CHAPTER 1 INTRODUCTION 1.1 Introduction: The dependency of secure power is increasing in the society which leads to higher demands on the availability of electric power. The availability can be defined as the fraction of time that the electric power is available in a certain point in the network during a given time interval. The complement to availability is called unavailability and is the fraction of time that the electric power is unavailable in a certain point in the network during a given time interval. Most of the electric power in Bangladesh is transmission through the 132 kV substations that are parts of the main grid. Many of the 132 kV substations in the main grid are today old and needs to be modernized. It has also in the last years occurred a number of faults in these substations that has increased the actuality of making the substations more reliable. The term reliability is closely related to the term availability and can be defined as the probability of failure-free operation of a system for a specified period of time in a specified environment. One major difference between the reliability concept and the availability concept is that the availability can be decreased by both planned and unplanned unavailability while the reliability concept only considers the equipments ability to function correctly when it is in service. The steam power stations which are owned by the company PDB, produces approximately 60% of the total consumption of electricity in Bangladesh. Transmission line of Bangladesh is as well connected to the 132 kV substation but through a 33 kV substation. The 400 kV substation needs now to be replaced due to its age and due to the upgrades of active power output capability of the generators in Bangladesh power grid. The suggested design consists of double bus bars and double disconnecting circuit breakers, DCBs, which has the disconnect or function integrated in the circuit breaker. The DCBs are meant to replace the conventional combination of circuit breakers and separate disconnectors. The existing substation consists of four bus bars of which one is a transfer bus bar used to bypass faults in the event of fault in any of the devices in the substation. The existing substation has a relatively large flexibility to change connection by operation of circuit breakers and disconnectors. 1 1.2 Problem Statement: It has been questioned by EEL if faults on the disconnecting circuit breaker in the proposed substation design will cause high unavailability for EEL. This thesis has investigated how the unavailability will be affected on the incoming lines to the substation, that are connected between EEL power transformers T1 and T2 and the 33 kV substation. However, when replacing an old system with a new one it is of importance to not only consider the improvement of the new system, but also consider the possible drawbacks. To do this it necessary to define the requirements on the system. The substation could be seen as a part of a larger electricity system that consists of generation, distribution and consumption of electricity. The demands on the larger electricity system is to continuously produce and distribute electricity of good quality to satisfy the instantaneous electricity consumption in each point of the grid. The quality of the electricity is of importance to make the equipment connected to the grid function correctly without being damaged. From this discussion it is possible to derive two requirements on the substation. First, it should under normal conditions continuously distribute and be able to switch the electric power that the generators are producing. Second, it should minimize the function loss of the substation when a failure occurs and it should help to maintain the quality of the electricity. For the first requirement, the substation needs to contain switching devices and control equipments for the switches. The function of the switches is to control the connection and disconnection of the incoming power from the high voltage stations at Simpevarp and to switch the connection to the outgoing lines. The switching can both be controlled by manual operation and by the protection system, which mainly consists of circuit breakers and protection systems. The circuit breaker can from a reliability point of view be seen as 1) an high voltage apparatus that can cause short circuit or earth faults and 2) a switching device that is used to break load and fault current. The purpose of the protection system is to sense if a fault condition occurs in the protected zone and send a tripping signal to the concerned circuit breakers around the protected zone. When a component has been disconnected it will be unavailable. To determine the unavailability in a point of the protection system it is necessary to consider the basic criteria‘s of a protective systems that commonly includes the following factors(1) selectivity, (2) speed of operation (3) reliability, (4) simplicity and (5) costs. 2 1.3 Objectives: The purpose of this thesis was to construct a program that can be used for reliability calculations and to use this program to compare and evaluate how the suggested two-breaker arrangement design and the existing 11 kV substation in Simpevarp will differ in terms of production availability for EEL. 1.4 Methodology: This chapter starts with a motivation of the chosen methodology. It continues with an explanation of how the study was performed and how the data was collected. Next follows a discussion of the validity of the study and in the end there is an explanation of how the factors that affect the availability have been measured. 1.4.1 Motivation for the Chosen Methodology: The determination of the expected future reliability (Li 2005) of a system is done by a risk evaluation. Power system risk evaluation normally includes these four tasks: 1. Determination of component outage model 2. Selecting possible states of the system and calculating the probabilities. 3. Evaluating the consequences of selected system states 4.Calculating the risk indices The purpose of a risk evaluation is often to manage the expected risk. Risk management normally includes: 1. A risk evaluation to determine the quantitive risk 2. Determination of measures to reduce risk 3. Evaluation and justification of an acceptable risk level. For power systems the acceptable risk level is always a balance between costs and the reliability of the system. There are a few different techniques that traditionally have been used in reliability evaluations of substations and their substations. These techniques can be divided into two categories. In the first category the failure states are selected deterministically, these are often referred to as state enumeration techniques and can include Marko chains, fault tree analysis, cut set methods and linear programming. In the second category the fault states are determined stochastically with a Monte Carlo analysis. The main advantage of state enumeration techniques over the stochastically technique is its simplicity and it is normally preferable when dealing with smaller systems. For larger systems that are more complex, Monte Carlo simulation are instead normally preferred. This study is based on a state enumeration technique given by Meeuwsen and Kling (1997) who introduced a technique to deal with the complex switching options in a substation. Many earlier studies have neglected 3 the complex switching option, which is to switch disconnectors to bypass faults, and instead chosen to evaluate simpler configurations which many times are inconsistent with the real case where switching normally has been possible. 1.4.2 Data Collection: The study has primarily been based on secondary data collected from EEL‘s intranet as well as from the databases and literature available through Chalmers library. Most of the reliability studies that have been used for comparison have been obtained from the database IEEE. For a general understanding of the function of the substation has one guided visit to the 132 kV substation at Simpevarp been made which included visit to the control house. The function of the substation and its protection system has been explained by personnel at EEL. 1.5 Validity of the Study: The validity of the result of this study is highly dependent on the quality of the data used to appreciate the failure rates and the repair times as well as on the model that is used to calculate the availability. The statistical data used in this study suffers from a few unavoidable problems. First, the number of faults that occurs in substation equipment is generally quite low and with a small statistical population follows a high uncertainty. Second, there are only a few sources available for fault statistics for the Swedish grid and these are in general limited to just showing the average fault values and no variance is for this reason possible to obtain. The data used for the fault frequencies and repair times are historical data collected from a grid with a large part of ageing components and might not be representative for a newly build substation. However, a large part of the uncertainties with the point estimation of the input data used in the model is avoided by performing a sensitivity analysis that makes it possible to make some more general conclusions about the relative advantages between the different designs. 1.6 Thesis Outline: This Project/thesis is organized as follows: Chapter1 Introduction Chapter 2 Literature Reviews Chapter 3 Substation of EEL Chapter 4 Hardware Development Chapter 6 Hardware Development 4 CHAPTER 2 LITERATURE REVIEWS 2.1 Introduction: A substation is a component of an electricity transmission or distribution system where voltage is transformed from high to low, or the reverse, using transformers. A transmission substation transforms the voltage to level suitable for transporting electric power over long distances. This is to minimize capital and operating costs of the system. Once it is transported close to where it is needed, a distribution substation transforms the voltage to a level suitable for the distribution system. So the assembly of apparatus used to change some characteristic of electric supply is called a substation In a substation using step up and step down transformer change AC voltages from one level to Another, change AC to DC or DC to AC. A substation may have one or more relates transformers, many protective equipment and switches. Substation is important part of power transmission and distribution system. Substations are the most critical part of any electrical supply grid. A failure of a single piece of substation equipment Can cause a total grid collapse which may take days or even longer to rectify. The continuity of power supply depends on successful operation of substations. It is therefore essential to exercise Extreme care while designing and building a substation. Specific functions of substation are Power transformer. Local network for Connection point. Switchyard, bus-bars, circuit breakers, disconnections. Measuring point for control center – Potential and current transformers. Fuses and other protection device. Classification of substations: There are the two most important ways of classifying a substation. According to1. Production requirement 2. Constructional features 2.2 According to Production requirement: A substation may be called upon to change voltage level or improve power factor or convert ac power into depower etc. According to service requirement 11kV substation is Transformer substation. In this substations using power transformer changes voltage level of electric supply. 5 2.3 According to constructional features: A substation has many components (e.g. Circuit breaker, switches, fuses, instrument etc.) Which must be used properly to ensure continuous and reliable service? According to constructional features the substation is outdoor type Substation. The outdoor equipment is installed under the sky. Outdoor type substations should be in fenced enclosures or located in special-purpose buildings. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions 11/.440 kV Substation Arrangement The arrangement of substations can be done in many ways. However the main sectors of arranging the substations are – At load center: Where voltage is getting down 11kV to 400volts using transformer and this is near to be load center. 2.4 Substation Layout: Figure: 11KV to 440V Substation Layout a) Principle of Substation Layouts Substation layout consists essentially in arranging a number of switch gear components in an ordered pattern governed by their function and rules of spatial Separation. b) Spatial Separation: i. Earth Clearance: This is the clearance between live parts and earthed structures, walls, screens and ground. ii. Phase Clearance: This is the clearance between live parts of different phases. 6 iii. Isolating Distance: This is the clearance between the terminals of an isolator and the connections. iv. Section Clearance: This is the clearance between live parts and the terminals of a work section. The limits of this work section, or maintenance zone, may be the ground or a platform from which the man works. c) Separation of maintenance zones Two methods are available for separating equipment in a maintenance zone that has been isolated and made dead. i. The provision of a section clearance ii. Use of an intervening earthed barrier the choice between the two methods depends on the voltage and whether horizontal or vertical clearances are involved. 2.5 Equipment Function: To provide reactive Power during low load Neutral Grounding Resistor Limit the earth current. Function of Substation: 1 – Supply of required electrical power. 2 – Maximum possible coverage of the supply of network. 3 – Maximum security of supply. 4 – Shortest possible fault-duration. 5 – Optimum efficiency of plants and the network. 6 – Supply of electrical power within targeted frequency limits (49.5 Hz and50.5 Hz). 7 – Supply of electrical power within specified voltage limits. 8 – Supply of electrical energy to the consumers at the lowest cost. 2.6 Elements of a Substation: Substations have one or more transformers, switching and control equipment. In a substation, circuits breakers are used to interrupt any short-circuit or overload currents that may occur on the network. Substations do not usually have generators, although a power plant may have substation nearby. Other devices such as power factor correction capacitors, synchronizer and voltage regulators may also be located at a substation. The main equipment‘s of substation are shown 11/.440 kV Substation equipment‘s details Transmission line set giving the rated voltage level up to 11 kV. This 11 kV lines are connected to the 2MVA transformer via 11 kV bus bars is further connected to LT switchgear. The equipment required for a transformer SubStation depends upon the type of Sub-Station, Service requirement and the degree of protection desired. 11kV Sub-Station has the following major equipment‘s. 7 2.6.1 Transformer: 2 MVA 33/11KV Main Transformer(4 MVA.3 MVA.2 MVA. &1.5 MVA) 2. Lightning arrestor 3. Isolator and Earth switches 4. Current Transformer 5. Potential Transformer 6. Duplicate type bus bar 7. Insulators 8. PFI Plant 9. LT Switchgear–3000Amps, 2500Amps, 1600Amps.1250Amps, 1000Amps. All LT Switchgear is Various ACB/ABB. 2.6.1.a Faraday’s law of induction, which states that: The induced electromotive force (EMF) in a y closed circuit is equal to the e time rate of change of the magnetic flux through the circuit. Or alternatively: The EMF generated is proportional to the rate of change of the magnetic flux. Where Vs. is the instantaneous voltage, Ns is the number of turns in the secondary coil and Equals the magnetic flux through one turn of the coil. If the turns of the coil are oriented perpendicular to the magnetic field lines, the flux is the product of the magnetic flux density B and the area A through which it cuts. The area is constant, being equal to the cross-sectional area of the transformer core, whereas the magnetic field varies with time according to the excitation of the primary. Since the same magnetic flux passes through both the primary and secondary coils in an ideal transformer, the instantaneous voltage across the primary winding equals Electrical power is transmitted from the primary circuit to the secondary circuit. 2.6.1.b Transformer EMF equation: If the flux in the core is purely sinusoidal, the relationship for either winding between its rms voltage Erm of the winding, and the supply frequency f, number of turns N, core crosssectional Area a and peak magnetic flux density B If the flux does not contain even harmonics the following equation can be used for half-cycle. There are only 4 possible transformer combinations: Delta to Delta – use: industrial applications Delta to Wye use: most common for step-up transformer; commercial and industrial Wye to Delta use: most common for step-down high voltage Wye to Wye use: rare, don‘t use causes harmonics and balancing problems. 8 2.6.2 Characteristics of Distribution Transformer: 1. According to method of cooling * Oil-immersed, combination self-cooled and fan -cooled 2. According to insulation between windings a. Windings insulated from each other * Autotransformers 3. According to number of phase‘s a Poly-phase 4. According to method of mounting a. Platform 5. According to purpose * Constant-voltage * Variable-voltage 6. According to service a. large power * Distribution 2.6.3 Bus-bar Trunking: Figure: Bus bar Connections Bus-bars are the important components in a substation. There is several bus-bar arrangements that can be used in substation. The choice of a particular arrangement depends upon various factors such as voltage, position of substation, degree of reliability, cost etc. These are made up of copper and aluminum to which the terminal of generators, transformers, distribution lines, loads etc. is connected. In an electrical power distribution system that conduct electricity within a switchboard, distribution board, substation, or other electrical apparatus. These busbars are insulated from each other and also from the earth. The size of the bulbar is important in determining the maximum amount of current that can be safely 9 carried. Bus bars can have a cross-sectional area of as little as 10 mm² but electrical substations may use metal tubes of 50 mm in diameter (1,963 mm²) or more as bus bars. 2.6.3.a The following are the important bus-bar arrangements used in substation. • Single Bus-bar • Single Bus-bar system with Sectionalization • Double/Duplicate bus-bar arrangement Duplicate type Bus-bar this system consists of two bus-burs, a main bar-bar and a spare busbar. Each bus bar has the capacity to take up the entire substation load .The incoming and outgoing lines can be connected to either bus-bar with the help of a bus bar coupler which consists of a circuit breaker and Isolators. The incoming and outgoing lines remain connected to the main bus bar. However, in case of repair of main bus-bar or fault occurring on it, the continuity of supply to the circuit can be maintained by transferring it to the spare bus-bar. Insulators the insulator serves two purposes. They support the conductor (or bus bar) and confine the current to the conductor. The most commonly used material for the manufactures of insulators is porcelain. There are several type of insulator (i.e. pine type, suspension type etc.) and there used in Sub-Station will depend upon the service requirement. 2.6.4 Insulators: The insulator serves two purpose. They support the conductor and on fine the current to the conductor. The most commonly used material for the manufactures of insulators is porcelain. There are several type of insulator (i.e. pine type, suspension type etc.) and there used in SubStation will depend upon the service requirement. 2.6.5 Earth system: Figure: Earth System of 11/.44kv Substation 10 2.6.5.a Why ground? 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 properly designed and maintained grounding system, they will find unintended paths that could include people. 2.6.5.b What is a ground and what does it do? The NEC, National Electrical Code, Article 100 defines a ground as: ―a conducting connection, whether intentional or accidental between an electrical circuit or equipment and the earth, or twosome conducting body that serves in place of the earth.‖ When talking about grounding, it is actually two different subjects: earth grounding and equipment grounding. Earth grounding is an intentional connection from a circuit conductor, usually the neutral, to a ground electrode placed in the earth. Equipment grounding ensures that operating equipment within a structure is properly grounded. These two grounding systems are required to be kept separate except for a connection between the two systems. 2.6.5.c What is a good ground resistance value? There is a good deal of confusion as to what constitutes a good ground and what the ground resistance value needs to be. Ideally a ground should be of zero ohms resistance. There is not one Standard ground resistance threshold that is recognized by all agencies. However the NFPA and IEEE have recommended a ground resistance value of 5.0 ohms or less. The NEC has stated to ―Make sure that system impedance to ground is less than 25ohms specified in NEC 250.56. In facilities with sensitive equipment it should be 5.0ohms or less. The Telecommunications industry has often used 5.0ohms or less as their value for grounding and bonding. 2.7 Components of a Ground electrode: Ground conductor • Connection between the ground conductor and the ground electrode • Ground electrode 2.7.1 Locations of Resistances: (a) The ground electrode and its connection the resistance of the ground electrode and its connection is generally very low. Ground rods are generally made of highly conductive / low resistance material such as steel or copper. 11 (b) The contact resistance of the surrounding earth to the electrode The National Institute of Standards(a governmental agency within the US Dept. of Commerce) has shown this resistance to be almost negligible provided that the ground electrode is free of paint, grease, etc. and that the ground electrode is in firm contact with the earth. (c) The resistance of the surrounding body of earth the ground electrode is surrounded by earth which conceptually is made up of concentric shells all having the same thickness. Those shells closest to the ground electrode have the smallest amount of area resulting in the greatest degree of resistance. Each subsequent shell incorporates a greater area resulting in lower resistance. This fin ally reaches a point where the additional shells offer little resistance to the ground surrounding the ground electrode. So based on this information, we should focus on ways to reduce the ground resistance when installing grounding systems. 2.7.1.a What affects the grounding resistance? First, the NEC code (1987, 250-83-3) requires a minimum ground electrode length of 2.5 meters (8.0 feet) to be in contact with soil. But, there are four variables that affect the ground resistance of a ground system: 1. Length/depth of the ground electrode 2. Diameter of the ground electrode 3. Number of ground electrodes 4. Ground system design 2.7.2 Length/depth of the ground electrode: One very effective way of lowering ground resistance is to drive ground electrodes deeper. Soil is not consistent in its resistivity and can be highly unpredictable. It is critical when installing the ground electrode that it is below the frost line. This is done so that the resistance to ground will not be greatly influenced by the freezing of the surrounding soil. Generally, by doubling the length of the ground electrode you can reduce the resistance levels by an addition an l0%. There are occasions where it is physically impossible to drive ground rods deeper— areas that are composed of rock, granite, etc. In these instances, alternative methods including grounding cement are viable. 2.7.3 Diameter of the ground electrode: Increasing the diameter of the ground electrode has very little effect in lowering the resistance. For example, you could double the diameter of a ground electrode and your resistance would only decrease by 10 %. 12 2.7.4 Number of ground electrodes: Another way to lower ground resistance is to use multiple ground electrodes. In this design, more than one electrode is driven into the ground and connected in parallel to lower the resistance. For additional electrodes to be effective, the spacing of additional rods needs to be at least equal to the depth of the driven rod. Without proper spacing of the ground electro des, their spheres of influence will intersect and the resistance will not be lowered. To assist you in installing a g that will meet your specific resistance requirements, you can use the table of ground resistances, below. Remember, this is to only be used as a rule of thumb, because soil is in layers and is rarely homogenous. The resistance values will vary greatly. 2.7.5 Ground system design: Simple grounding systems consist of a single ground electrode driven into the ground. The use of a single ground electrode is the most common form of grounding and can be found outside your home or place of business. Complex grounding systems consist of multiple ground rods, connected, mesh or grid network, ground plates, and ground loops. These systems are typically installed at power generating substations, central offices, and cell tower sites. Complex networks dramatically increase the amount of contact with the surrounding earth and lower ground resistances. 2.7.5.a How do I measure soil resistance? To test soil resistivity, connect the ground tester as shown below. As you can see, four earth ground stakes are positioned in the soil in a straight line, equidistant from one another. The distance between earth ground stakes should be at least three times greater than the stake depth. So if the depth of each ground stake is one foot(.30meters), make sure the distance between stakes is greater than three feet(.91meters). The Fluke 1625 generates a known current through the two outer ground stakes and the drop in voltage potential is measured between the two inner Ground stakes. Using Ohm‘s Law (V=IR), the Fluke tester automatically calculates the soil resistance. Because measurement results are often distorter and invalidated by underground pieces of metal, underground aquifers, etc. additional measurements where the stake‘s axis are turned 90 degrees is always recommended. By changing the depth and distance several times, a profile is produced that can determine a suitable ground resistance system. Soil resistivity measurements are often corrupted by the existence of ground currents and their harmonics. To prevent this from occurring, the Fluke 1625 uses an Automatic Frequency Control (AFC) System. This automatically selects the testing frequency with the least amount of noise enabling you to get a clear reading. 13 2.8 Fall-of-Potential measurement: The Fall-of-Potential test method is used to measure the ability of an earth ground system or an individual electrode to dissipate energy from a site. 2.8.a How does the Fall-of-Potential test work? First, the earth electrode of interest must be disconnected from its connection to the site. Second, the tester is connected to the earth electrode. Then, for the 3-pole Fall-of-Potential test, two earth stakes are placed in the soil in a direct line-away from the earth electrode. Normally, Spacing of 20 meters (65feet) is sufficient. For more detail on placing the stakes, seethe next section. A known current is generated by the Fluke 1625 between the outer stake (auxiliary earth stake) and the earth electrode, while the drop in Voltage potential is measured between the earth stake and the earth electrode. Using Ohm‘s Law (V=IR), the tester automatically calculates the resistance of the earth electrode. Connect the ground tester as shown in the picture. Press START and read out the RE (resistance) value. This is the actual value of the ground electrode under test. 2.9 Specification of Substation: 11/0.415kV, 2MVA Sub-station of Instrument is … a) 2MVA 33/11Kv Main Transformer, b) LT Switchgear-5000A 4000A 3000A c) Dropout Fuse, Rated Voltage (Nominal) -11kV, Rated Current RMS -100A, 1Set d) Lightning Arrester, Rated Voltage (RMS) – 9kV, 1Set e) 2Mva – 3 Units1500 KVAR-tow units, 150KVAR 3-Units Automatic PFI Plant. f) Current Transformer, Ratio 2500/5A 1000/5A-500/5A, g) Potential Transformer, Ratio -11//110V, 3pcs. 14 2.9.1 Setup Information: Factory requirement following as there instrument will be ascension. There Instrument of the figure same as connection from the distribution line. First time we are connected Isolator. It connect to main line include drop out Fuse 15A, 3pcs, for 3phase. Isolator one terminals connect to main distribution line (11kv line) another terminal connects to current transformer of primary side. This Isolator attach on the front of poll after transformer connection. Transformer connected is delta to star, primary side is delta and secondary side is star. Primary side voltage is 11kV and secondary voltage is 415V. Secondary side of common terminal connected to grounding and also transformer body connected to grounding. Primary side bushing number is 3pcs. And secondary side bushing number is 4pcs. Transformers all of check, oil level, silica gel, temperature, and all nut bolt. Transformer secondary side connects to LT switch gear (Low tension) its medium is LT cable. There have 4 LT of (Radiator room, Transformer Tank Section, miscellaneous section, Machine-Shop and Paint section), Nickel section. All are connects by 185 and 95 RM cables. Now Low tension switch gear connection Low tension indoor type switchgear with hard drawn copper bus-bars, TPN&E equipped with incoming connected is 1no. 1600A, 36kA, TP MCCB with adjustable thermal overload and adjustable magnetic short-circuit releases. 3 Current transformer, ratio: 690/5A with suitable accuracy and burden. 3 -Ammeter 0-500A, 1Voltmeter 0-500V, with selector switch. 2 indicating lamp on/off and 1 set control fuse. Outgoing is 1, 2 and 3 CB 1250A, 36kA, TP MCCB with adjustable thermal overload and adjustable magnetic short circuit releases. No. of 4 CB, 690A, 18kA, TP MCCB with adjustable thermal overload and adjustable magnetic short circuit releases. Distributed to factory machineries by some low tension switchgear by the. And power factor improvement plant connected from low tension switchgear. Substation last pert is 250kVAR automatic power factor improvement plant connected. It is flour mounting, 415V, 50HZ, 250kVAR indoor type Automatic power factor improvement plant. To direct connections connected by another CB. 2.9.2 Comprising: 1500kVAR bank of TP dry type power 3-capacitor with built-in discharge resistor (Direct). With 1500kVAR bank of TP dry type power capacitor Built-in to discharge. Automatic power factor correction relay. Use some TP air contractors of adequate ratings. HRC fuses with base of adequate rating. Indicating lamps and 1 set control fuses. And other gives the connected on Distribution Box, Switching board etc. There are connected from low tension switchgear. Connect the Machineries line from Distribution Box. And all Instrument & machineries of body connected to earth grounding. 15 2.10 Operation of Sub-station: At many places in the line of the power system, it may be desirable and necessary to change some characteristic (voltage, frequency, power factor etc.) of electric supply. This is accomplished by suitable apparatus called substation. The sub-station operation explained as under: 1) The 3-phase, 3-wire of 300rm 11kV line is tapped and brought to the gang operating switch installed near the sub-station. The G.O. Switch consists of isolators connected in each phase of the 3-phase line. 2) From the G.O. Switch, the 11kV line is brought to the indoor sub-station as underground cable of 185RM. It is fed to the H.T. Side of the transformer (11kV/400V) via the 11kV O.C.B. The transformer steps down the voltage to 400V, 3-phase, 4 wires per wires are 120Rm, but together 630Rm. 3)The secondary of transformer supplies to the bus-bars via the main OCB. From the bus- bars, 400V, 3phase, 4-wire supply is given to the various consumers via 400V OCB. The voltage between any phase and neutral it is 230V. The single phase residential load is connected between any one phase and neutral whereas 3-phase, 400V motor load is connected across 3phase lines directly. 4) The CTs are located at suitable place in the sub-station circuit and supply for the metering and indicating instruments and relay circuits. Total 23 DB, and many Circuit barkers. All DB connects by 95RM Cables. 2.11 Maintenance and Trouble shutting: Symmetrical Fault the symmetrical fault rarely the symmetrical fault is occurs in practice as most severe and imposes more majority of the fault are of heavy duty on the circuit unsymmetrical nature breaker. The reader to understand the problems that, short circuit conditions present to the power system. Single line to ground, any line with short to the Separate to the line from short fault. Ground faults. Designed Circuit for solve the problem. 1. Line to line fault. One line with another line to separate to the line from short. Circuit for solving the problem. Insulation problem. 2. Double line to ground two line with short to the Separate to the line from short fault. 2.11.1 Problems: 3. Arc phenomenon when a short-short circuit Arc resistance is made to occurs, a heavy current increase with time so that flows the contacts of the current is reduced to a value circuit breaker. Insufficient to maintain the arc. 16 4. The ionized particles between the contacts tend to maintain the arc. Transformer open circuit an open circuit in one phase open phase connected with to fault. Of a 3-phase transformer circuit may cause undesirable heating. 2.11.2 Relay protection is not provided against open circuits: 5. On the occurrence of such because this condition is fault, the transformer can be relatively harmless. Disconnected manually from the system. 6. Transformer overheating over heating of the relay protection is also fault. Transformer is usually not provide against this caused by sustained contingency and thermal overloads or short-circuit accessories are generally used and very occasionally by the two sound an alarm or control failure of the cooling the bank of fans. 2.11.3 System: 7. Transformer Winding short-circuit (also the transformer must be short circuit fault. Called internal faults)on the disconnected quickly from the 8. Transformer arises from system because a prolonged deterioration of winding arc in the transformer may insulation due to cause oil fire. 2.11.4 Overheating or mechanical injury: 9. Lightning for over voltage the surges due to internal Surges due to internal causes fault. Causes hardly increase their taken care of by providing. 10. System voltage to twice the proper insulation to the normal value. Equipment in the power system. 11. A lightning arrester is a protective device which conducts the high voltage surges on the power system to the ground. 12. Low voltage Supply voltage is low. Transformer tap changing turn to move after solved the problem. 2.12 Switchgear: The term switchgear, used in association with the electric power system, or grid, refers to the electrical equipment like isolators, fuses, circuit breakers which intended to connect and disconnect power circuits are known collectively as switchgear. Switchgear is used in connect with generation, transmission, distribution and conversion of electric power for controlling, metering protecting and regulating devices. A basic function of switchgear power systems is protection of short circuits and overload fault currents while simultaneously providing service continuously to unaffected circuits while avoiding the creation of an electrical hazard. Switchgear power systems also provide important isolation of various circuits from different 17 Power supplies for safety issues. There are many different types and classifications of switchgear power systems to meet a variety of different needs. Switchgear power systems can vary, depending on several factors, such as power need, location of system and necessary security. Therefore, there are several different types of switchgear Power systems and each has their own unique characteristics to meet the specific needs of the system and its location. Figure: LT Switchgear 2.12.1 Switchgear instruments of Factory: Factory has low voltage (up to 380 volt) and medium voltages (up to 400V) switch gear. It is indoor type and switch gear instruments are: 1) Circuit breaker:-Miniature circuit breaker, Vacuum circuit breaker, MCC breaker. 2) Relay– Distance Relay, over current and Earth fault relay, Under/Over voltage relay, Trip circuit supervision relay, Differential protection relay, Static relay: 3) Current transformer (C T) 4) Potential transformer (PT) 5) Fuse 6) Lightning arrestor 7) Isolator and Earth switches 8) Magnetic conductor 2.13 Current transformers (CT): Current Transformers are used in current circuits in protection systems employing secondary relays. This transformer is to measure large currents and differs in phase from it by an angle 18 which is approximately zero for an appropriate direction of the connections. This highlights the accuracy requirement of the current transformer but also important is the isolating function. Figure: Current Transformer (CT) The Primary which is usually of few turns or even a single turn or thick copper or brass bar is inserted into the core of the transformer is connected in series with the load. The secondary current is normally rated for 5A or 1A and the number of turns in the secondary will be high. When the current transformer has two secondary windings then one winding is connected to the protective Relay system and there is to indicating metering circuit. Current transformer windings are polar in nature. The current transformers with 1A rating secondary can handle 25 times more burden than the current transformers of 5A secondary. Current Transformers of 1ASecondaries are normally used in the protection of 11 kV–132 kV. Transmission lines where the substation apparatus is located at a considerable distance from the control room, where the relays are situated. The magnitude of the current which flows through the secondary winding of a CT is a function of the primary current, the transformation ratio and also the impedance of the secondary circuit. CT‘s normally operate under Conditions close to shortcircuit conditions. The Secondary winding burden further depends upon the method of connection of the CT secondary, the relay windings and the kind of short circuit experience. CT‘s used for extra high voltage network protection must be capable of accurately transmitting currents both during steady state process and under transient conditions in order to permit operation of the protective devices correctly. The reasons for choosing proper CT‘s for extra high voltage network protection are; 1. The time constants of DC components in the short circuit currents of EHV networks are large. 2. The ratio of the short circuit current to the rated current is very high, due to increased concentration. 19 3. High Speed relaying is essential to protect electrical equipment during fault and to increase system stability The accuracy of a CT is directly related to a number of factors including: Burden Burden class/saturation class Rating factor Load External electromagnetic fields Temperature and Physical configuration. The selected tap, for multi-ratio CT‘s Ratio 5000/5A 2.14 Potential Transformers (PT): Figure: potential transformers (PT) Instrument Transformers are of means of extending the range of AC in strumpets like ammeters, voltmeters, V.A.R. Meters, Walt-meters. They are two types of potential transformers. The primary of the potential transformers is connected across the transmission line whose voltage may range from 2.4 kV to 220 kV. The secondary voltage is standardized at 110 kV. The load connected to the secondary is referred to as burden Small leakage reactance. The leakage reactance is due to the leakage of the magnetic fluxes of the primary & secondary voltages. They can be minimized by keeping the primary, secondary windings as close as possible subject to insulation problem as the primary is at high voltage. Small Magnetic 20 current. This can be achieved by making the reluctance of the core as small as possible and flux density in the core is also lowland it is very less than 1wb/m. 2.14.1 Minimum Voltage Drop: The resistance of the windings is made as small as possible. The Primary as it carries high voltage should be heavily insulated. Hence it is immersed in oil and the terminals are brought out to porcelain bushing. Now-a-days synthetic rubber insulation like styrene is used avoiding oil and porcelain. When the load or burden on the secondary is increased, the secondary current increases with corresponding increase in primary current so that transformation ratio remains the same. 2.14.2 Specification of Potential Transformer • Manufacturer : Energypac • Device maximum operating voltage : 2000kV • Rated frequency : 50-60Hz • Rated voltage : 11 kV • Rated output : 440V 2.14.3 Lightning Arrestor: Figure: Lightning arrester A lightning arrester is a device used on electrical power systems to protect the insulation on the system from the damaging effect of lightning. Metal oxide arrestors (MOVs) have been used for power system protection since the mid1970s. The typical lightning arrester also known as surge arrester has a high voltage terminal and a ground terminal. When a lightning surge or switching surge travels down the power system to the arrester, the current from the surge is diverted around the protected insulation in most cases to earth. 21 2.14.4 Specification of Lightning Arrestor: • Manufacturer : ABB • Rated voltage : 11kV • Rated current : 50-60Hz • Rated discharging current : 10kA • Continuous operating voltage : 100.8kV • Residual voltage under thunder in : 10kA 615kV peak • Standard discharging current : 1.0kAPeak 2.15 Circuit Breaker: Figure: Circuit Breakers A circuit breaker is an automatically-operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and these by interrupting continuity, to immediately discontinue electrical flow principle of Operation All circuit breakers have common features in their operation, although details vary substantially depending on the voltage class, current rating and type of the circuit breaker. The circuit breaker must detect a fault condition in low-voltage circuit breakers this is usually done within the breaker enclosure. Circuit breakers for large currents or high voltages are usually arranged with pilot devices to sense a fault current and to operate the trip opening mechanism. The trip Solenoid that releases the latch is usually energized by a separate battery, although some high-Voltage circuit breakers are self-contained with current transformers, protection relays and an internal control power source. Once a fault is detected, contacts within the circuit breaker must op en to interrupt the circuit. Some mechanically22 stored energy (using something such as springs or compressed air) contained within the breaker is used to separate the contacts, although some of the energy required may be obtained from the fault current itself. The circuit breaker contacts must carry the load current without excessive heating, and must also withstand the heat of the arc produced when interrupting the circuit. Contacts are made of copper or copper alloys, silver alloys and other materials. Service life of the contacts is limited by the erosion due to interrupting the arc. Miniature circuit breakers are usually discarded when the contacts are worn, but power circuit breakers and high-voltage circuit breakers have replaceable contacts. 2.15.a Classification of Circuit Classification of Circuit Breaker According to the voltage level circuit breaker are classified into three categories, such as 1. Low Voltage Circuit Breaker (Up to 619 volt) 2. Medium Voltage Circuit Breaker (Up to 11kV) 3. High Voltage Circuit Breaker (Up to 145kV) 2.15.1 Low Voltage Circuit Breaker: • A contact system with arc-quenching and current-limiting means • A mechanism to open and close the contacts • Auxiliaries which provide additional means of protection and indication of the switch positions. The MCCB may be used as an incoming device, but it is more generally used as an outgoing device on the load side of a switchboard. It is normally mounted into a low-voltage switchboard or a purpose-designed panel board. In addition to the three features listed at the start of this section, it also includes: • An electronic or thermal/electromagnetic trip sensing system to operate through the tripping mechanism and open the circuit breaker under overload or fault conditions • All parts housed within a plastic molded housing made in two halves • Current ratings usually from 10A to 1600A. 2.15.2 Medium Voltage Circuit Breakers: Medium-voltage circuit breakers rated between 440 Voltage and 11kV assemble into metalenclosed switchgear line ups for indoor use in MPS substation. Medium voltage circuit breakers are also operated by current sensing protective relays operated through current transformers. Medium-voltage circuit breakers nearly always use separate current sensors and protective relays, instead of relying on built-in thermal or magnetic over current sensors. 23 2.15.3 High-voltage circuit breakers: Electrical power transmission networks are protected and controlled by high-voltage breakers. The definition of high voltage varies but in power transmission work is usually thought to be 72.5 kV or higher. In MPS used SF6 circuit breaker for high voltage in substation. Highvoltage Breakers are always solenoid-operated, with current sensing protective relays operated through current transformers. In substations the protective relay scheme can be complex, protecting equipment and busses from various types of overload or ground /earth fault. 2.15.4 Miniature Circuit Breaker (MCB): Miniature circuit breakers rated current not more than 100A trip characteristic normally not adjustable. The miniature circuit breaker (MCB) has a contact system and means of arc quenching, a mechanism and tripping and protection system to open the Circuit breaker under fault conditions. Early devices were generally of the ‗zero-cutting‘ type, and during a short circuit the current had to pass through a zero before the arc was extinguished; this provided a short circuit breaking capacity of about 3kA. Most of these early MCBs were housed in Bakelite moldings. The modern MCB is a much smaller and more sophisticated device. All the recent developments associated with molded case circuit breakers have been incorporated into MCBs to improve their performance, and with breaking capacities of 10kA to 16kA now available, MCBs are used in all areas of commerce and industry as a reliable means of protection. Most MCBs are of single-pole construction for use in single-phase circuits. 2.15.5 Vacuum circuit breaker: Figure: Vacuum circuit breakers Vacuum circuit breaker with rated current up to 3000A, these breakers interrupts the current by creating and extinguishing the arc in a vacuum container. These are generally applied for 24 voltages up to about 35,000V but PS use vacuum circuit breaker for 11KV which corresponds roughly to the medium voltage range of power systems. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers. Vacuum circuit breakers tend to have longer life expectancies between overhaul than do air circuit breakers. In a vacuum circuit breaker, two electrical contacts are enclosed in a vacuum. One of the contacts is fixed, and one of the contacts is movable. When the circuit breaker detects a dangerous situation, the movable contact pulls away from the fixed contact, interrupting the current. Because the contacts are in a vacuum, arcing between the contacts is suppressed, ensuring that the circuit remains open. As long as the circuit is open, it will not be energized. Vacuum recluses will automatically reset when conditions are safe again, closing the circuit and allowing electricity to flow through it. Re-closers can usually go through several cycles before they will need to be manually reset Vacuum interrupters, mounted vertically within the circuit breaker frame, perform the circuit. 2.15.6 Specification of Vacuum circuit breaker: • Rated frequency-50 -60Hz • Rated making Current-10 Peak kA • Rated Voltage-11kV • Supply Voltage Closing-220 V/DC • Rated Current-1250 A • Supply Voltage Tripping-220 V/DC • Insulation Lev el-IMP 75 KVP • Rated Short Time Current-40 kA (3 SEC) 2.16 Fuses: 25 Figure: Fuses A fuse is a short piece of wire or thin strip which melts when excessive current flows through it for sufficient time. It is inserted in series with the circuit to be protected. Under normal operating conditions the fuse element it at a temperature below its melting point. Therefore, it carries the normal load current without overheating. However when a short circuit or overload occurs, the current through the fuse element increases beyond its rated capacity. This raises the temperature and the fuse element melts (or blows out), disconnecting the circuit protected by Inuit Electronics and electrical engineering a fuse (short for fusible link) is a type of sacrificial over current protection device. Its essential component is a metal wire or strip that melts when too much current flows, which interrupts the circuit in which it is connected. Short circuit, overload or device failure is often the reason for excessive current. 2.16.1 Fuse Ratings: 2.16.1.a Ampere Rating: Each fuse has a specific ampere rating, which is its continuous current-carrying capability. There are different types of fuse used in MPS, rating start from 2A. Voltage rating the voltage rating of a fuse must be at least equal to the circuit voltage. The voltage rating of a fuse can be higher than the circuit voltage, but never lower. A 500 volt fuse, for example, could be used in a 450 volt circuit, but a 350 volt fuse could not be used in a 500 volt circuit. 2.16.1.b Magnetic Contactor: A magnetic contactor is a relay-controlled switch used to turn a power control circuit on and off. It is electrically controlled and uses less power than other circuits. A magnetic contactor comes in different forms and capacities. Magnetic contactors are a form of electrical relay found on most electrically powered motors. They act as a go-between for direct power sources, and high-load electrical motors in order to homogenize or balance out changes in electrical 26 frequency which may come from a power supply as well as to act as a safeguard Components a magnetic contactor has three parts: power contacts, contact springs and auxiliary contacts. The power contact creates, carries and breaks the current in a magnetic contactor. Input a basic magnetic conductor has a coil input that is driven by either a DC or AC supply, and it can be energized at the same voltage as the motor. It can also be controlled separately using programmable controllers and low voltage pilot devices. Most contactors handle lighting, Heating, electric motors and capacitor banks 2.16.1.c Function of Magnetic conductor: Figure: Magnetic conductors Contactors are usually fitted on open contacts, and are designed to suppress and control electric arcs which are produced by interrupting heavy motor currents. They work on the principle of electromagnetism and the electricity runs through the coil from the core of the contactor. While the core is moving, a force is developed that allows the electromagnet to carry charge and hold the contacts together. Once the contactor coil is de-energized, the spring of the electromagnet returns to its original position. 2.17 Summary of This Chapter: I joint to EnergyPac Engineering Ltd. Factory for internship. This factory of all worker and management is very good and helpfully. Their help for my internship period happen be easy, therefore I would highly obliged. There, I learn to 11kv/.440kv Sub-station, Generator, Switchgear and factory maintenance division of equipment‘s are working principle, competent and possible cause‘s solution. Hare, many new machine, instruments & tools about understood. There are many problem to face and there with try to solution. This is my new experience. I am very enjoy the internship period, because it a new situation, new work and new problem solution invention and new learn of goods are characters. I hope this experience will need of my service and my future. 27 CHAPTER 3 ANALYSIS AND SIMULATION 3.1 Introduction Electrical utility has three functional areas namely generation, transmission and distribution. In the distribution network there are two main distribution network lines namely, primary distribution lines (33kV/22kV/11kV) and secondary distribution lines (415 volts line voltage). Primary distribution lines feed the HT consumers and distribution transformers. The distribution transformers feed the low voltage distribution networks which are the secondary distribution lines. Hence low voltage distribution network (LV network) is the last link connecting the consumers. Each of the primary distribution line leaves the sub-station as a three-phase circuit and supplies a number of distribution transformers. On the secondary side of the distribution transformer, the Secondary distribution lines are connected. The distribution transformers and secondary distribution lines are rated to maintain the voltage received by consumers within a prescribed tolerance over the full range of loading conditions. The Figure 1 shows the distribution system prevalent in EEL. The main part of distribution system includes:- 3.2 Analysis of Distribution System Figure : Distribution System 28 Figure : Sub-transmission lines. Figure : Secondary circuits on the LV side of the distribution transformer. Figure : Service mains with metering arrangement. The existing system in the distribution network is manually controlled. The distribution transformers are located at convenient places in the load area. At the distribution transformer, the voltage is stepped down to 415V and power is fed into the secondary distribution systems. The secondary distribution system consists of distributors (Pillar Boxes) which are laid along the road sides. The service connections to consumers are tapped off from the distributors. The 29 main feeders, distributors may consist of overhead lines or cables or both. The distributors are 3-phase, 4 wire circuits, the neutral wire being necessary to supply the single phase loads. There are single phase and three phase services given by the electrical utility depending on the requirement of consumers. The service connections of consumer are known as service mains. A ground connectionism normally provided, connected to conductive cases and other safety equipment, to keep current away from equipment and people. For single phase services the phase and neutral conductors from distribution transformer is connected to Figure 1. 3.3 Performance of distribution system The distribution system requires more attention as it is very difficult to standardize due to its complexity. As it involves consumers, power quality becomes paramount consideration in feeding the power supply. With a quality power there is need for uninterrupted supply of power. To avoid shortage of power one important consideration is reduction of transmission and distribution losses. Transmission and distribution losses (t & d losses) in Bangladesh have been consistently on the higher side between the ranges of 21–25%. Out of these losses, 19% is at the distribution level in which 14% is contributed by technical losses. This is due to inadequate investments for system improvement work. The detailed analysis of distribution transformers with different types of loading patterns have to be studied for solving power quality issues. Such data are not available in scientific way for analysis across the country. Therefore, DSP has been developed. At most care has been taken to collect the data of LV network from different places like metro, urban and rural populated regions to be analysed using DSP. Valentina Cecchi and et al (2007) have designed instrumentation and measurement configuration for network configuration and meter placement. The work is in primary distribution network and it is not extended up to secondary distribution network. Aderiano Galindo Leal and et al (2009) describes artificial neural network approach for loss evaluation of distribution transformer. The authors have recorded seven-day load profiles of distribution transformers of Brazilian distribution utility which proves uniformity in stochastic nature of the loads. That is, load profiles of different categories of consumers can be grouped under clusters as residential, commercial and industrial. But it requires daily load profiles of consumers whereas daily load profiles of distribution transformer are only available in developed country‘s scenario. Hence development of DSP for developing countries‟ is significant contribution for distribution sector. 30 3.4 Development of distribution simulation package In the existing system of distribution network, energy meters are provided for energy accounting. There is no means of sensing unbalance currents, voltage unbalance and power factor correction requirement for continuous 24 hours in three phases of LT Feeder. In other words, load curves, voltage curves, energy curves and power factor curves for individual three phases for full day are not available for monitoring, analyzing and controlling the LV network. To solve this, the modules developed in the DSP are listed below along with their associated function. Load Survey Module: Collection of 30 minute readings on the daily load pattern of distribution transformer. Power measurement Module: The measurement of power (Real, Reactive and Apparent) and display of voltage –current (vi-profile), power factor and power in the front panel for each phase R, Y and B. Display Module: The display of voltage graph showing all three phases, current graph with all three phases, power graph showing all three phases and total power for any selected day for the low voltage distribution network. These graphs are effectively utilized for load analysis and to study the power quality performance of low voltage distribution network. Unbalance Prediction Module: Prediction of unbalance in the network for the day selected and display them with LED indicators. 3.4.1 Design of Simulation Package In the existing system of distribution network, the distribution transformers are fixed with energy meters in the secondary of the distribution transformer and energy meter readings are downloaded with CMRI. The energy meter reading includes voltage current profile (vi–profile) and power factor. It can because for power measurement. With additional functionalities developed in this work like plotting of all electrical parameters on graph, prediction of unbalance current, LabVIEW based system can be termed as effective management and monitoring Module (MMM)for low voltage distribution networks. 3.5 Load analysis Load analysis of distribution transformers with different loading patterns deduces Significant inferences for the work presented in this paper. For performing load analysis, distribution simulation package developed as discussed in Section 3.1 is effectively utilized. One number sample transformer has been presented in this paper as case study. Typical loads on low voltage networks are stochastic by nature. However it has to be ensured that there is 31 similarity in stochastic nature throughout the day. For arriving at practical and effective conclusion for formulating this pragmatic methodology and to solve the power quality issues,the study has been undertaken using DSP. Balanced Condition Unbalanced condition Balanced Condition in two phases Unbalanced Condition in two phases Input to the DSP Low voltage distribution album of distribution transformer. Meter readings of distribution transformer (30minute readings). Output from the DSP Power measurement Voltage graph Load (Current Graph) Power graph and Total power 3.5.1 Load Analysis of Sample Distribution Transformer The schematic diagram with low voltage distribution album is shown in Figure 8. Energy Measurement for one day is shown in Figure 9 and load graphs for 2 different days taken at random are shown in Figure10 and Figure 11. 3.6 Results and Discussions The detailed analysis of distribution transformers with different types of loading patterns leads to very interesting findings as summarized below. Though the load utilization of individual consumers is a variable factor, there is uniformity found in stochastic nature. The per-capita consumption of electricity is high in urban compared to rural areas. The peaks and valleys in load graphs tend to follow similarity though not identical in all types of distribution network. The high peak occurs in approximately same band of one hour every day, the major variation may be two hours which is also very rare. The peak load of the transformer occurs when majority of the consumers connected to distribution network utilizes most of the loads. It occurs in the same time band of peak load of the distribution transformer due to prevailing culture and 32 habit of the people. The percentage of unbalance between phases is observed to be proportionate and hence value of unbalance will be maximum during peak loads. 3.7 Summary: All the inferences made out of low voltage distribution network load analysis prove that optimization of low voltage distribution network can be achieved by proper planning and successful reconfiguration of consumers to avoid unbalance of loads in distribution LV network. This distribution package serves as a backbone for the load analysis. With this method of load analysis and load reconfiguration there is a possibility of energy saving in terms of millions of rupees to the nation. 33 CHAPTER 4 HARDWARE DEVELOPMENT 4.1 Introduction: There are 3 substations within campus – Substation A,B,C. The 11kv 3phase supply is directly received at Substation A. From there we have 11kV feeder running to Sub B and C. For the purpose of distribution it is first step down to 440V. Single phase lines of 230 V are distributed among the various loads (department, hostel, lab, library etc.). For the purpose of backup each substation is equipped with 250kVA diesel generator. Its kVA rating is not enough to drive both the departments as well as the hostel at the same time. So it is usually switched between them depending upon the requirement. During day time, most of are gone for classes so in case of power failure, diesel generator if run supplies to the department only. Whereas at night, the authorities give priority to water pumping and then to the mess and the hostel. In VNIT you won‘t find overhead cables. The large number of trees falling during rainy season leading to interruption in power supply plus increased maintenance expenditure had forced the authorities to shift all the cables underground. 4.2 Main Component of 11kV substation: 4.2.1 Circuit breakers: Circuit breaker does the task of on load making and breaking of the circuit. This task can't be done in open air for high voltage as it leads to arcing as a result of ionization of air. There are different type of circuit breakers : 1. Gas Circuit Breakers 2. Oil Circuit Breakers 3. Vacuum Circuit Breakers 4. Air Circuit Breakers For 11kV, Vertical isolation VCB's are installed at VNIT substation. It uses spring energy for making and breaking of the circuit. It is 3 pole VCB having a rated current rating of 800A.It has a breaking capacity up to 20kA. The making and breaking time of the installed VCB is 3 cycles. Panel installed above it are for monitoring of relay protection, tripping etc. 34 4.2.2 Other advantages of Vertical isolation VCB are : 1. A time tested product for high reliability 2. Motorized spring charging mechanism 3. Cost effective solution by add-on protective devices like Series tripping coils. 4. Portable earthing device for bus bar feeder earthing 3-pole Air Circuit Breaker is installed on the 400V incomer coming form the transformer. For lower voltage level Ie. at the distribution end, MCCB of 250A rating are installed on each phase (Phase Voltage =230V). There are air circuit breakers which uses arc chutes for extinguishing of the arc produced during the process. Figure: Vacuum Circuit Breaker 4.2.3 Current transformers: Current transformers are installed at various stages in a network. Current transformers serve two basic purposes : Measurement of current Driving Protection relays Current transformers usually have 1 -5A on the secondary side. 5A is usually required for driving protection relays. As for example the CT installed on the 400V incomer from transformer is 600:5. For measurement purposes we 200:1 or 400:1 etc rating. 4.3 Transformer : Figure: Transformer 35 Transformer is a constant flux device used for changing the ac voltage level. Each substation has two 400kVA 11kV/400V delta - star step down transformer of which one of them is standby in case some problem occurs. It is provided with ONAN type of cooling. 4.4 Auxiliaries attached to transformer: 3.4.1 Conservator: The above described transformer is filled with 310Kg of oil for the purpose of cooling. Due to heating of the oil, oil will expand. If space is not provided to allow expansion, the transformer will burst. To prevent such scenario from being created, Conservators are installed on the top of the transformer to allow expansion. 4.4.2 Buchholz relay: Buchholz relay is a mechanical actuator, which trips the transformer in case of internal faults like insulation breakdown, short circuit of winding etc. Due to these faults the temperature rises and there is decomposition of insulating oil and gases are produced which get accumulated in the upper part of the Buchholz relay. This leads to tilting of the mercury switch. Based upon the level, the alarm is produced and relay comes in action. 4.4.3 Silica Jell: One of the biggest enemy of transformer is the moisture. To deal with these, the transformer are provided with silica jell box. It is blue in color. As it absorbs moisture it becomes pinkish. Then it is required to be replaced. They can be reused after placing them in light for some time. They lose their moisture content and become blue again and ready to be reused. However with every use the silica content keeps on getting reduced so it can't be used infinitely and needs to be replaced with new one. 4.4.4 Various: Various other components such are thermometer, pressure release valve, tap changer are present to ensure smooth functioning of the transformer. 4.5 Capacitor Bank: Capacitor Bank is installed in each substation to improve the power factor. Power factor is keep close to 1 usually .99 by using different combination of 25kVAR and 50kVAR of Capacitors. 36 Figure: Capacitor Bank 4.6 Distribution Panel: Distribution Panel contains panels containing Circuit Breakers for each department, hostel etc. so that the can be switched on and off depending upon the requirement. Figure: Distribution Bord(DB) 4.7 Diesel Generator Set: To ensure uninterrupted supply in case of power failure within the campus, each substation has a 250kVA Diesel Generator Set. Since it is less than 400kVA transformer that are meant to power the campus, the DG set is incapable of powering the normal load at once. So usually power is restored depending upon the requirement as stated earlier. Figure: Diesel Generator 37 DG set installed is a GPW250 model Greaves power 50Hz DG set of dimension 4*1.8*2.15 (m). It is a 3 phase generator set with operating power factor of 0.8 and maximum load current capacity of 347.8A at this power factor. It has 24V, 2*180AH battery required for its starting up. Integral fuel tank capacity provided is 420L. It uses water as coolant. 4.8 DG set comprises of two main parts: 1. Diesel Engine 2. Alternator 4.8.1 Diesel Engine: Figure: Diesel Engine It is a 6 cylinder, TBD3V8 Greaves model rated at 1500rpm and output of 313HP. It has lub oil sump capacity of 29L and Greaves Maxtherm API CF is used as lub oil. For starting the engine 24V battery is installed. AMF Panel is provided with a start button to start the generator remotely. However before this the Changeover should be moved from the transformer side to the diesel engine side. It is used as prime mover for the alternator. 4.8.2 Alternator: Figure: Alternator 38 The shaft of the diesel engine is connected to a 3 phase, 4 poles brush-less generator of 250 kVA, 415V and power factor of 0.8. It is provided with IP23 protection and class-H insulation. 4.9 Other features: Ease of maintenance with integrated components and outboard Exciter/Rotating Rectifier. A reliable long life with superior class 'H' insulation. Higher motor starting capability. Compact, light and sturdy die cast aluminum stator for frames up to 250, offer superior finish. High thyristor load withstand capability for Cell-Phone and Telecom applications. Short circuit withstand capability. Wide range of coupling discs/adaptor for single bearing construction suitable for wide range of Engine makers. Additionally there is air cleaner pumps for proper functioning of the DG set. Also we have various meters for oil level, water temperature, battery charge indicator etc for monitoring the health of the set. The whole DG set is provided with an acoustic enclosure to prevent noise pollution. Also regular maintenance is done to extend the life of the DG set. And lastly an important question!!Ever wondered what's the electrical bill of our campus?? It's 30 lakhs per months!! So when they ask us to save electricity, they do actually mean it. 4.10 Summary of The Chapter: A substation in which the apparatus is equipped inside the substation building is called indoor substation. Such type of substation is mainly used for the voltage up to 11000 v, but when the surrounding air is contaminated by impurities such as metal corroding gases and fumes, conductive dust, etc., their voltage can be raised up to 33000v to 66000v. The indoor substation is subdivided into several compartments like control compartment, indicating and metering instruments and protective device compartment main bus-bar compartment, current transformer and cable sealing box compartment as shown in the figure below. For the protection of feeders usually, reverse power relay is used. For the protection of oil filled transformer Buchholz relay is used. The accessories of the indoors type substations are a storage battery, fire fighting equipment such as water, buckets, and fire extinguisher, etc., The battery is used for the operation of protective gear and switching operating solenoids and emergency lighting in substations in the case of failure of supply. Indoor substations and transformer substation, as well as, high voltage switchboards consist of a series of open and enclosed chamber or compartments. The main equipment for this 39 installation is arranged in these compartments. The chamber space within which the equipment of the main bus-bar is connected is called a compartment or a cubical cell. 4.10.a Substations of the Integrity Built Type: In such type of substation, the device is equipped on site. In such substation, the cell structure is constructed of concrete or bricks. 4.10.b Substations of the Composite Built-Up Type: In such type of substations factory or workshop are built but are assembled on site within a substation switch gear room. The compartments of such substations take the form of metal cabinets or enclosures, each of which contains the equipment of one main connection cell. In such cabinets, an oil circuit breaker, a load interrupter switch, and one or more voltage transformers may be mounted. 4.10.c Unit Type Factory Fabricated Substations and Metal Clad Switchboards: These are built in electrical workshops and are carried to the site of installations fully pre-assembled. After installations of substations and switchboards only connection to the incoming and outgoing power circuits are required to be made 40 CHAPTER 5 RESULTS AND DISCUSSIONS 5.1 Introduction: Electricity is the basic necessity for the economic of a country. The industrial development and the increase of living standard of people are directly related to the more use of electricity. Without it is not possible to drive industrial machine, pump for irrigation and possible to develop living slandered of people. It is quit impossible for us to study on protective devices in a distribution substation. So in short we have tried to discuss the main components of the power protection system of the distribution substation. It has seen that fuses, circuit breakers, relays, lightning arresters, isolators, ear-thing and current limiting reactors are used as protective devices of distribution substation system protection. 5.2 Discussion of Low Voltage Substation: The reason that we are at 400KV is to reduce the line currents. If we stepped down to 33 KV then the current would be 12–13X and the wires would have to be correspondingly heavier. Usually a step down is tied together with splitting of the power to multiple branches so each branch will be carrying less power and hence lowers the current to manageable amounts at the lower branch voltages. You need lower voltages in the branches because you are going from large transmission towers with wide arms for air insulation to smaller towers and smaller insulators. It's all a matter of scale. A distribution transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer. The invention of a practical efficient transformer made AC power distribution feasible; a system using distribution transformers was demonstrated as early as 1882. Distribution transformers normally have ratings less than 500 kVA, although some national standards can describe up to 5000 kVA as distribution transformers. Since distribution transformers are energized for 24 hours a day (even when they don't carry any load), reducing iron losses has an important role in their design. As they usually don't operate at full load, they are designed to have maximum 41 efficiency at lower loads. To have a better efficiency, voltage regulation in these transformers should be kept to a minimum. Hence they are designed to have small leakage reactance. 5.3 Result of Low Voltage Substation: Result of Substation and the thesis paper. The substations are constructed under roof is called indoor type substation. Generally 11 KV and sometime 33 KV substation are of this type. 5.3.1 Step Down Substation: The stepped up voltages must be stepped down at load centers, to different voltage levels for different purposes. Depending upon these purposes the step down substation are further categorized in different sub categories. 5.3.2Primary Step Down Substation: The primary step down sub stations are created nearer to load center along the primary transmission lines. Here primary transmission voltages are stepped down to different suitable voltages for secondary transmission purpose. 5.3.3 Secondary Step Down Substation: Along the secondary transmission lines, at load center, the secondary transmission voltages are further stepped down for primary distribution purpose. The stepping down of secondary transmission voltages to primary distribution levels are done at secondary step down substation. 5.3.4 Distribution Substation: Distribution substation are situated where the primary distribution voltages are stepped down to supply voltages for feeding the actual consumers through a distribution network. 5.3.5 Underground Substation: The substation are situated at underground is called underground substation. In congested places where place for constructing distribution substation is difficult to find out, one can go for underground sub-station scheme. 5.3.6 Pole Mounted Substation: Pole mounted substation are mainly distribution substation constructed on two pole, four pole and sometime six or more poles structures. In these type of substation fuse protected distribution transformer are mounted on poles along with electrical isolator switches. 42 5.4 Summary of The chapter: A distribution substation transfers power from the transmission system to the distribution system of an area. The input for a distribution substation is typically at least two transmission or sub transmission lines. Distribution voltages are typically medium voltage, between 2.4 and 33 kV depending on the size of the area served and the practices of the local utility. Besides changing the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line. Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching, and switching and backup systems on the low-voltage side. 43 CHAPTER 6 SUBSTATION OF EEL 6.1 Introduction: Energypac Engineering Ltd. (EEL) is one of the leading power engineering companies in Bangladesh. Continual research and development, state of the art production facility, quality products, competent services, and countrywide operations have made it warmly acceptable to the customers. Energypac was incorporated in 1982 as a private limited business enterprise. It is powered by 1200 skilled manpower of which 150 are graduated engineers. The relentless efforts and dedication of these people are providing continual help to improve technology to innovate and develop new products, just in time delivery, pre and post sales services to maintain a long term business relationship with the customers. To meet countrywide demand of its products and services, Energypac has extensive distribution network throughout Bangladesh with full-fledged offices in the major cities like Chittagong, Khulna, Rajshahi, Sylhet, and Bogura. In an effort to introduce its products globally, Energypac has established its offices in India, Nepal, Italy and China. Energypac has already experienced its products and service supply to India, Nepal, Yemen, Ghana, Uganda, Nigeria, Saudi Arabia, Vietnam, Korea, Philippines and United Kingdom. 6.2 Description: Energypac is an ISO 9001:2008 and 14000:2004 certified company. Energypac enhances the business of its customers by providing them with complete solutions. While creating better and environmentally compatible technologies, Energypac focuses on the customers. Energypac is one of the leading power engineering companies in Bangladesh. Continual research and development, state of the art production facility, quality products, competent services, and countrywide operations have made it warmly acceptable to the customers. Energypac was incorporated in 1982 as a private limited business enterprise. It is powered by 1200 skilled manpower of which 150 are graduated engineers. The relentless efforts and dedication of these people are providing continual help to improve technology to innovate and develop new products, just in time delivery, pre and post sales services to maintain a long term business relationship with the customers. To meet countrywide demand of its products and services, 44 Energypac has extensive distribution network throughout Bangladesh with full-fledged offices in the major cities like Chittagong, Khulna, Rajshahi, Sylhet, and Bogura. In an effort to introduce its products globally, Energypac has established its offices in India, and China. Energypac has already experienced its products and service supply to India, Yemen, Ghana, Uganda, Nigeria, Saudi Arabia, and United Kingdom. 6.3 Construction: Energypac provides complete solution to the customers including engineering, procurement and construction (EPC). We deliver the keys of a commissioned plant to the owner for an agreed amount, just as a builder hands over the keys of a flat to the purchaser. It is also a way that needs the better understanding of customer‘s need for perfect execution of a project. An EPC contract is important for several vital reasons. EEL provides total solution to the customers for their power requirement from the project solution activities. Two broad categories of the project solutions are Turnkey, and EPC (Engineering, Procurement, and Construction). It entails project development, BOP development, project engineering, and project management. Our knowledgeable, skilled and experienced engineers are getting involved with the customers through all the stages of project conception, development, implementation, and continuation. It happens from a tiny substation to a large project implementation. Energypac Engineering Ltd. has an established department for taking care of the projects. It starts with the concept development, and finishes with the successful implementation of the project. It includes the preparation of project profile, feasibility study, detailed engineering, outsourcing needful, managing resources, and draw fine finishes to the projects. Desirous to be a cutting-edge engineering organization, EEL is relentlessly trying for acquiring excellence in engineering knowledge, preparing BOQ & feasibilities, and practical implementations of the projects. Relevant books are collected and consulted, trainings are received, and hands on experience are achieved for establishing proven confidence. Stringent calculations on different engineering aspects of product characteristics, performance analysis, and environmental considerations have been made it possible to reengineer the product, improvement in the quality of installations, and enhancement of performance. 6.4 Installation and Commissioning: Energypac Engineering Ltd is acquainted with the installation and commissioning of thousands of transformers, switchgears, and complete substations in its nearly 3 decade‘s long experience. The entire process of installation and commissioning involves the preparation of BOQ, 45 ensuring availability of the right materials at the right quantity at the right time, physical arrangements of transformer, switchgear, ancillary materials, and arrangement of all equipment‘s as per the regulatory authority‘s requirement. The ultimate focus on this job is to ensure that safety of the power transmission/distribution equipment, efficient power transmission/distribution, and maximum care for the installation and the environment. The commissioning of the equipment‘s has specific and stringent procedures to avoid premature or wrong operations. It includes the checking of all connections and settings of equipment‘s and ancillaries, load management, test run, and finally supplying power to the grid/loads. Energypac has highly expert engineers who are designated only to perform the commissioning jobs. They have received intensive trainings and are of highly experienced to demonstrate perfections in their jobs. 6.5 Spare Parts: Energypac Engineering Limited supports customers through the supply of genuine spare parts from the globally reputed and authentic sources. We cater the demand of spare parts through our business network all over Bangladesh as well as in abroad. The storage place at our factory premise is a well-managed and controlled temperature and humidity warehouse. Each part is entered, stored, and exited through modern software system. We maintain a huge stock of parts for our own consumption as well as warranty and life time support of the equipment‘s. 6.6 Service and Maintenance: Figure: Maintenance of Transformer Energypac Engineering Ltd is involved with the service and maintenance of its own supplied equipments and solutions. Basically it happens in two ways, on call basis, and contract basis. 46 On Call method allows the customers to call EEL‘s After Market Care (AMC) people to provide service and maintenance when it requires, and are charged for the completion of that particular job. We provide service and maintenance on 24/7 basis. Expert engineers and technicians are just a call away from our valued customers to respond to their complaints and act promptly. For Contracted Services, customers have to sign formal agreements with EEL for a certain period of time with specified terms and conditions of service and maintenance. Service and Maintenance Contracts may include schedule maintenance, unscheduled maintenance, health-check and troubleshooting. During the contracted period, EEL people will visit customer‘s facility as per the agreed schedule and provide service and maintenance. It is divided into two categories. LTSA (Long Termed Service Agreement): Under LTSA customers are contracted for long term services which may include spare parts also. It gives the customers comfort in keeping the price of parts and services valid for several years. STSA (Short Termed Service Agreement): STSA facilitates customers to take the benefits of taking discounted price on services and spares for a reasonable short period of time. 47 CHAPTER 7 CONCLUSION 7.1 Conclusion 7.2 Limitation of work 7.3 The Future of 11kV/440V Substation 7.1 Conclusion: For a technical service provider plant O&M activities are very important as its service mostly depends on the availability of its equipment. To maintain properly it requires very efficient O&M activities with minimum costing. By using proper O&M schedule of substation costcan be reduced and supply can be increased. O&M is traditionally classified as a part of output that comes from the system. There are many diverse ways of evaluating O&M of power system, as well as different objectives. O&M comprises all measures for maintaining and restoring the target condition as well as determining and assessing the actual condition of the technical equipment in a system. During this study, it has been observed from the organizational point of view where it has been implemented. There are so many improvements and applications that can be offered through this substation which of course would have direct benefit for the organization. 7.2 Limitation of Work: The researched problem in this thesis was to analyze how the suggested two-breaker arrangement design will differ from the existing substation considering the following aspects: The following points should be developed All instruments should be clearance between two equipment. Bus-bar should be used 20% or 30% ampere greater than the load current. Every circuit breaker really has time setting option from 0-1sec. If circuit breaker is more than one the time setting should be from 10ms to 80ms or 10ms to 1sec from load circuit breaker to generator circuit breaker. Transformer oil and silica gel should be checked after one month or any types of fault occurs any time . Oil should be changed if it is decomposed. 48 All cable should be cheek before use or any kinds They use manually based equipment, if they use PLC based equipment then the system will be easier. If they use new technology then the system loss will be reduce. High system loss, it will be reducing. Expected unavailability due to faults and maintenance Thesis Problem: The researched problem in this thesis was to analyze how the suggested twobreaker arrangement design will differ from the existing substation considering the following aspects: • Expected unavailability due to faults and maintenance • Fault and maintenance frequencies • Costs of the different substation designs Purpose: The purpose of this thesis was to construct a program that can be used for reliability calculations and to use this program to compare and evaluate how the suggested two-breaker arrangement design and the existing 11 kV substation in Simpevarp will differ in terms of production availability for EEL. Other factors that earlier have been mentioned as being of importance for the choice of substation design are the space that the construction will require and the possible affect the construction will have on the environment. This thesis will not further consider space limitation of the substation configurations. The new DCBs (ABB 2015) contain the gas SF6 which is a gas that contributes to the greenhouse effect. The handling of the gas needs to be done in an environmental friendly way which increase the demands on the maintenance, like for example refilling of the gas and testing of the gas pressure. The environmental aspect has not been considered further in this thesis. 7.3 Future Scopes of the Work: It takes smart approaches and powerful technologies to meet the world‘s soaring demand for electrical energy and to transmit power to where it is needed most. High-voltage substations are the node points of the increasingly complex power transmission infrastructure. They play a key role in enabling the reliable transmission of large amounts of power. As a turnkey contractor, Siemens offers the one-stop planning and construction of customized, state-of-theart high-voltage substations worldwide. Finally, it could also be of interest to investigate the different interests of the users of the substation. This can be divided into four groups. First, EEL that supply the grid with power. Second, BPDB that owns the substation and has the main responsibility for the function of the 49 main grid. Third, the electricity consumers that is dependent of the supply of electric power and fourth, the other electricity producers that is dependent on a well functioning grid to be able to deliver and sell their production of electric power. All of these can be assumed to be interested in a well functioning substation. However, EEL and BPDB can have different priorities concerning where in the grid it is important to have high availability. BPDB have responsibility for the availability in the whole main grid while EEL interest is more concerned with the ability to deliver its produced electricity to the main grid. This thesis will only concentrate on how the production availability for EEL is affected by the suggested new substation design. The factors that are important for EELs ability to deliver its power is both the unavailability on the incoming lines and on the outgoing lines. The incoming lines are the lines connected between the power transformers and the substation. The outgoing lines are the transmission lines leaving the substation and they are included in the study because the loss of load might force EEL to limit its production of electricity. For this reasons the study will concentrate on determining the unavailability in both the incoming and outgoing lines in the substation. 7.3.1 Other Important Factors to Consider: Other factors that earlier have been mentioned as being of importance for the choice of substation design are the space that the construction will require and the possible affect the construction will have on the environment. This thesis will not further consider space limitation of the substation configurations. The new DCBs (ABB 2015) contain the gas SF6 which is a gas that contributes to the greenhouse effect. The handling of the gas needs to be done in an environmental friendly way which increase the demands on the maintenance, like for example refilling of the gas and testing of the gas pressure. The environmental aspect has not been considered further in this thesis. 50 REFERENCES [1] Books i. Principlesof Power System by V.K Mehta & Rohit Mehta A Electrical Textbook of Electrical Technology_Vol.2 by B.L Theraja. Transmission & Distribution by B.L. Theraja ii. Protection and Switchgear by U.A.Bakshi and M.V. Bakshi Theraja. ii. J.B. Gupta, ―A Course in Electrical Installation, Estimating & Costing‖; Published by S.K Kataria & Sons, 9thEdition. iii. POWER SYSTEM ANALYSIS 5th ed iv. A.S.Pabla, ―Electrical Power Distribution System‖, Tata McGraw-Hill, 5th Edition [2] Journal International Journal of Advanced Research in Electrical, Electronics and Instrumentation Engineering(An ISO 3297: 2007 Certified Organization)Vol. 3, Issue 11, November2014 Valentina Cecchi, Xiaoguang Yang, Karen Miu and Chipka Nwankpa, ―Instrumentation and Measurement of a Power Distribution System laboratory for Meter Placement and Network reconfiguration Studies‖, IEEE Instrumentation & Measurement Magazine, October 2007 [3] Online Links http://www.assignmentpoint.com/science/eee/report-on-building-aspects-of-electric-substation.html http://www.school-for-champions.com/science/electrical_generation.htm http://electronics-polytech.wikispaces.com/DC+Generator http://dc349.4shared.com/doc/spDAwc9C/preview.html http://www.school-for-champions.com/science/dc_circuits.htm http://ytcphyssci.wikispaces.com/Ohm's+Law http://qiszqaiszmama.blogspot.com/2012/05/electricity-parallel-series-circuit.html http://kiran111.hubpages.com/hub/electrical-substation http://heag.en.alibaba.com/product/325207748200603654/JW_252_Outdoor_High_Voltage_Earthing_Switch.htm http://en.wikipedia.org/wiki/Capacitor_voltage_transformer https://www.google.com.bd/search?q=11kv/440v+substation+layout http://commonelectricaldoubts.blogspot.com/2014/10/vnit-11kv-indoor-substation.html http://www.electricaltechnology.org/2013/07/how-to-calculatefind-rating-of.html https://iiteeeestudents.wordpress.com/2011/11/14/key-diagram-11kv400v-indoor-substation/ http://eblogbd.com/basic-of-substation-substation-design-part-1/ http://winsol.info/substation-hardware/ https://electricalnotes.wordpress.com/2011/06/08/11kv415v-overhead-line-specification-rec http://macao.communications.museum/eng/exhibition/secondfloor/moreinfo/2_4_1_ACGenerator.html http://circuitglobe.com/indoor-substation.html#ixzz4NGu8T3ts http://www.oneschool.net/Malaysia/UniversityandCollege/SPM/revisioncard/physics/electromagnetism/induction. html 51 Appendix A AC : Alternating Current Alternator : A synchronous AC generator Alternator rotor: The rotor consists of a coil of wire wrapped around an iron core B Bus-bar : The metal (often copper) bar system which is the distribution media for the 3-phase high voltage system in the power plant C Current Transformer: In electrical engineering, a current transformer (CT) is used for measurement of electric currents. Current transformers are also known as instrument transformers. Capacitor : A device capable of storing electric energy. It consists of two conducting surfaces separated by insulating material. It blocks the flow of direct current while allowing alternating current to pass. Conductor : A wire or cable for carrying current. CT : Short for Current Transformer. An AC current measuring the generators to share the reactive component of the Current: The rate of flow of electricity. The unit of the ampere (A) defined as 1 ampere = 1coulomb per second. Circuit Breaker: An automatic switch that stops the flow of electric current in a suddenly over loaded or otherwise abnormally stressed electric circuit. F Frequency: Number of cycles over a specified time period over which an event occurs. 52 Feeder : The temperature to which oil must be heated in order to give sufficient vapor to forma flammable mixture with air under the conditions of the test. The vapor will ignite but will not support combustion G Generator: A device that produces electric current, usually by rotating a conductor in a magnetic field, thereby generating current through electromagnetic induction. H Hertz (Hz): Units in which frequency is expressed. Synonymous with cycles per second values. Machine language programs are often written in hexadecimal notation. HT: High Temperature (cooling water circuit) I Isolator: A passive attenuator in which the loss in one direction is much greater than that in the opposite direction; a ferrite isolator for waveguides is an example. L Load: The electrical demand of a process expressed as power (watts), current (amps) or resistance (ohms). Load sharing: The way in which two or more alternators are run to accommodate the load demands from the electrical network. LT side: Low tension side. O O&M: Operation and Maintenance. 53 P Parallel operation : More than one unit supplying power to the same network. Phase line: A line in an electrical network having system voltage potential. PLC : Programmable Logic Controller. Power factor : The extent to which the voltage zero differs from the current zero. (p.f = kW /kVA) 54
