the 69kV system through a single 500 to 69kV transformation or through a series of cascading transformation at 230,138,and 69Kv. 2- Since the phase-to-neutral voltage is only 57.7% of the phase-to-phase voltage, the windings of a Y-Y transformer require fewer turns to produce the same level of excitation in the core compared to windings connected across the phases. 3- If the neutral end of a Y-connected winding is grounded, then there is an opportunity to use reduced levels of insulation at the neutral end of the winding. A winding that is connected across the phases requires full insulation throughout the winding. 4- A Y-Y transformer may be constructed as an autotransformer, with the possibility of great cost savings compared to the two-winding transformer construction. 2.3.3 Disadvantages of the Y-Y connection The Y-Y transformer connection was poorly understood in the early days of power engineering and it received a very bad reputation when it was first used; in fact, this connection was avoided for a long time until its limitations were overcome by good 8

practice. Some of the inherent disadvantages of the Y-Y connection are below: 1. The presence of third (and other zero-sequence) harmonics at an ungrounded neutral can cause overvoltage conditions at light load. 2. There can be a large voltage drop for unbalanced phase-to-neutral loads. 3. Under certain circumstances, a Y-Y connected three-phase transformer can produce severe tank overheating that can quickly destroy the transformer. This usually occurs with an open phase on the primary circuit and load on the secondary.

Figure 2.4:Overvoltage condition produced by a fault at the primary of a Y-Y connection 4. Series resonance between the third harmonic magnetizing reactance of the transformer and line-to-ground capacitance can result in severe over voltages. 5. If a phase-to-ground fault occurs on the primary circuit with the primary neutral grounded, then the phase-to-neutral voltage on the unfaulted phases increases to 173% of the normal voltage. This would almost certainly result in over excitation of the core, with greatly increased magnetizing currents and core losses. This is illustrated in Figure 2.4. A bold X marks the location of a B phase-to ground fault with the neutral of the voltage source either ungrounded or connected to ground through a large impedance. The voltage across the B phase winding collapses and the applied voltages across the A phase and C phase windings are now equal in magnitude to the phase-to-phase voltages.

6. If the neutrals of the primary and secondary are both brought out, then a phase- to-ground fault on the secondary circuit causes neutral fault current to flow in the primary circuit.Ground protection relaying in the neutral of the primary circuit may then operate for faults on the secondary circuit. This is illustrated in the Figure 2.5.

Figure 2.5:Primary neutral current for a fault on the secondary side of a Y-Y connection. 2.3.4 The Y-∆ connection and the ∆-Y connection

Connecting the primary or secondary winding across the phases of a three-phase transformer can eliminate many of the major drawbacks of the Y-Y connection. A winding connected phase-to-phase is called a delta-connected winding because of its resemblance to the Greek letter ∆ when it is depicted in a vector diagram. Since a ∆ winding has no connection to its neutral point, it is usually left ungrounded, so it „„floats‟‟ above ground potential. The ∆-Y connection is shown in Figure 2.6 The left-hand part of Figure 2.6 shows the physical winding connections as a set of two- winding transformers. The primary winding of each transformer is connected phase-to-phase and the secondary of each transformer is connected phase-to-neutral with the neutrals grounded. The right-hand part of Figure 2.6 shows the winding connections shown as a vector diagram. As usual, the direction of phase rotation is assumed to be A-B-C in a counterclock-wise direction. laying in the neutral of the primary circuit may then operate for faults on the 10

secondary circuit. This is illustrated in the Figure 2.5.The two magnetically coupled windings are connected by a dotted line. Any fault current in the secondary neutral is transformed into neutral current in the primary circuit through the second transformer law. The obvious remedy for some of the disadvantages of the Y-Y trans-former connection would be to simply solidly ground both the primary and secondary neutrals. In fact, this is standard practice for virtually all Y-Y transformers in systems designed by utility companies. Unfortunately, solidly grounding the neutrals alone does not solve the problem of tank overheating, ferroresonance, and operating primary ground protection during secondary faults. One of the major advantages of the ∆-Y connection is that it provides harmonic suppression. Another important advantage of the ∆-Y connection is that it provides ground current isolation between the primary and secondary circuits. A transformer-connected ∆-Y with the neutral of the Y grounded is sometimes referred to as a grounding bank, because it provides a local source of ground current at the secondary that is isolated from the primary circuit. The ∆-Y transformer connection is used universally for connecting generators to transmission systems because of two very important reasons. First of all, generators are usually equipped with sensitive ground fault relay protection. The ∆-Y transformer is a source of ground currents for loads and faults on the transmission system, yet the generator ground fault protection is completely isolated from ground currents on the primary side of the transformer. Second, rotating machines can literally be shaken apart by mechanical forces resulting from zero-sequence currents. The ∆-connected winding blocks zero-sequence currents on the transmission system from the generator.

Figure 2.6 : ∆-Y transformer connection and vector diagram

Figure2.7 : Measurement of resistance of a ground grid

2.3.5 The Y-∆ grounding bank There are times when a grounded Y-∆ transformer is used for no other purpose than to provide a good ground source in an otherwise ungrounded system. Take, for example, a distribution system supplied by a ∆-connected (i.e., un-grounded) power source. If it is required to connect phase-to-ground loads to this system a grounding bank is connected to the system, as shown in Figure 2.8. Note that the ∆-connected winding is not connected to any external circuit in Figure 2.8. With a load current equal to 3 times i, each phase of the grounded Y winding provides the same current i, with the ∆-connected secondary winding of the grounding bank providing the ampere-turns required to cancel the ampere-turns of the primary winding. Note that

the grounding bank does not supply any real power to the load; it is there merely to provide a ground path. All the power required by the load is supplied by two phases of the ungrounded supply.

Figure2.8 :∆-Y transformers as a local source of ground current (grounding bank) 2.3.6 The Zigzag connection The zigzag connection is also called the interconnected star connection. This connection has some of the features of the Y and the ∆ connections, combining the advantages of both. The zigzag connection is a three-phase connection and is constructed as shown in Figure 2.9. There are three pairs of windings, each having a 1:1 turns ratio. The left-hand set of windings shown in the figure is a conventional Y connection, a′-b′-c′, with the neutral N brought out. The open ends of the Y are electrically connected to the right-hand set of windings as follows: a′ connects to the right-hand winding paired with to the b′-N winding, b′ connects to the right-hand winding paired to c′-N winding, and c′ connects to the right-hand winding paired to the a′-N winding. The opposite ends of the right-hand windings are brought out as the phase terminals a, b, and c. The vector diagram shown on the right of Figure 2.9 makes it is obvious why this is called a zigzag connection. a ∆ winding used as a primary winding, the ∆-zigzag connection is created, as shown in Figure 2.10.

Figure 2.9: interconnected star or zigzag winding connection and vector diagram

Figure 2.10: ∆-Zigzag Transformer Vector Diagrams 2.3.7 Autotransformer connections The autotransformer is both the most simple and the most fascinating of the connections involving two windings. It is used quite extensively in bulk power transmission systems because of its ability to multiply the effective KVA capacity of a transformer. Autotransformers are also used on radial distribution feeder circuits as voltage regulators. The connection is shown in Figure 2.11.The autotransformer shown in Figure 2.11 is connected as a boosting autotransformer

because the series winding boosts the output voltage. Care must be exercised when discussing „„primary‟‟ and „„secondary‟‟ voltages in relationship to windings in an autotransformer. In two-winding transformers, the primary voltage is associated with the primary winding, the secondary voltage is associated with the secondary winding, and the primary voltage is normally considered to be greater than the secondary voltage. In the case of a boosting autotransformer, however, the primary (or high) voltage is associated with the series winding, and the secondary (or low) voltage is associated with the common winding; but the voltage across the common winding is higher than across the series winding.

Figure 2.11: The boosting autotransformer connection. The output terminals operate at a higher voltage than the input terminals. The other possible connection for an autotransformer is shown in Figure 2.12 The autotransformer shown in Figure 2.12 is connected as a bucking autotransformer because the series winding bucks, or opposes, the output voltage. The key feature of an autotransformer is that the KVA throughput of the transformer, i.e., its capacity, is different than the KVA transformed by the common and series windings. The common and series windings are wound on the same core leg.

Figure 2.12:The bucking autotransformer connection. The output terminals operate at a lower voltage than the input terminals

2.4 Component of Transformer Of the below, laminated soft iron core, windings and insulating material are the primary parts and are present in all transformer.  Core The core acts as support to the winding in the transformer. It also provides a low reluctance path to the flow of magnetic flux. It is made of laminated soft iron core in order to reduce eddy current loss and Hysteresis loss. The composition of a transformer core depends on such as factors voltage, current, and frequency. The diameter of the transformer core is directly proportional to copper loss and is inversely proportional to iron loss. If the diameter of the core is decreased, the weight of the steel in the core is reduced, which leads to less core loss of the transformer and the copper loss increase. When the diameter of the core is increased, the vice versa occurs. shown in appendix(c).  Winding Two sets of winding are made over the transformer core and are insulated from each other. Winding consists of several turns of copper conductors bundled together, and connected in series.

- Winding can be classified in two different ways: 1- Based on the input and output supply 2- Based on the voltage range - Due to supply classification, winding are categorized: 1-Primary winding - These are the winding of the input voltage applied. 2-Secondary winding - These are the winding of the output voltage applied. - Due to voltage range classification, winding are categorized: 1-High voltage winding - It is made of copper conductor. The number of turns made shall be the multiple of the number of turns in the low voltage winding. The conductor used will be thinner than that of the low voltage winding. 2-Low voltage winding - It consists of fewer number of turns than the high voltage winding. It is made of thick copper conductors. This is because the current in the low voltage winding is higher than that of high voltage winding. Input supply to the transformers can be applied from either low voltage (LV) or high voltage (HV) winding based on the requirement. - Insulating Materials Insulating paper and cardboard are used in transformers to isolate primary and secondary winding from each other and from the transformer core.  Transformer oil Transformer oil is another insulating material. Transformer oil performs two important functions: in addition to insulating function, it can also cool the core and coil assembly. The transformer's core and winding must be completely immersed in the oil. Normally, hydrocarbon mineral oils are used as transformer oil, shown in appendix (d). Oil contamination is a serious problem because contamination robs the oil of its dielectric properties and renders it useless as an insulating medium.

 Conservator The conservator conserves the transformer oil. It is an airtight, metallic, cylindrical drum that is fitted above the transformer. The conservator tank is vented to the atmosphere at the top, and the normal oil level is approximately in the middle of the conservator to allow the oil to expand and contract as the temperature varies. The conservator is connected to the main tank inside the transformer, which is completely filled with transformer oil through a pipeline.  Breather The breather controls the moisture level in the transformer. Moisture can arise when temperature variations cause expansion and contraction of the insulating oil, which then causes the pressure to change inside the conservator. Pressure changes are balanced by a flow of atmospheric air in and out of the conservator, which is how moisture can enter the system. If the insulating oil encounters moisture, it can affect the paper insulation or may even lead to internal faults. Therefore, it is necessary that the air entering the tank is moisture-free. The transformer's breather is a cylindrical container that is filled with silica gel. When the atmospheric air passes through the silica gel of the breather, the air's moisture is absorbed by the silica crystals. The breather acts like an air filter for the transformer and controls the moisture level inside a transformer. It is connected to the end of breather pipe  Tap Changer The output voltage of transformers vary according to its input voltage and the load. During loaded conditions. In order to balance the voltage variations, tap changers are used. Tap changers can be either on-load tap changers or off-load tap changers. In an on-load tap changer, the tapping can be changed without isolating the transformer from the supply. In an off-load tap changer, it is done after disconnecting the transformer. Automatic tap changers are also available.

 Cooling Tubes Cooling tubes are used to cool the transformer oil. The transformer oil is circulated through the cooling tubes. The circulation of the oil may either be natural or forced. In natural circulation, when the temperature of the oil rises the hot oil naturally rises to the top and the cold oil sinks downward. Thus the oil naturally circulates through the tubes. In forced circulation, an external pump is used to circulate the oil.  Buchholz Relay The Buchholz Relay, shown in appendix(e), is a protective device container housed over the connecting pipe from the main tank to the conservator tank. It is used to sense the faults occurring inside the transformer. It is a simple relay that is operated by the gases emitted during the decomposition of transformer oil during internal faults. It helps in sensing and protecting the transformer from internal faults.  Explosion Vent The explosion vent is used to expel boiling oil in the transformer during heavy internal faults in order to avoid the explosion of the transformer. During heavy faults, the oil rushes out of the vent. The level of the explosion vent is normally maintained above the level of the conservatory tank. 2.5 Grounding Grounding is a conducting connection, whether intentional or accidental between an electrical circuit or equipment and the earth, or to some conducting body that serves in place of the earth. Grounding is connecting to a common point which is connected back to the electrical source. It may or may not be connected to earth. An example where it is not connected to earth is the grounding of the electrical system inside an airplane.

2.6 Earthing Earthling is a common term used outside the US and is the connection of the equipment and facilities grounds to Mother Earth. This is a must in a lightning protection system since earth is one of the terminals in a lightning stroke. 2.7 Terms used in Electrical Earthing

 Earth: The proper connection between electrical installation systems via conductor to the buried plate (the earth electrode) in the earth is known as Earth.

 Earthed: When an electrical device, appliance or wiring system connected to the earth through earth electrode, it is known as earthed device or simple “Earthed”.

 Solidly Earthed: When an electric device, appliance or electrical installation is connected to the earth electrode without a fuse, circuit breaker or resistance/Impedance, It is called “solidly earthed”.

 Earth Electrode: When a conductor (or conductive plate) buried in the earth for electrical earthing system. It is known to be Earth Electrode. Earth electrodes are in different shapes like, conductive plate, conductive rod, metal water pipe or any other conductor with low resistance.

 Earthing Lead: The conductor wire or conductive strip connected between Earth electrode and Electrical installation system and devices in called Earthing lead.

 Earth Continuity Conductor: The conductor wire, which is connected among different electrical devices and appliances like, distribution board, different plugs and appliances etc. in other words, the wire between earthing lead and electrical device or appliance is called earth continuity conductor. It may be in the shape of metal pipe (fully or partial), or cable metallic sheath or flexible wire.

 Sub Main Earthing Conductor: A wire connected between switch board and distribution board i.e. that conductor is related to sub main circuits.

 Earth Resistance: This is the total resistance between earth electrode and earth in Ω (Ohms). Earth resistance is the algebraic sum of the resistances of earth continuity conductor, earthing lead, earth electrode and earth. 2.8 The grounding system serves three primary functions which are listed below: 1- Personnel Safety Personnel safety is provided by low impedance grounding and bonding between metallic equipment, chassis, piping, and other conductive objects so that currents, due to faults or lightning, do not result in voltages sufficient to cause a shock hazard. Proper grounding facilitates the operation of the overcurrent protective device protecting the circuit. 2- Equipment and Building Protection Equipment and building protection is provided by low impedance grounding and bonding between electrical services, protective devices, equipment and other conductive objects so that faults or lightning currents do not result in hazardous voltages within the building. Also, the proper operation of overcurrent protective devices is frequently dependent upon low impedance fault current paths. 3- Electrical Noise Reduction Proper grounding aids aim electrical noise reduction and ensures: - The impedance between the signal ground points throughout the building is minimized. 2. The voltage potentials between interconnected equipment are minimized. 3. That the effects of electrical and magnetic field coupling are minimized. 4. Provides a reference for circuit conductors to stabilize their voltage to ground during normal operation.

2.9 Ungrounded systems By definition, an electrical system, which is not intentionally connected to the ground at any point, is an ungrounded system. However, it should be noted that a connection to ground of sort does exist due to the presence of capacitances between the live conductors and ground, which provides a reference. But these capacitive reactance are so high that they cannot provide a reliable reference. Figure 2.13 illustrates this point. In some cases, the neutral of potential transformer primary windings connected to the system is grounded, thus giving a ground reference to the system.

Figure 2.13 A virtual ground in an ungrounded system 1-Advantages of ungrounded system The main advantage cited for ungrounded systems is that when there is a fault in the system involving ground, the resulting currents are so low that they do not pose an immediate problem to the system. Therefore, the system can continue without interruption, which could be important when an outage will be expensive in terms of lost production or can give rise to life-threatening emergencies. The second advantage is that one need not invest on elaborate protective equipment as well as grounding systems, thus reducing the overall cost of the system. (In practice, this is however offset somewhat by the higher insulation ratings which this kind of system calls for due to practical considerations.) - The disadvantages of ungrounded system • In all but very small electrical systems, the capacitances, which exist between the system conductors and the ground, can result in the flow of capacitive current at the

faulted point which can cause repeated arcing and buildup of excessive voltage with reference to ground. This is far more destructive and can cause multiple insulation failures in the system at the same instant. • The second disadvantage in practical systems is that of detecting the exact location of the fault, which could take far more time than with grounded systems. This is because the detection of fault is usually done by means of a broken delta connection in the voltage transformer circuit (Figures 2.14 and 2.15). This arrangement does not tell where a fault has occurred and to do so, a far more complex system of ground fault protection is required which negates the cost advantage we originally talked about. • Also, a second ground fault occurring in a different phase when one unresolved fault is present, will result in a short circuit in the system.

Figure 2.14: Detection of ground fault using a broken delta connection – under normal condition. 2.10 Types of grounding electrical systems 1- Solidly grounded systems As is evident from the name, a solidly grounded system is one where the neutral of the system is directly connected to ground without introducing any intentional resistance in the ground circuit. With appropriate choice of the type and number of grounding electrodes, it is possible to obtain a very low-impedance ground connection, sometimes as low as 1 Ω. A solidly grounded system clamps the neutral tightly to ground and ensures that when there is a ground fault in one phase, 23

the voltage of the healthy phases with reference to ground does not increase to values appreciably higher than the value under the normal operating conditions.

When there is an Earth fault in line A it assumes Earth Potential . Therefore Voltage across PT primary windings become : , ,

Figure 2.15:Detection of ground fault using a broken delta connection – under Ground Fault condition The advantages of solidly grounded systems :

• A fault is readily detected and therefore isolated quickly by circuit protective devices. Quite often, the protection against short circuit faults (such as circuit breakers or fuses) is adequate to sense and isolate ground faults as well.

• It is easy to identify and selectively trip the faulted circuit so that power to the other circuits or consumers can continue unaffected (contrast this with the ungrounded system where a system may have to be extensively disturbed to enable detection of the faulty circuit). • No possibility of transient over-voltages.

The disadvantage of solidly grounded systems: The main disadvantage is that when applied in distribution circuits of higher voltage (5 kV and above), the very low ground impedance results in extremely high fault currents almost equal to or in some cases higher than the system‟s three-phase short circuit currents. This can increase the rupturing duty ratings of the equipment to be selected in these systems. Such high currents may not have serious consequences if the failure happens in the distribution conductors (overhead or cable). But when a fault happens inside a device such as a motor or generator such currents will result in extensive damage to active magnetic parts through which they flow to reach the ground. For these reasons, use of solid grounding of neutral is restricted to systems of lower voltage (380 V/480 V) used normally in consumer premises. In all the other cases, some form of grounding impedance is always used for reducing damage to critical equipment components. 2- Impedance grounding using neutral reactor In this method of grounding, an inductor (also called a grounding reactor) is used to connect the system neutral to ground. This limits the ground fault current since it is a function of the phase to neutral voltage and the neutral impedance. It is usual to choose the value of the grounding reactor in such a way that the ground fault current is restricted to a value between 25 and 60% of the three-phase fault current to prevent the possibility of transient over-voltages occurring. Even these values of fault current are high if damage prevention to active parts (as seen above in Figure 2.15) is the objective. 3- Resonant grounding using neutral reactor To avoid the problem of very high ground fault currents, the method of resonant grounding can be adopted. Resonant grounding is a variant of reactor grounding with the reactance value of the grounding reactor chosen such that the ground fault 25

current through the reactor is equal to the current flowing through the system capacitances under such fault condition. This enables the fault current to be almost canceled out resulting in a very low magnitude of current, which is in phase with the voltage. This serves the objectives of low ground fault current as well as avoiding arcing (capacitive) faults, which are the cause of transient over-voltages. The action is explained in Figure 2.16.

Figure 2.16: Resonant grounding This type of grounding is common in systems of 15 kV (primary distribution) range with mainly overhead lines but is not used in industrial systems where the reactor tuning can get disturbed due to system configuration changes caused by switching on or off cable feeders (with high capacitive currents) frequently. 4- Impedance grounding through neutral resistance This is by far the most common type of grounding method adopted in medium voltage circuits. The system is grounded by a resistor connected between the neutral point and ground. The advantages of this type of grounding are as follows: • Reducing damage to active magnetic components by reducing the fault current. • Minimizing the fault energy so that the flash or arc blast effects are minimal thus

ensuring safety of personnel near the fault point. • Avoiding transient over-voltages and the resulting secondary failures. • Reducing momentary voltage dips, which can be caused if, the fault currents were higher as in the case of a solidly grounded system. • Obtaining sufficient fault current flow to permit easy detection and isolation of faulted circuits.

Figure 2.17: Grounding of a turbine generator neutral through a high neutral resistance

2.11 Resistance grounding can again be sub-divided into two categories: A- High-resistance grounding B- low-resistance grounding High-resistance grounding limits the current to about 10 A. But to ensure that transient over-voltages do not occur, this value should be more than the current through system capacitance to ground. As such, the applications for high-resistance grounding are somewhat limited to cases with very low tolerance to higher ground fault currents. A typical case is that of large turbine generators, which are directly

connected to a high voltage transmission system through a step up transformer. The capacitance current in generator circuits is usually very low permitting values of ground fault currents to be as low as 10 A. The low current ensures minimal damage to generator magnetic core thus avoiding expensive factory repairs. Figure 2.17 illustrates a practical case of grounding the neutral of a generator of this type. On the other hand, low-resistance grounding is designed for ground fault currents of 100 A or more with values of even 1000 A being common. The value of ground fault current is still far lower than three-phase system fault currents. This method is most commonly used in industrial systems and has all the advantages of transient limitation, easy detection and limiting severe arc or flash damages from happening. 2.12 Point of grounding In most three-phase systems, the neutral point at source is connected to ground. This has the advantage of minimum potential of the live terminals with reference to ground. In the case of generators, which are almost always star connected, the neutral point is available for grounding. However, in the case of transformer substations, a neutral may not always be available as the winding may be delta connected. In such cases, it will be necessary to obtain a virtual neutral using a device called grounding transformer. Grounding transformers are generally of two types viz. zig-zag connected transformer with no secondary winding and a wye- delta transformer. Figure 2.18 shows a zig-zag grounding transformer.The transformer primary winding terminals are connected to the system, which has to be grounded. The neutral point of the transformer is grounded solidly or through an impedance depending on the type of grounding selected. Under normal conditions, the transformer behaves like any other transformer with open circuited secondary and draws a small magnetizing current from the system. The impedance of the transformer to ground fault currents is however extremely small. When one of the lines develops a ground fault, the current is only 28

Figure 2.18 Zig-zag grounding transformer restricted by the grounding impedance. Thus, the system behaves virtually in the same manner as any system with grounded source neutral. Figure 2.19 shows this behavior. The ground fault current flowing in the faulted line divides itself into three equal parts flowing through each phase winding of the transformer. The other type of grounding transformer is a wye-delta connected transformer. The primary winding terminals of the transformer are connected to the system, which is to be grounded, the neutral of the primary is connected to the ground and the secondary delta is either kept open or can be connected to a three-phase three-wire supply system as required (refer to Figure 2.20). This type of transformer too presents a low-impedance path to the flow of zero sequence currents due to the circulating path offered by the secondary delta winding. This enables the ground fault current to flow through the primary and to the ground through the grounding impedance. Figure 2.21 illustrates this action.

Figure 2.19: Behavior of a zig-zag connected transformer during a ground fault

Figure 2.20: Star–delta grounding transformer

Figure 2.21: Behavior of star–delta grounding transformer during ground faults

CHAPTER THREE GROUNDING RISESTANCE 3.1 Introduction In this chapter, we will discuss about the design of grounding system and the materials used for this purpose. The practices adopted in different countries follow the national standards/codes that are specified by the appropriate authority and can be significantly different. The construction of grounding electrodes depends on the local codes applicable. The purpose however is common. It is to establish a low-resistance (and preferably low-impedance) path to the soil mass. It can be done using conductors that are exclusively meant for this function or by structures/conductors used for other functions but which are essentially in contact with soil. However, while using the latter category, it must be ensured that the ground connection is not inadvertently lost during repair works or for any other reason.(" Practical Grounding, Bonding, Shielding and Surge Protection") 3.2 Grounding electrodes 3.2.1 Ground electrodes and factors affecting their efficacy : A common thread in the foregoing discussions is the need for a good ground connection in power sources, consumer installations and for structures prone to lightning strokes. The connection to groundmass is normally achieved by a ground electrode. Several types of ground electrodes using different materials, physical configurations and designs are in widespread use and follow usually the local standards that govern electrical installations. In most standards, a metallic rod driven into the ground to a depth where adequate moisture is available in the soil throughout the year in both wet and dry seasons is recommended for use as a ground electrode. A typical electrode is shown in Figure (3.1). 31

Figure 3.1: A typical ground electrode used in electrical installations The performance of such electrodes (considering the ground resistance of the electrode as an indicator) depends on the type of soil, its composition, conductivity, presence of moisture, soil temperature, etc. Several ground electrodes bonded together to form a cluster are usually provided for achieving satisfactory results. The general requirements that influence the choice of earth electrodes are as follows:  The type of soil where the grounding is carried out (in particular, its electrical resistivity).  The need for achieving minimum acceptable earth resistance appropriate to the installation involved.  The need to maintain this resistance all round the year in varying climatic conditions.  Presence of agents that can cause corrosion of elements buried in ground. The electrode design and methods of installation will be dependent on these requirements. These will be taken up in detail in this chapter. To improve the

conductivity of ground electrodes, several forms of electrode construction are in use in which the layer of soil surrounding the electrode is treated with chemical substances for improving the conductivity. These are known as chemical electrodes. The basic principle of these electrodes is the use of substances that absorb moisture and retain it over long periods. These are packed as backfill around the electrode. Materials containing carbon (charcoal/coke) and electrolyte salts such as sodium chloride are typically used as backfill. Figure (3.2) shows such an electrode construction. It may also be noted that in this construction, a provision has been made to add water externally to keep the backfill material wet during prolonged dry weather conditions

Figure 3-2: A typical chemical ground electrode 3.2.2 Factors contributing to ground electrode resistance : The resistance of a ground electrode is made up of the following components: • The resistance of electrode material • Contact resistance of the electrode with soil 33

• Resistance of the soil itself. The values of the first two are quite low compared to the last and can be neglected. 3.3 Soil resistance Though the ground itself being a very large body can act as an infinite sink for currents flowing into it and can be considered to be having very low resistance to current flow, the resistance of soil layers immediately adjacent to the electrode can be considerable. Soil has a definite resistance determined by its resistivity that varies depending upon the type of soil, presence of moisture and conductive salts in the soil and the soil temperature. Soil resistivity can be defined as the resistance of a cube of soil of 1 m size measured between any two opposite faces. The unit in which it is usually expressed is ohmmeter.

Area A = B*H Resistance R between face P and Q=

Figure 3-3: Soil resistivity Resistance of the sample of soil shown in Figure (3.3) can be arrived at by the formula: R= ρL/A Where 34

R: is the resistance between faces P and Q Ohm A: Area pr faces P and Q (mz) L: Length of soil sample in meter ρ : Soil reisistivity Ohm-m Soil resistivity for a given type of soil may vary widely depending on: • The presence of conducting salts • Moisture content • Temperature • Level of compaction. Conducting salts may be present naturally in the soil or added externally for lowering the resistivity. Chlorides, nitrates and sulfates of sodium, potassium, magnesium or calcium are generally used as soil additives. However, the addition of such salts can be corrosive and in some cases undesirable from the environmental point of view. Especially, the presence of calcium sulfate in the soil is detrimental to concrete foundations and in case it is to be used for electrode quality enhancements, it should be limited to electrodes situated well away from such foundations. Also, over a period, they tend to leach away from the vicinity of the electrode. Moreover, these additive salts have to first get dissolved in the moisture present in the soil in order to lower the resistivity, and provision should be made for addition of water to the soil surrounding the electrode to accelerate this process particularly in dry locations. Moisture is an essential requirement for good soil conductivity. Moisture content of the soil can vary with the season and it is advisable for this reason to locate the electrodes at a depth at which moisture will be present throughout the year so that soil resistivity does not vary too much during the annual weather cycle. It is also possible that moisture evaporates during ground faults of high magnitude for long duration. The electrode design must take care of this aspect. We will cover this in 35

more detail later in this chapter. Temperature also has an effect on soil resistivity but its effect is predominant at or near 0ºC when the resistivity sharply goes up. Similarly, compaction condition of the soil affects resistivity. Loose soil is more resistive in comparison to compacted soil. Rocky soil is highly resistive and where rock is encountered, special care is to be taken. One of the methods of increasing soil conductivity is by surrounding the electrode with bentonite clay, which has the ability to retain water and also provides a layer of high conductivity. Unlike salts mentioned earlier, bentonite is a natural clay, which contains the mineral montmorillonite formed due to volcanic action. It is non-corrosive and does not leach away as the electrolyte is a part of the clay itself. It is also very stable. The low resistivity of bentonite is mainly a result of an electrolytic process between water and oxides of sodium, potassium and calcium present in this material. When water is added to bentonite, it swells up to 13 times of its initial volume and adheres to any surface it is in contact with. Also, when exposed to sunlight, it seals itself off and prevents drying of lower layers. Any such enhancement measures must be periodically repeated to keep up the grounding electrode quality. A section later in this chapter describes about electrodes, which use these principles to dramatically lower the resistance of individual electrodes under extreme soil conditions. Such electrodes are commonly known as „chemical electrodes‟. The IEEE 142 gives several useful tables, which enable us to determine the soil resistivity for commonly encountered soils under various conditions which can serve as a guideline for designers of grounding systems. These are shown in Table (3.1) and (3.2). 3.4 Measurement of soil resistivity Soil resistivity can be measured using a ground resistance tester or other similar instruments using Wenner‟s 4-pin method. The two outer pins are used to inject current into the ground (called current electrodes) and the potential developed as a 36

result of this current flow is measured by the two inner pins (potential electrodes) (figure 3.4).

Table 3.1: Effect of moisture content on soil resistivity

Resistivity ( in ohmmeter ) Moisture Content (%) Top Soil Sandy Loam Red Clay 2 *** 0581 *** 4 *** 011 *** 0 0081 051 *** 5 011 251 *** 01 011 221 0511 02 001 071 881 04 281 041 211 00 211 021 211 05 081 011 041 21 021 01 011 22 011 51 01 24 011 71 51

Table 3.2: Effect of temperature on soil resistivity

Temperature ( Resistivity ( in ohmmeter )

-5 700 0 300 0 100 10 80 20 70 30 60 40 50 50 40

The general requirements for ground resistance testing instruments are as follows: • The instrument should be suitable for Wenner‟s 4-pin method. It should give a direct readout in ohms after processing the measured values of current injected into the soil and the voltage across the potential electrodes. • The instrument should have its own power source with a hand-driven generator or voltage generated using batteries. The instrument will use an alternating current for measurement. • Direct reading LCD type of display is preferable. Resistance range should be between 0.01 and 1999 ohm with range selection facility for 20, 200 and 2000 ohm for better accuracy. • Indications should preferably be available for warning against high current through probes, high resistance of potential probes, low source voltage and excessive noise in the soil.

• A minimum of four (4) steel test probes of length 0.5 m and sectional area of 140 mm2 along with the necessary insulated leads (a pair of 30 m and another pair of 50 m) should be supplied with the instrument. All the pins should be located in a straight line with equal separating distance between them and the pins driven to a depth of not more than 10% of this distance. Care should be taken to ensure that the connections between the pins and the instrument are done with insulated wires and that there is no damage in the insulation. The resistance of the soil between potential electrodes is determined by Ohm‟s law (R = V/I) and is computed and displayed by the instrument directly. The resistivity of the soil is given by the formula : ρ = (2πSR) Where ρ: Soil resistivity in Ohm-m S: Distance shown in table 3.1 in meters R: Resistance measured in Ohms Since the soil is usually not very homogeneous especially near the surface, the depth to which the pins are driven and the separation between the pins will cause resistivity figures to vary and can indicate the type of soil at different depths. The calculated value of resistivity can be taken to represent the value at the depth of 0.8S where S is the electrode spacing. The test is repeated at different values of S viz. 1, 2, 3, 5, 10 and 15m. They can also be plotted in the form of a graph. A study of the values will give some indication of the type of soil involved. A rapid increase of resistivity at increasing D values shows layers of soil with higher resistivity. A very rapid increase may indicate the presence of rock and will possibly prevent use of vertical electrode. On the other hand, decrease of soil resistivity as D increases will indicate lower-resistivity soils in deeper layers where vertical electrodes can be installed with advantage. 39

Figure 3.4: Soil resistivity measurements P1 and P4: Current electrodes P2 and P3: Potential electrodes I: Current (AC) injected by the instrument into ground V: Voltage drop in soil 3.4.1 Errors due to stray currents : Stray currents in the soil may be the result of one or more of the following reasons: • Differential salinity • Differential aeration of the soil • Bacteriological action • Galvanic action (more on this later in the chapter) • Ground return currents due to electric traction systems nearby • Currents from multiple grounding of distribution system neutrals.

These stray currents appear as potential drop across the voltage electrodes without a corresponding current from the instrument‟s current source. Thus, they result in exaggerated resistivity measurements. This can be avoided by selecting an instrument source frequency, which is different from the stray currents, and providing filters that reject other frequencies. 3.4.2 Coupling between test leads Improper insulation may give rise to leakage currents between the leads, which will result in errors. Ensuring good insulation and running the current and potential leads with a gap of at least 100 mm will prevent errors due to leakage. 3.4.3 Buried metallic objects Buried metallic objects such as pipelines, fences, etc. may cause problems with readings. If presence of such objects is known, it will be advisable to orient the leads perpendicular to the buried object. 3.5 Resistance of a single rod electrode The resistance of a ground electrode can be calculated once the soil resistivity is known. For a rod driven vertically into ground, the electrode resistance is given by the following formula :

A simplified formula for an electrode of 5/8 in. (16 mm) diameter driven 10 ft (3 m) into the ground is :

Knowledge of the soil resistivity alone is thus adequate to assess the electrode resistance to a reasonable degree of accuracy. The IEEE 142 gives the following (Table3.3) for ready reference and can be used to arrive at the resistance value of the standard ground rod for different types of soil.

Table 3.3: Soil resistivity for different soil types

Resistance of Rod Dia. 5/8 in. Length Soil Type Average Resistivity (Ω m) 10 ft in ohms

Well-graded gravel 600-1000 180-300 Poorly graded gravel 1000-2500 300-750 Clayey gravel 200-400 60-120 Silty sand 100-800 30-150 Clayey sand 50-200 15-60 Silty or Clayey sand 30-80 9-24 with slight plasticity

Fine sandy soil 80-300 24-90 Gravelly clays 20-60 17-18 Inorganic clays of high 10-55 3-16 plasticity

 Resistance distribution in soil surrounding a single electrode The resistance of the soil layers immediately in the vicinity of the soil is significant in deciding the electrode resistance. To illustrate this let us see Figure 4.5. A current that flows into the ground from a buried electrode flows radially outward from the electrode. It is therefore reasonable to assume for the purpose of calculating the soil resistance that the soil is arranged as concentric shells of identical thickness with the electrode at the center. The total resistance can thus be taken as the sum of the resistance of each shell taken in tandem.

Figure 3.5: Soil resistance distribution around a vertically driven electrode The resistance of each shell is given by the formula

Where

R: Resistance of the shell

L: Length of the shell

A:Surface are (inner) of the shell

The area of the shells keeps increasing as we move away from the electrode. Thus, the resistance of the shells keeps reducing in value. The IEEE 142 has tabulated this variation (Table 3.4).It can be seen from Figure 4.9 that the first 0.1 ft accounts for 25% of the resistance value and the first 1 ft for 68%. At 10 ft (equal to the rod length), 94% of the resistance value has been achieved. For this reason, lowering of soil resistivity in the immediate vicinity of the electrode is the key to lowering the electrode resistance. Also, placing more ground electrodes in the vicinity will only interfere with the conduction of current since the current from one electrode will increase the ground potential, which will have the effect of decreasing the current flow from the other nearby electrode (and vice versa). table 3.4: Radial variation of soil resistance around a rod electrode Distance from Electrode (in feet) App. % of Resistance 0.1 25 0.2 38 0.3 46 0.5 52 1 68 5 86 10 94 15 97 20 99 25 100 100 104 10,000 117

3.6 Current-carrying capacity of an electrode When current flow through a ground electrode into ground is low, the heat generated in the ground layers gets dissipated fairly fast and does not lead to any appreciable temperature rise. On the other hand, for a high current flow as happens during faults in solidly grounded systems, the effect would be quite different. As we saw earlier, the bulk of the resistance is concentrated in the immediate vicinity of the electrode. Without adequate time for the heat generated to be conducted away, the temperature of the ground layers surrounding the ground electrode rises

Where

sharply and causes evaporation of soil moisture around the electrode. If this persists, the soil around can become dry losing all the moisture present in it resulting in arcing in the ground around the electrode. Thus, a smoking or steaming electrode results in an electrode that is ineffective. To prevent this from happening, it is essential to limit the flow of current flowing into the ground through an electrode as indicated by the following formula: 3.7 Use of multiple ground rods in parallel When it is not possible to obtain the minimum resistance stipulations or the ground fault current cannot be dissipated to the soil with a single electrode, use of multiple ground rods in parallel configuration can be resorted to. The rods are generally

arranged in a straight line or in the form of a hollow rectangle or circle with the separation between the rods not lower than the length of one rod. As we have seen earlier in this chapter, the soil layers immediately surrounding the electrode contribute substantially to the electrode resistance. More than 98% of the resistance is due to a soil cylinder hemisphere of 1.1 times the electrode length. This is called the „critical cylinder‟. Placing electrodes close to each other thus interferes with the conduction of current from each electrode and lowers the effectiveness. It is also of interest to note that the combined ground resistance of multiple rods does not bear a direct relationship to the number of rods. Instead, it is determined by the formula: R = R/N × F R: Combined ground resistance of the electrode system having N electrode (Ohms). R: Resistance of a single typical electrode (Ohms). F: Factor F in the table shown in Figure 4.5 for Number of Roads = N. The Table (3-5) shows the value of the factor F used above. Table 3-5: Factor F for multiple ground rods

No. of Rods F 2 1.16 3 1.29 4 1.36 8 1.68 12 1.80 16 1.92 20 2.00 24 2.16

3.8 Measurement of ground resistance of an electrode The resistance of a single ground electrode (as well as small grounding systems using multiple rods) can be measured using the 3-point (or 3-pin) method. The apparatus for this purpose is the same that is used for soil resistivity, viz. the ground resistance tester (see Figure 3.6). This method, however, may not yield correct results when applied to large grounding systems of very low resistance. The measurement of electrode resistance is done in order to: • Check on correctness of calculations and assumptions made • Verify the adequacy after installation and • Detect changes in an existing installation. In this case, the ground electrode itself serves both as a current and potential electrode. The other electrode farther from the electrode is the other current electrode and the nearer one is the second potential electrode. The resistance can directly be read off the instrument. To get correct results, the current electrode must be placed at a distance of at least 10 times the length of the electrode being measured and the potential electrode at half the distance. A very similar method can be adopted for the measurement of ground grids, which are used commonly in HV substations (usually outdoor switchyards) refer (Figure 3.7).

E: Electrode under measurement P1:Potential electrode P2: Potential electrode 2S Figure 3.6: Measurement of electrode resistance by 3-point method

Figure 3.7: Measurement of resistance of a ground grid

3.9 Concrete-encased electrodes Concrete foundations below ground level provide an excellent means of obtaining a low resistance electrode system. Since concrete has a resistivity of about 30 ohm meter at 20 °C, a rod embedded within a concrete encasement gives a very low electrode resistance compared to most rods buried in ground directly. Since buildings are usually constructed using steel-reinforced concrete, it is possible to use the reinforcement rod as the conductor of the electrode by ensuring that an electrical connection can be established with the main rebar of each foundation.

The size of the rebar as well as the bonding between the bars of different concrete members must be done so as to ensure that ground fault currents can be handled without excessive heating. Such heating may cause weakening and eventual failure of the concrete member itself. Alternatively, copper rods embedded within concrete can also be used. Concrete electrodes are often referred to as „Ufer‟ electrodes in honor of Mr Ufer, who performed a large amount of research into concrete-encased electrodes. The rebars used are required to be either bare or zinc coated. Normally, the following applies to a rebar used as an earthing electrode: • Minimum length of 6 m • Minimum diameter 13 mm. and installed: • In a minimum of 50 mm of concrete • Concrete is in direct contact with earth • Located within and near the bottom of a concrete foundation or footing • Permitted to be bonded together by the use of steel tie wire. With respect to the last point, steel tie wire is not the best means of ensuring that the rebars make good continuity. Excellent joining products are available in the market, which are especially designed for joining construction rebars throughout the construction. By proper joining of the rebars in multi-level buildings, exceptionally good performance can be achieved. An extremely low-resistance path to earth for lightning and earth fault currents is ensured as the mass of the building keeps the foundation in good contact with the soil. Some examples of splicing products available in the market for jointing of rebars are shown in Figures(3-8ac). A recent advancement for solving difficult earthing problems is the use of conductive concrete to form a good earth. Normally, this form of concrete is a special blend of carbon and cement that is spread around electrodes of copper.

These are normally installed in a horizontal configuration by digging a trench of approximately a half a meter wide and 600 mm deep. The flat copper or rods are then installed in the center of the trench. The conductive concrete is then applied dry to the copper and spread to approximately 4 cm thick over the copper to the edges of the trench. The trench is then backfilled and the conductive concrete then absorbs moisture from the soil and sets to about 15 Mpa. It is also possible to install these electrodes vertically. However, in this case, the conductive concrete has to be made up as a slurry and pumped to the bottom of the hole to displace water or mud.

Figure 3-8a: Threaded splice joint using a coupler

Figure 3.8b: A comparison between lap splice using tie wire and threaded (mechanical) splice

Figure 3.8c: A welded splice joint with sleeve 3.10 Corrosion problems in electrical grounding systems Buried electrode systems bonded to other facilities embedded in ground such as piping/conduits can form galvanic cells when they involve dissimilar metals having differing galvanic potential. These cells, which are formed from the dissimilar metals as electrodes and the ground as the electrolyte, set up galvanic current through the bonding connections (refer Figure 3.9). For example, copper electrodes and steel pipes used as a part of grounding system can cause cells of 0.38-V potential difference with copper as the positive electrode.

This circulates a current, which causes corrosion of the metal in the electrode from which current flows into the ground. A galvanic current of 1-A DC flowing for a 1- year period can corrode away about 10 kg of steel. This can be avoided by the use of materials with the same galvanic potential in the construction of ground electrode systems. Other methods such as use of sacrificial materials as anodes and injection of DC currents help to control this type of corrosion.

Figure 3.9: Galvanic action of a ground electrode system 3.11 Maintenance of grounding system A properly scheduled and executed maintenance plan is necessary to maintain a grounding system in proper order. This is essential because the efficacy of the system can be affected over a period of time due to corrosion of metallic electrodes and connections. Periodic measurement of the ground electrode resistance and recording them for comparison and analysis later is a must. In the case of any problems, repairs or soil treatment must be taken up to bring the ground electrode system resistance back to permissible values.

3.12 Chemical electrodes We have seen earlier in this chapter that the resistance of the ground electrode is influenced by the soil immediately surrounding the electrode. It is also influenced by the ambient conditions of the soil such as moisture and temperature. Thus, it is difficult to obtain acceptable values of grounding resistance in areas where: • Natural soil is of very high resistivity such as rocky material, sand without vegetation, etc. • During part of the year, the resistance may become excessive because of the absence of moisture. • Soil temperature remains extremely low as in the case of polar regions or those close to the polar circle (called as permafrost condition, where the ground is below freezing temperature). It thus follows that the performance of an electrode can be improved by using chemically treated soil to lower the soil resistivity and to control the ambient factors. While the soil temperature cannot be controlled, it is possible to ensure presence of moisture around the electrode. Soil treatment by addition of hygroscopic materials and by mechanisms to add water to the soil around the electrode are common methods of achieving this objective. Also, the resistivity behavior in permafrost conditions can be improved by soil conditioning, thus improving the electrode resistance dramatically. Tests performed by the US Corps of Engineers in Alaska have proved that the resistance of a simple conventional electrode can be lowered by a factor of over twenty (i.e. 1/20). The treatment involved simply replaces some of the soil in close proximity with the electrode by conditioned backfill material. Refer Figure 3.10 for the result of tests conducted at

Point Barrow, Alaska, which illustrates that the electrode resistance has dropped from a high of 20 000 Ω to a maximum of 1000 Ω by soil treatment.

Figure 3.10: Result of soil treatment on electrode resistance The principle of improving the soil conductivity has been applied for a long time in ground electrode construction. The example shown earlier in Figure 3.2 belongs to this category. In this example, the hollow earthing tube contains sodium chloride, which absorbs moisture from surrounding air and leaches out to the soil to lower its resistivity. The backfill is soil mixed with charcoal and also sodium chloride. Since moisture in air is essential for this construction, means are provided to externally add water during dry weather. These basic principles are used by several vendors who manufacture electrodes for applications involving problem areas. In these cases, both the electrode fill material and the augmented backfill are decided based on the soil properties so that moisture can be absorbed from surrounding soil itself and preserved in the portion immediately surrounding the electrode. In some systems, automated moisture addition devices are provided to augment this effect. A typical system by a vendor incorporating a solar powered moisture control mechanism is shown in Figures (3.11a and b).

Figure 3.11 a Arrangement of chemical electrode with moisturizing mechanism

Figure 3.11 b Control system for moisture addition

CHAPTER FOUR 4.1 Introduction:

 Earth/Ground tester 1625 used to measure grounding resistance values, pictured in appendix(f).  Clampmeter used to measure voltages and currents values, pictured in appendix(g).

4.2 Result:  Comparison between earth resistance and neutral to earth voltages:

Figure 4.1 shows the relation between the earth resistance and neutral to earth voltage.  it is clear from the analysis that, whenever there is a high value of grounding resistance, the voltage between neutral and ground will increase to high

value and sometimes it may reaches the line voltage values if there is a line to ground fault or the neutral is not grounded.  Most of the transformers have a very high amount of grounding resistance, which results from the poor grounding.  whenever the transformer Case is grounded and neutral point is not, and there is a ground fault to a line or more, the transformer windings will not be affected instantaneously but with time, due to failure of insulation between windings of the high and low voltage.Measurements of each transformer had been taken during this period (1-4)pm. Table 4.1 shows measurements of each line to neutral and value of grounding resistance.

No AREA Type of KVA Readings transformer L1-N L2-N L3-N E-N E(Ω) VOLT VOLT VOLT VOLT 1 GABRA Ground mounted 1000 230.3 240 216.8 226 0.9 BLOCK NO.(12) 2 GABRA Ground mounted 1500 88.4 55.9 57.5 215 1.2 BLOCK NO.(10) 3 GABRA Ground mounted 750 239.6 237 237.2 229 13.9 BLOCK NO.(8) 4 GABRA Pole mounted 200 250 249.5 246.3 70 241 BLOCK NO.(6) 5 GABRA Ground mounted 750 256 167.6 167 9 132 BLOCK NO.(4) 6 OUMDURMAN Ground mounted 750 248.3 248.5 248.4 20 12.8 BIT ALMAAL 7 OUMDURMAN Ground mounted 500 247.7 246.8 247 7.4 190 ALMOLAZMEEN 8 OUMDURMAN Ground mounted 1500 246 247 247 6.3 235 ALSHOHADA 9 OUMDURMAN Ground mounted 1000 242 244 245 1.8 18 ALMESHKA 10 OUMDURMAN Ground mounted 750 247.5 248 247 12.8 11.3 HOOSH ALKHALIFA

TABLE 4.2 : Shows The Voltage Readings Between The Earth And Each Line

NO AREA L1-E L2-E L3-E Volt volt volt 1 GABRA 236 238 7.4 BLOCK NO.(12) 2 GABRA 381 389 9.2 BLOCK NO.(10) 3 GABRA 428 427 5 BLOCK NO.(8) 4 GABRA 242 262 3.1 BLOCK NO.(6) 5 GABRA 190 283.8 255 BLOCK NO.(4) 6 OUMDURMAN 178 243.6 248 BIT ALMAAL 7 OUMDURMAN 243 249 249 ALMOLAZMEEN 8 OUMDURMAN 246.7 247 247 ALSHOHADA 9 OUMDURMAN 246.6 241.7 247 ALMESHKA 10 OUMDURMAN 250 248 249 HOOSH ALKHALIFA

TABLE 4.3 : Shows The Currents Readings Between The Earth And Each Line

NO AREA I1(A) I2(A) I3(A)

1 GABRA 60 30.8 46 BLOCK NO.(12) 2 GABRA 66 36 64 BLOCK NO.(10) 3 GABRA 104 79 90 BLOCK NO.(8) 4 GABRA 160 142 190 BLOCK NO.(6) 5 GABRA 120 95 37 BLOCK NO.(4) 6 OUMDURMAN 80.4 148.6 106.3 BIT ALMAAL 7 OUMDURMAN 32 106 21 ALMOLAZMEEN 8 OUMDURMAN 80 50 67 ALSHOHADA 9 OUMDURMAN 62 86 70 ALMESHKA 10 OUMDURMAN 135 149 79 HOOSH ALKHALIFA

4.3 Analysis : 1\ Gabra (Block no.12) Transformer: Form measurements of this transformer it is clear that there is a fault between line 3 and ground, therefore this causes the voltage to fall down to (7.4) and rises voltage between neutral and ground to (226), although of the very small value of the ground resistance (0.9). 2\ Gabra (Block no.10) Transformer: This transformer has the same problem of the above transformer, and as a result of the problem line 3 voltages has fall to (9.2), and neutral to ground voltage rises to a value near to voltage of the line (215), although of the small value of the grounding resistance (1.2). 3\ Gabra (Block no.8) Transformer: This transformer has the same problem of the above transformers, and as a result of the problem voltage of line 3 has fall to (5), and neutral to ground voltage rises to a value near to the voltage of the line (229), this transformer has slightly high value of grounding resistance (13.9). 60

4\ Gabra (Block no.6) Transformer: There is a fault between line 3 and ground so that, line 3 voltage fall to (3.1).This transformer suffers from the high value of the grounding resistance (241). 5\ Gabra (Block no.4) Transformer: This transformer has a suitable voltages and grounding resistance (132) measurement, but the problem is that, the load distribution is not good, which that most of loads are on the first and the second lines, and thus rises the neutral current to this value (73.74 A ). 6\ OUMDURMAN (BIT ALMAAL) Transformer: There is a fault between line 1 and the ground which results a drop voltage to this value (178) and rises neutral to ground voltage to this value (20), and also has a high value of grounding resistance (12.8). 7\ OUMDURMAN (ALMOLAZMEEN) Transformer: This transformer has a suitable voltages measurement, but has a very high amount of grounding resistance (190). 8\ OUMDURMAN (ALSHOHADA) Transformer: This transformer has good voltage measurements, but has a very high amount of grounding resistance (235). 9\ OUMDURMAN (ALMESHKA) Transformer: This transformer has good voltage measurements, but has a high amount of grounding resistance (18). 10\ OUMDURMAN (HOOSH ALKHALIFA) Transformer: This transformer has good voltage measurements, but has a high amount of grounding resistance (11.3). Neutral current of each transformer obtained from the below equation:

IN = I1∟0 + I 2∟120 + I 3∟-120 1\ Gabra (Block no.12) Transformer:

IN1 = 60∟0 + 30.8∟120 + 46∟-120

IN1 = 25.29 A 2\ Gabra (Block no.10) Transformer:

IN2 = 66∟0 + 38∟120 + 64∟-120 61

IN2 = 27.05 A 3\ Gabra (Block no.8) Transformer:

IN3 = 104∟0 + 79∟120 + 90∟-120

IN3 = 21.7 A 4\ Gabra (Block no.6) Transformer:

IN4 = 160∟0 + 142∟120 + 190∟-120IN4 = 42 A 5\ Gabra (Block no.4) Transformer:

IN5 = 120∟0 + 95∟120 + 37∟-120

IN5 = 73.74 A 6\ OUMDURMAN (BIT ALMAAL) Transformer:

IN6 = 80.4∟0 + 148.6∟120 + 106.3∟-120

IN6 = 59.63 A 7\ OUMDURMAN (ALMOLAZMEEN) Transformer:

IN7 = 32∟0 + 106∟120 + 21∟-120

IN7 = 80.06 A 8\ OUMDURMAN (ALSHOHADA) Transformer:

IN8 = 80 ∟0 + 50∟120 + 67∟-120

IN8 = 26.05A 9\ OUMDURMAN (ALMOSHKA) Transformer:

IN9 = 62∟0 + 86∟120 + 70∟-120

IN9 = 21.17 A 10\ OUMDURMAN (HOOSH ALKHALIFA) Transformer:

IN10 =135∟0 + 149 ∟120 + 79∟-120

IN10 = 64.15A

CHAPTER FIVE CONCLUSION AND RECOMMENDATION 5.1 Conclusion:  From this study which done on some of distribution transformers in city of Khartoum, it shows clear that some of the transformers have a very high grounding resistance due to ignorance of maintenance programs.  These transformers that have been studied have high neutral current due to the unequal distributed loads. 5.2 Recommendation:  Installation of neutral to ground must be done in a proper way to keep the ground fault current away from transformer windings.  Periodic check and maintenance must be considered, besides reviewing of the load distribution.

5.3 References 1. Practical Grounding, Bonding, Shielding and Surge Protection, G.Vijayaraghavan, Mark Brown, Malcolm Barnes, 1th published 2002 2. Practical Troubleshooting Electrical Equipment (Mark Brown, Jawahar Rawtani and Dinesh Patil) First published 2004.

APPENDIX PICTURES

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(c)

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(e)

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(g)