1) Siting Hydroelectric plants are located in geographic areas where they will make economic use of hydraulic energy sources. Hydraulic energy is available wherever there is a flow of liquid and accumulated head. Head represents potential energy and is the vertical distance through which the fluid falls in the energy conversion process. The majority of sites utilize the head developed by freshwater; however, other liquids such as saltwater and treated sewage have been utilized. The siting of a prospective hydroelectric plant requires careful evaluation of technical, economic, environmental, and social factors. A significant portion of the project cost may be required for mitigation of environmental effects on fish and wildlife and relocation of infrastructure and population from flooded areas. 2) Hydroelectric Plant Schemes There are three main types of hydroelectric plant arrangements, classified according to the method of controlling the hydraulic flow at the site: 1. ...
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Hydroelectric power generation involves the storage of a hydraulic fluid, water, conversion of the hydraulic (potential) energy of the fluid into mechanical (kinetic) energy in a hydraulic turbine, and conversion of the mechanical energy to electrical energy in an electric generator. The first hydroelectric power plants came into service in the 1880s and now comprise approximately 20% (875 GW) of the worlds installed generation capacity (World Energy Council, 2010). Hydroelectricity is an important source of renewable energy and provides significant flexibility in base loading, peaking, and energy storage applications. While initial capital costs are high, the inherent simplicity of hydroelectric plants, coupled with their low operating and maintenance costs, long service life, and high reliability, makes them a very cost-effective and flexible source of electricity generation. Especially valuable is their operating characteristic of fast response for start-up, loading, unloading,...
Successfully practice electrical engineering to serve state and regional industries, government agencies, or national and international industries. Work professionally in one or more of the following areas: analog electronics, digital electronics, communication systems, signal processing, control systems, and computer-based systems. Achieve personal and professional success with awareness and commitment to their ethical and social responsibilities, both as individuals and in team environments. Maintain and improve their technical competence through lifelong learning, including entering and succeeding in an advanced degree program in a field such as engineering, science, or business. Electrical Engineering Student Outcomes Student outcomes are statements that describe what students are expected to know and are able to do by the time of graduation, the achievement of which indicates that the student is equipped to achieve the program objectives. The generalized outcomes for ...
A current-carrying coil produces a magnetic field that links its own turns. If the current in the coil changes the amount of magnetic flux linking the coil changes and, by Faraday’s law, an emf is produced in the coil. This emf is called a self-induced emf. Let the coil have N turns. Assume that the same amount of magnetic flux F links each turn of the coil. The net flux linking the coil is then NF. This net flux is proportional to the magnetic field, which, in turn, is proportional to the current I in the coil. Thus we can write NF µ I. This proportionality can be turned into an equation by introducing a constant. Call this constant L, the self-inductance (or simply inductance) of the coil: As with mutual inductance, the unit of self-inductance is the henry. The self-induced emf can now be calculated using Faraday’s law: The above formula is the emf due to self-induction. Example Find the formula for the self-inductance of a solenoid of N turns, length l, and cross-s...
Suppose we hook up an AC generator to a solenoid so that the wire in the solenoid carries AC. Call this solenoid the primary coil. Next place a second solenoid connected to an AC voltmeter near the primary coil so that it is coaxial with the primary coil. Call this second solenoid the secondary coil. As shown in figure. The alternating current in the primary coil produces an alternating magnetic field whose lines of flux link the secondary coil (like thread passing through the eye of a needle). Hence the secondary coil encloses a changing magnetic field. By Faraday’s law of induction this changing magnetic flux induces an emf in the secondary coil. This effect in which changing current in one circuit induces an emf in another circuit is called mutual induction. Let the primary coil have N1 turns and the secondary coil have N 2 turns. Assume that the same amount of magnetic flux F 2 from the primary coil links each turn of the secondary coil. The net flux linking the secondary...
Since the equivalent circuit contains two winding resistances and a core-loss resistance then power is lost as heating energy inside the transformer. Hence the conversion of power through the transformer cannot be 100%, a small loss of efficiency occurs. This is usually less than about 2% for power transformers. Assume all resistances and reactances are referred to the secondary winding. The efficiency can be expressed as, Where cosØ is the power factor of the load Pc is the core-loss Is is the secondary current Vs is the secondary voltage Es is the secondary emf. This formula applies to single-phase transformers, or to one phase of a three-phase transformer.
A single-phase power system transformer consists basically of two windings wound onto an iron core. The iron core concentrates the flux and restricts it to a defined path. It also creates the maximum possible amount of flux for a given excitation. In order to maximize the mutual coupling the two windings are wound concentrically on to the same part of the iron core. Figure 6.1 shows the typical winding arrangement of a single-phase transformer. This is called shell-type construction. Not all the flux created by one winding couples with the other winding. Furthermore the flux which does not couple both windings does not flow completely round the iron core, some of it flows in the air close to the windings. The common flux in the iron circuit is called the mutual or magnetizing flux. The flux that escapes into the air and does not couple the windings is called the leakage flux. One winding is referred to as the primary winding and is connected to the source of supply voltage. The second ...
When the maximum kW rating of an induction motor is reached for direct-on-line starting, it becomes necessary to introduce an alternative method of starting the motor. There are several methods used in the oil industry. The object is to reduce the starting current drawn from the supply during all or part of the run-up period. There are two basic approaches that can be used:- • Select special-purpose designs for the motor in which the winding arrangements are modified by external switching devices that are matched to the motor, e.g. star-delta motor and starter. • Select conventional motors but use special external starting devices, e.g. Korndorfer starter, autotransformer starter, ‘soft-starter’ using a controlled rectifier-inverter system. In all cases of reduced voltage starting, care must be taken to check that the motor will create sufficient torque at the reduced voltage to accelerate the load to the desired speed in as short a time as possible. Excessive run-up times must be avoi...
There are two important time periods that are critical in the application of induction motors . One is the allowable run-up or starting time and the other is the maximum stalling time. The run-up time is determined by the static torque versus speed characteristic, and the moment of inertia of the load. High inertia loads can cause very long run-up times. However, a long runup time in itself is not usually a problem for the driven machine. Most induction motors in the oil industry are started direct-on-line and the starting and run-up currents drawn by the motor can be in the range between about 4 and 7 times the rated current. When these currents exist for, say, 20 seconds, the amount of heat created in the stator windings and the rotor bar conductors is considerable. The surface temperature of these conductors can reach values high enough to cause damage to the winding insulation and slot wedges. With hazardous area applications this temperature rise can be very significant for some ...
Induction motors have two main components, the stator and the rotor. The stator carries a three-phase winding that receives power from the supply. The rotor carries a winding that is in the form of a set of single-bar conductors placed in slots just below the surface of the rotor. The slots have a narrow opening at the surface of the rotor, which serves to lock the conductor bars in position. Each end of each bar conductor is connected to a short-circuiting ring, one at each end of the rotor. The stator winding is a conventional type as found in three-phase generators and synchronous motors. The three-phase stator winding produces a rotating field of constant magnitude, which rotates at the speed corresponding to the frequency of the supply and the number of poles in the motor. The higher the number of poles the lower the speed of the rotation. Assume that the rotor is stationary and the motor has just been energized. The magnetic flux produced by the stator passes through the rotor an...
Main Exciter The exciter (sometimes called the main exciter) is a synchronous generator that has its stator and rotor windings inverted. Its field winding is fixed in the stator, and the rotor carries the armature or AC . In addition the rotor carries the semiconductor bridge rectifier that converts the armature voltages to a two-wire DC voltage system. The AC voltages and currents in the armature are often alternating at a higher frequency than those in the main generator, e.g. 400 Hz. The higher frequency improves the speed of response of the exciter. The DC power circuit is coupled to the field of the main generator by the use of insulated conductors that pass coaxially inside the rotor of the exciter and the rotor of the main generator. This eliminates the use of slip rings, which were traditionally used before shaft mounted rectifiers were developed. A slight disadvantage of this technique is that the derivative feedback cannot be taken from the output of the exciter. Howeve...
Following phasor is phsor diagram of a two-axis salient pole generator . The following points apply to the drawing of phasor diagrams of generators and motors:- • The terminal voltage V is the reference phasor and is drawn horizontally. • The emf E lies along the pole axis of the rotor. • The current in the stator can be resolved into two components, its direct component along the ‘direct or d-axis’ and its quadrature component along the ‘quadrature or q-axis’. The emf E leads the voltage V in an anti-clockwise direction when the machine is a generator. Each reactance and resistance in the machine has a volt drop associated with it due to the stator current flowing through it. Consider a generator. The following currents and voltages can be shown in a phasor diagram for both the steady and the dynamic states. E the emf produced by the field current If . V ...
The stator, also called the armature, carries the three-phase AC winding. The rotor, also called the field, carries the DC excitation or field winding. The field winding therefore rotates at the shaft speed and sets up the main magnetic flux in the machine. The fundamental magnetic action between the stator and rotor is one of tangential pulling. In a generator, the rotor pole pulls the corresponding stator pole flux around with it. In a motor, the stator pole pulls the rotor pole flux around with it. The action is analogous to stretching a spring, the greater the power developed, the greater the pull and greater the corresponding distance that is created between the rotor and stator flux axes. When a machine is not connected to the three-phase supply but is running at rated speed and with rated terminal voltage at the stator, there exists rated flux in the iron circuit and across the air gap. This flux cuts the stator winding and induces rated emf in winding and hence rated voltag...
Again assume that the generator is loaded and operating in a steady state. In this situation the magnitude of the stator current is allowed to change rapidly, as in the case of a short circuit in the stator circuit. The additional flux produced by the stator winding will try to penetrate the surface of the rotor poles. Most oil industry generators are provided with damper bars to reduce the excursions in rotor speed during major disturbances. The bars are made of copper or copper alloy and placed longitudinally in the face of the rotor poles. They function in a manner similar to a squirrel cage induction motor when there is a transient change in rotor speed relative to the synchronous speed. As soon as the additional flux passes through the pole faces it will induce currents in the damper bars and the solid pole tips, by the process of transformer induction. These induced currents will set up flux in opposition in order to maintain constant flux linkages with the stator. During this...
Assume the generator is loaded and operating in a steady state. If the peak-to-peak or rms value of the stator current changes in magnitude then its corresponding change in magneto-motive force (mmf) will try to change the air-gap flux by armature reaction. Relatively slow changes will allow the change in flux to penetrate into the rotor. When this occurs an emf is induced in the field winding. This emf drives a transient current around a circuit consisting of the field winding itself and the exciter that is supplying the winding. The induction of current is by transformer action. An increase in stator current will be matched by an increase in field current during the transient state. A voltage drop will occur in the machine due to the armature reaction and the reduction in air-gap flux. Reactance is associated with this type of armature reaction. When the rotor poles are coincident with the stator coils axis the armature reaction is a maximum and the reactance is called the direct ...
The rotating field in the air gap of a synchronous machine is generally considered to be free of space harmonics, when the basic operation of the machine is being considered. In an actual machine there are space harmonics present in the air gap, more in salient pole machines than a cylindrical rotor machine. It is acceptable to ignore the effects of space harmonics when considering armature reaction and the associated reactance. Therefore the flux wave produced by the rotating field winding can be assumed to be distributed sinusoidally in space around the poles of the rotor and across the air gap. If the stator winding, which consists of many coils that are basically connected as a series circuit, is not connected to a load then the resulting emf from all the coils is the open circuit emf of the phase winding. Closing the circuit on to a load causes a steady state current to flow in the stator coils. Each coil creates a flux and their total flux opposes the field flux from the rotor...
The stator, also called the armature, carries the three-phase AC winding. The rotor, also called the field, carries the DC excitation or field winding. The field winding therefore rotates at the shaft speed and sets up the main magnetic flux in the machine. The fundamental magnetic action between the stator and rotor is one of tangential pulling. In a generator, the rotor pole pulls the corresponding stator pole flux around with it. In a motor, the stator pole pulls the rotor pole flux around with it. The action is analogous to stretching a spring, the greater the power developed, the greater the pull and greater the corresponding distance that is created between the rotor and stator flux axes. When a machine is not connected to the three-phase supply but is running at rated speed and with rated terminal voltage at the stator, there exists rated flux in the iron circuit and across the air gap. This flux cuts the stator winding and induces rated emf in winding and hence rated voltage at...
The theoretical operation of synchronous generators and synchronous motors is almost the same. The main differences are the direction of stator current and the flow of power through these machines. The construction of generators and motors, of the same kW ratings, used in the oil and gas industry is very similar. Variations that are noticeable from the external appearance exist mainly due to the location of the machine and its surrounding environment. It is uncommon for generators to be placed in hazardous areas, whereas it is occasionally necessary to use a synchronous motor in a hazardous area, e.g. driving a large gas compressor. Large induction motors are often used for driving oil pumps and gas compressors that need to operate in hazardous areas. The rotor of generators may be either ‘cylindrical’ or ‘salient’ in construction. Synchronous motors nearly always have salient pole rotors. Machines with four or more poles are always of the salient pole rotor type. Cylindrical pole roto...
In all power systems the requirement is that the steady state speed deviation, and hence frequency, is kept small for incremental changes in power demand, even if these power increments are quite large – 20%, for example. There are two main methods used for speed governing gas turbines, a) Droop governing. b) Isochronous governing. Droop governing requires a steady state error in speed to create the necessary feedback control of the fuel value. ‘Droop’ means that a fall in shaft speed (and hence generator electrical frequency) will occur as load is increased. It is customary that a droop of about 4% should occur when 100% load is applied. Droop governing provides the simplest method of sharing load between groups of generators connected to the same power system . In control theory terminology this action is called ‘proportional control’. This method of governing is the one most commonly used in power systems because it provides a reasonably accurate load sharing capabi...
Gas turbines are usually started by a DC motor or an air motor. Either system is available for most turbines up to about 20 MW. Occasionally AC motors are used. Beyond 20 MW, when heavy industrial machines tend to be used, it becomes more practical to use air motors or even diesel engine starters. DC motors require a powerful battery system. The DC motor and battery systems tend to be more reliable and less space consuming, which is important for offshore systems. Air motors require air receivers and compressors. The compressors require AC motors or diesel engines. Air start and diesel start systems are more popular for onshore plants especially remote plants, e.g. in the desert. This is partly due to the fact that batteries tend to suffer from poor maintenance in hot, dry locations. Air systems require regular maintenance and must be kept fully charged in readiness for a quick start. Air system receivers can become very large if more than three successive starting attempts are require...
The electrical engineer should take full account of the site location and environmental conditions that a gas turbine generator will need to endure. These conditions can seriously affect the electrical power output that will be achievable from the machine. The starting point when considering the possible output is the ISO rating. This is the declared rating of the machine for the following conditions:- • Sea level elevation. • 15 0 C (59 0 F) ambient temperature. • Basic engine, no losses for inlet or exhaust systems, no losses for gearbox and mechanical transmission. • Clean engine, as delivered from the factory. The gas turbine manufacturer provides a standardized mechanical output power versus ambient temperature characteristic. (Some manufacturers also give the electrical output power versus ambient temperature characteristic. Therefore care must be exercised to be sure exactly which data are to be given and used.) The following derating factors should be used...
The fuels usually consumed in gas turbines are either in liquid or dry gas forms and, in most cases, are hydrocarbons. In special cases non-hydrocarbon fuels may be used, but the machines may then need to be specially modified to handle the combustion temperatures and the chemical composition of the fuel and its combustion products. Gas turbine internal components such as blades, vanes, combustors, seals and fuel gas valves are sensitive to corrosive components present in the fuel or its combustion products such as carbon dioxide, sulphur, sodium or alkali contaminants. The fuel can generally be divided into several classifications:- • Low heating value gas. • Natural gas. • High heating value gas. • Distillate oils. • Crude oil. • Residual oil.
There are basically two gas turbine driving methods, known as ‘single-shaft’ and ‘two (or twin) shaft’ drives. In a single-shaft gas turbine, all the rotating elements share a common shaft. The common elements are the air compressor, the compressor turbine and the power turbine. The power turbine drives the generator. In some gas turbines, the compressor turbine and the power turbine are an integral component. This tends to be the case with heavy-duty machines. The basic arrangement is shown in Figure 2.3. In a two-shaft gas turbine the compressor is driven by a high pressure turbine called the compressor turbine, and the generator is driven separately by a low pressure turbine called the power turbine The basic arrangement is shown in Figure 2.4. Two-shaft systems are generally those which use aero-derivative engines as ‘gas generators’, i.e. they produce hot, high velocity, high pressure gas which is directed into the power turbine. Some light industr...