Genset

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Note: The source of the technical material in this volume is the Professional

Engineering Development Program (PEDP) of Engineering Services.

Warning: The material contained in this document was developed for Saudi

Aramco and is intended for the exclusive use of Saudi Aramco’s employees. Any material contained in this document which is not already in the public domain may not be copied, reproduced, sold, given, or disclosed to third parties, or otherwise used in whole, or in part, without the written permission of the Vice President, Engineering Services, Saudi Aramco.

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Engineering Encyclopedia Electrical Generator Fundamentals

Content Page

INTRODUCTION... 1

ELEMENTS OF A POWER GENERATOR... 2

Basic Generator Principles... 2

Mechanical to Electrical Conversion ... 2

Sinusoidal Voltage Output ... 2

Motor Versus Generator Comparison ... 7

Single-Phase Generators...10 Components...10 Operation...11 Three-Phase Generators ...12 Components...12 Operation...13

ELEMENTS OF A POWER GENERATION SYSTEM...14

Purpose and Types of Prime Movers ...14

Purpose ...14

Types of Prime Movers...14

Prime Mover Governors/Speed Control...21

Purpose of Governors/Speed Control...21

Methods of Speed Control ...23

Purpose and Types of Generators/Alternators...26

Purpose ...26

Types of Generators/Alternators ...27

Purpose and Types of Voltage Regulators ...30

Purpose ...30

Types of Voltage Regulators...30

MAJOR AC GENERATOR COMPONENTS...33

Stator ...33

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Wye Configuration...39

Delta Configuration ...39

Types of Rotors...40

Salient Pole...41

Cylindrical Pole...42

Types of Cooling Systems ...44

Air-Cooled...44

Air-To-Water Heat Exchanger ...45

Gas-To-Water Heat Exchanger ...45

Types of Bearings and Lubrication Systems ...45

Types of Bearings ...45

Types of Lubrication Systems ...48

GENERATOR EXCITATION ...49

Purposes of Generator Excitation ...49

Power to the Rotating Electromagnetic Field ...49

Locking Rotor To Stator ...49

Means of Regulating Voltage ...49

Types of Generator Excitation ...49

DC Exciters ...50

Static Excitation ...53

Brushless Excitation...55

Concept of Response Time Versus Voltage Levels ...56

GENERATOR GROUNDING ...58

Purposes of Generator Grounding ...58

Personnel Safety ...58

Equipment Protection ...59

Methods of Generator Grounding...59

Solidly-Grounded...61

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Source System Ground Versus Plant Generator Ground...68

SAES-P-114 Grounding Requirements ...69

ELEMENTS OF GENERATOR PROTECTION...75

Temperature Protection...75 Rotor ...75 Stator ...76 Bearing ...77 Lubrication ...77 Electrical Protection ...78 Overcurrent ...81

Differential Current (Fault) ...81

Ground Fault ...82 Loss of Field...83 Phase Unbalance ...84 Frequency/Overspeed...85 Voltage...85 Sync Verification ...86

Reverse Power (Motoring)...86

Zone Protection Concepts ...87

Overlap...87

Backup ...87

AC GENERATOR PERFORMANCE CHARACTERISTICS ...89

Nameplate Ratings ...89 kVA ...89 kW ...90 Power Factor ...90 Terminal Voltage ...92 Field Current...92 Speed ...92 Temperature Rise...92

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Insulation Class...93

Ratings Interrelationships ...93

kVA, kW, Power Factor ...93

kVA, kW, Voltage...94

Saturation Curve Parameters ...95

Excitation Current (IE) ...98

Terminal Voltage (Vt)...99

Load ...100

Power Factor ...100

Generator Performance vs. Saturation ...100

Load vs. Voltage Relationship...100

Effects of Load and Excitation Current Changes ...101

Reactance Values ...108

Synchronous Reactance (Xd) ...110

Transient Reactance (X’d) ...111

Subtransient Reactance (X’’d) ...114

Decrement Curve Model ...114

Synchronous Reactance (Xd) ...116

Transient Reactance (X’d) ...117

Subtransient Reactance (X”d) ...118

Capability Curve Parameters...119

Prime Mover...120

Load kW ...121

Load kVAR ...121

Load kVA...121

Underexcitation-Leading Power Factor ...121

Overexcitation-Lagging Power Factor...122

Generator Performance vs. Capability...122

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VAR Flow / Unbalanced Voltages ...126 VAR Compensation ...126 GLOSSARY...127

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Table of Figures Page

Figure 1. Sinusoidal Voltage Output... 3

Figure 2. Rotating Flux Fields... 4

Figure 3. Single-Phase Generator... 5

Figure 4. Elementary Three-Phase Generator... 6

Figure 5. One Cycle of Three-Phase Sinusoidal Waveshape ... 7

Figure 6. Torque Angle at No-Load ... 9

Figure 7. Torque Angle at Rated Load...10

Figure 8. Equivalent Circuit...11

Figure 9. Vector Diagram...11

Figure 10. Cross-Section View of Stator Windings for Two-Pole and Four-Pole Generators ...12

Figure 11. Three-Phase Generator and Sine Wave Relationship ...13

Figure 12. Generation and Delivery of Mechanical Energy vs. Generation and Delivery of Kinetic Energy ...16

Figure 13. Generation and Delivery of Mechanical Energy ...17

Figure 14. Simple (Open) Cycle Gas Turbine Engine ...19

Figure 15. Energy Flow Simple (Open) Cycle ...20

Figure 16. Electric Controls for Single Unit Generators ...24

Figure 17. Basic Electric Paralleling System...24

Figure 18. Electric Load Sharing System ...25

Figure 19. Fully Automated Electric Load Sharing System...26

Figure 20. Generator Winding ...27

Figure 21. Torque - Slip Curves of Squirrel Cage Induction Motor...28

Figure 22. Rotating Armature...29

Figure 23. Stationary Armature ...30

Figure 24. Electro-Mechanical Voltage Regulator...31

Figure 25. Electronic Regulator ...32

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Figure 29. Stator Winding End-Turn Support ...37

Figure 30. Temperature Detector Locations...38

Figure 31. Wye Configuration ...39

Figure 32. Salient Pole Rotor...40

Figure 33. Cylindrical Pole Rotor ...41

Figure 34. Cylindrical Pole Rotor ...42

Figure 35. Rotor End-Turn Conductors ...43

Figure 36. Cross-Section Views of a Typical Sleeve Bearing Assembly...47

Figure 37. Induced EMF In A Rotating Coil ...50

Figure 38. Rectification By Two Segment Commutator ...51

Figure 39. Current Flow Through Commutator ...52

Figure 40. Current Flow Through Commutator (Cont’d) ...52

Figure 41. Static Excitation ...54

Figure 42. Brushless Excitation System ...55

Figure 43. Brushless Exciter With Permanent Magnet Pilot Exciter...56

Figure 44. Typical Response Time Versus Voltage for Sudden Application of Load ...57

Figure 45. Characteristics of Different Methods of Grounding ...60

Figure 46. Recommendations for Applying Different Methods of Grounding ...61

Figure 47. Solidly-Grounded Generator ...62

Figure 48. Low Resistance Grounded Generator...64

Figure 49. High Resistance Grounded Generator ...66

Figure 50. Reactance-Grounded Generator...68

Figure 51. System Ground Versus Generator Ground ...69

Figure 52. Grounding of Large Direct-Connected Synchronous Generators ...70

Figure 53. Grounding of Large Unit-Transformer Connected Generators...71

Figure 54. Grounding of Medium Direct-Connected Generators ...72

Figure 55. Grounding of Low Voltage Generators- Separately Derived System ...73

Figure 56. Grounding of Low Voltage Generators-Not Separately Derived System ...74

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Figure 58. Stator RTD Protection...77

Figure 59. Large Direct-Connected Generator Protection Scheme ...79

Figure 60. Protection Device Legend...80

Figure 61. Voltage Restraint Relay ...81

Figure 62. CA Generator Differential Relay ...82

Figure 63. CWC Ground Fault Relay ...83

Figure 64. Generator Current Locus ...84

Figure 65. Generator k-Values...85

Figure 66. Zones of Protection ...88

Figure 67. Typical Generator Nameplate Data ...89

Figure 68. Generator kVA Formulas...90

Figure 69. Examples of Generator Power Factor ...91

Figure 70. Generator Speed and Frequency Relationship...92

Figure 71. Typical Generator Saturation Curve...96

Figure 72. Example of Manufacturer Saturation Curves for a Specific Generator...98

Figure 73. Generator Field Circuit ...99

Figure 74. Formulas Showing Relationship between Generator Voltage and Load ...101

Figure 75. Equivalent Circuit for 60 kVA, 480 V Generator ...102

Figure 76. Saturation Curve for 60 kVA, 480 V Generator...103

Figure 77. Vector Diagram of Generator Voltage with 0.8 Lagging Power Factor ...104

Figure 78. Vector Diagram of Generator Voltage with 0.8 Leading Power Factor ...106

Figure 79. Vector Diagram of Generator Voltage with 1.0 Power Factor...107

Figure 80. Magnetic Axis’ in a Single Phase Generator ...109

Figure 81. Vector Diagram Illustrating Affect of Synchronous Reactance on Generator Voltage ...110

Figure 82. Fault Current Waveform ...111

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Figure 86. Typical Oscillogram of a Sudden Three-Phase Short Circuit...116 Figure 87. Envelope of a Synchronous Generator’s Short-Circuit Current...117 Figure 88. Subtransient and Transient Currents Plotted to Semi-logarithmic

Coordinates...118 Figure 89. Generator Capability Curve...120 Figure 90. Generator Capability Curves with Identified Cooling Limit Sources ...123

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INTRODUCTION

The generation of electric power requires several transformations of energy that involves many components and procedures. The process of generating electric power begins with the conversion of fuel energy into a form of heat or combustion energy. The heat energy is then converted into mechanical energy with the use of a prime mover. The mechanical energy is used to turn the shaft of a generator, which, in turn, converts the mechanical energy into electrical energy.

This Module describes the fundamental components and procedures that are used for the

generation of electric power. Included in this Module are descriptions of: the elements of a power generator, the elements of a power generation system, the major components of an AC generator, generator excitation, generator grounding methods, generator protection methods, and

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ELEMENTS OF A POWER GENERATOR Basic Generator Principles

Mechanical to Electrical Conversion

The conversion of mechanical energy into electrical energy occurs in the generator via the rotation of a magnetic field that intersects windings, inducing a voltage. The transformation is similar to the transformation that occurs in a transformer, except that the growth and collapse of the

magnetic field in a transformer is accomplished through the application of an alternating current to the primary winding, which induces a voltage in the secondary winding. In a generator, the

growth and collapse of the magnetic field is accomplished by the physical movement (revolution) of the fixed field (the primary) past the conductors (secondary winding). A stronger field results in a higher voltage; however, the energy transformed is limited to the input mechanical energy minus the losses that occur in the transformation process.

Sinusoidal Voltage Output

The sinusoidal field voltage shown in Figure 1 is induced in the conductors of a stationary armature by the flux of the two poles of the rotating field structure. The voltage induced in the coil loop will have a sine-wave pattern if the field poles move (turn) at a constant speed. The reason that the voltage is sinusoid is illustrated in Figure 2 by the vectors that represent the field flux that intersects the windings and that grows and collapses with rotation. The frequency of the generated power is directly related to the speed of the generator, which, in turn, is directly related to the prime mover. The formula to calculate frequency is:

f = (n x p)/120 where:

f = frequency in cycles-per-second (expressed in hertz) n = the speed of the generator in revolutions-per-minute p = number of poles in the generator

120 = a constant

For example, in order to obtain a sinusoidal voltage output with a frequency of 60 hertz from a two pole machine, the machine must run at 3600 rpm, and, in order to obtain a frequency of 50 hertz, the machine must run at 3000 rpm.

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Single-Phase Output is obtained by having one set of armature windings in the stator as illustrated in Figure 3. This figure shows a two pole, single-phase generator. Note that the two poles consist of one North pole and one South pole. Also, note that the (a) and (-a) conductors are part of a continuous armature winding conductor that fills the slots in the stator. The stator slots for this example are separated both mechanically and electrically by 180°. When the flux from the North pole intersects the (a) side of the conductor, the flux returning to the South pole intersects the (-a) side of the conductor, resulting, then, in the generation of a peak voltage between (a) and (-a). When the North and South poles are perpendicular to the plane of the (a) and (-a) conductors, no lines of force are intersecting the conductors, and the voltage difference between (a) and (-a) is zero. One complete cycle is one complete revolution (360o) of the rotor.

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Three-Phase Output - Although the single-phase machine shown in Figure 3 will work quite well, the efficiency of the mechanical to electrical energy transformation is improved by use of polyphases and, most commonly, by three phases, shown in Figure 4 as a, b, and c. Figure 4-A shows a two-pole, three-phase generator. The rotating field in this figure has only one North and one South pole; however, there are three sets of conductors, a and -a, b and -b, and c and -c. Each set of conductors is located 120 mechanical degrees apart. Arranged in this order, each group of conductors generates a single-phase voltage. Because the groups are spaced 120° apart, the single-phase voltage of each group is electrically spaced 120° from the other two. The

combined output of the three single-phase voltages results in a three-phase output. Figure 4-B illustrates the conductor arrangement for a four-pole, three-phase generator. The construction of this generator requires two North poles and two South poles on the rotor and another complete three-group set of conductors added to the stator. Figure 4-C shows the electrical connection for the three-phase, four-pole generator shown in Figure 4-B.

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Figure 5 shows the voltage output waveshapes for the three-phase generator shown in Figure 4.

Figure 5. One Cycle of Three-Phase Sinusoidal Waveshape Motor Versus Generator Comparison

In comparing a generator to a motor, several differences and similarities can be identified. To begin with, a generator is a machine that converts the mechanical power of a prime mover, in the form of horsepower, into electrical energy, in the form of kilowatts (kW). In comparison, a motor is a machine that converts electrical energy into mechanical energy and that delivers this energy in the form of horsepower to the shaft of a mechanical load.

A typical synchronous generator consists of a rotating magnet, called the field, that is mounted and turns inside of a stationary winding, called the armature. The generator shaft is turned by a mechanical prime mover. As the generator shaft turns, the magnetic field is rotated, causing its flux to intersect the armature winding thereby inducing an electromotive force (emf). The rotation of the field causes the induced emf to increase and decrease at a sinusoidal rate that produces a sine wave voltage at the terminals of the armature winding. Connecting the terminals of the armature winding to an electrical load causes an alternating current to flow.

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In comparison, a typical synchronous motor consists of the same components as the generator; that is a rotating magnetic field mounted inside of a stationary armature winding. However, the motor additionally has an induction winding mounted on the surface of the rotor that is used for starting. During motor start-up, no current is applied to the field winding, instead, an alternating current is supplied to the terminals of the armature winding, which, in turn, causes a magnetic field to be established in the winding. Because this field is supplied by an alternating current, it travels around the armature winding at the same frequency as the supply current. The rotating armature field induces a current in the winding on the surface of the rotor developing a torque that causes the rotor to turn and the motor to start as an induction motor. When the speed of the motor is close to synchronous speed, current is applied to the rotating field and the motor is brought up to synchronous speed.

The rotating field windings used for the generator and motor are similar in that both are low voltage windings; however, a major difference between the two types is the size of conductors required for their windings. The level of current used in the generator field is relatively high compared to the level of current used in the motor field. For this reason, the conductors in the field winding of the generator are much larger than those conductors that are used for the motor field winding.

Another difference between a generator and a motor is the voltage rating of the armature winding. Typically, generators are rated at higher voltages than are motors. For this reason, the armature winding of a generator, in accordance with its voltage rating, uses insulation with a higher dielectric strength than the insulation used for the typically lower rated voltage motor armature winding.

For many applications, synchronous machines are preferred in place of induction machines. When generator applicationsare considered, the reason why synchronous generators are normally

applied in preference to induction generators is because induction generators operate only at a fixed power factor value. As a result of this characteristic, the induction generator must always be operated in parallel with either synchronous machines or capacitors so that together they act as power factor correction devices. A synchronous generator, on the other hand, is able to correct the power factor and, with an adjustment of the field current, to deliver a constant frequency power.

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For the case of motors, the synchronous motor is preferred over the induction motor for applications where constant speed is important. The reason for this preference is because an induction motor will decrease in speed as mechanical load is applied. The decrease of speed results in a decrease of counter electromotive force, which, in turn, causes more current to be supplied by the source. However, a synchronous motor always runs at synchronous speed, even when the load is increased. The reason the synchronous motor is able to operate at a constant synchronous speed is because it operates differently from other types of motors. Figures 6 and 7 illustrates how a synchronous motor responds to a change in load with a change in the position of the stator poles in relation to the position of the rotor poles. Figure 6 shows a synchronous motor operating at no-load. For this case, there is little torque on the motor, and the torque angle between the stator and rotor poles is almost zero. As the load is increased to full-rated load (refer to Figure 7), the torque angle increases, and the phase angle between the impressed voltage and the counter emf also increases. This action causes more current to flow into the stator winding to meet the demands of the load.

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Figure 7. Torque Angle at Rated Load Single-Phase Generators

Components

A single-phase generator consists of a rotating magnet, called the field, inside a stationary

winding, called an armature, that is wound in a laminated iron core. The laminated iron core has a number of functions. It firmly supports the armature winding to prevent it from shifting or

vibrating in the magnetic field. It directs the lines of flux from the field, so that they will cross the armature at a perpendicular angle for maximum efficiency, and it serves to dissipate the heat generated as a byproduct of the alternating current flow. The armature winding and the iron core are called the stator. The rotating magnet is generally an electro-magnet that is wound on a cylindrically shaped shaft, called the rotor. The rotor rests in the bearings and is elongated on one end, to which a coupling is attached to connect the generator to a prime mover. The stator core is contained in a steel enclosure called the generator frame, and the bearings are mounted on heavy steel end plates, called bearing brackets. The armature winding exits the generator frame through insulated terminals, called bushings. These terminals are located in a compartment attached to the generator frame, called the lead box.

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Operation

As the magnetic field rotates across the armature winding, an electromotive force (emf) is induced in the armature winding. The rotation of the field causes this induced emf to increase and

decrease at a sinusoidal rate, thereby, producing a waveform, called a sine wave, at the terminals of the armature winding. If the terminals of the armature winding are connected to form a complete circuit through a load, like a light bulb or the primary winding of a transformer,

alternating current will flow between these terminals. Figure 8 is an equivalent circuit of a single-phase generator. The combination of the inductive reactance (XL) and resistance (R) is called the synchronous impedance.

Figure 8. Equivalent Circuit

Figure 9 is a vector diagram showing the relationship between voltage and current for a single-phase generator that is delivering current to a load with a unity power factor. With reference to this figure, it is seen that a generator can operate at a different power factor from its load.

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Three-Phase Generators Components

The components of a three-phase generator are basically the same as the components of a single-phase generator; however, a three-single-phase generator has three sets of armature windings in its stator winding. Figure 10, by showing a cross-sectional view of two three-phase stators with windings, one for a two-pole generator and the other for a four-pole generator, illustrate how the three sets of windings are arranged in generator stators. Each set of armature windings, referred to as a phase, is located 120 electrical degrees from each of the other two sets. Each phase has its own set of leads connected to terminals in the lead box. The windings of all commercial

generators are connected in a "wye" configuration, meaning that one end of each phase is

connected directly to the other two. This type of connection is called the neutral connection (T4, T5 and T6 as shown in Figure 11). The other three ends of each phase are called the line leads (T1, T2 and T3 as shown if Figure 11). In an actual stator core, the windings of each phase are distributed evenly around the core. In a two-pole machine, each phase winding is separated into two parallel groups, and, in a four-pole machine, each phase is separated into four parallel phase groups. These phase groups are connected to the main and neutral leads by parallel rings, located around the circumference of the stator winding, at the collector end of the generator.

Figure 10. Cross-Section View of Stator Windings for Two-Pole and Four-Pole Generators

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Operation

Three-phase generators, such as the one illustrated in Figure 11, are much more efficient than comparably sized single-phase generators. As the magnetic field rotates across the armature winding, an electromotive force (emf) is induced in the armature winding. The rotation of the field causes this induced emf to increase and to decrease at a sinusoidal rate and thereby produce a waveform, called a sine wave, at the terminals of the armature windings. As the rotor spins in a three-phase generator, three sets of AC voltages are generated in the stator windings as illustrated by the sine waves shown in Figure 11. These voltages are equal in amplitude, but they are shifted in phase from each other by 120 electrical degrees. When the terminals of the armature winding are connected to form a complete circuit through a load, such as the primary winding of a three-phase transformer, an alternating current will flow.

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ELEMENTS OF A POWER GENERATION SYSTEM

This section will describe the following four elements of a power generation system:

_ Prime Mover

_ Prime Mover Governors (Speed Control) _ Generator/Alternator

_ Voltage Regulator

Purpose and Types of Prime Movers Purpose

As mentioned above, conversion of fuels to usable work via electrical power requires multiple transformations. Power generation requires a prime mover that converts fuel energy into heat or combustion, which, in turn, is transformed into mechanical energy (torque). The prime mover is coupled to a generator, and the generator converts the mechanical energy into magnetic energy, and then, the magnetic energy into electrical power. The prime mover is the energy source for power generation.

Speed/torque curves are used to communicate information when an engine or prime mover is operated over a broad speed range. Prime movers for power generation operate within a narrow speed range. If the generator is not to be paralleled, the speed is regulated to a single speed and is called isochronous operation. The operating speed of a prime mover is selected for optimization in the application. Steam turbines can be operated between 1800 to over 10,000 rpm, and diesels and spark-fired gas engines are usually operated below 4000 rpm. Most diesel engines are directly coupled to their generators, and these diesel engines operate at 1800 rpm. Because the maximum speed for a generator that will produce 60 hertz (a two-pole machine) is 3600 rpm, a prime mover that is operated above 3600 rpm must have reduction gearing between it and the power generator.

Types of Prime Movers

The following three types of prime movers and their advantages are described below: _ Steam Turbines

_ Gas Turbines

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Steam Turbines- A steam turbine is essentially a series of calibrated nozzles in which the stored thermal energy (or heat energy) of steam is converted into mechanical energy (or work). Because steam is the energy source used to produce mechanical energy, a steam turbine is flexible with regard to the types of fuels used. Steam turbines also offer the advantages of low initial cost per kilowatt capacity, low maintenance cost, economy of foundation and building cubical content, high efficiency when operated far into the low-pressure range, and uniform angular velocity with freedom from vibration. In addition, steam turbine units can be built in sizes from fractional horsepower to over 1000 MW and for speeds to over 20,000 rpm. Designs can be tailored to fit the cycle and economics of each installation.

The conversion of the stored thermal (or heat) energy of steam into mechanical energy (or work) of a rotating shaft is accomplished by expansion of the steam through alternating rows of both stationary nozzle vanes and rotating blades. The geometry of the nozzle vanes and of the blades determines the pressure distribution throughout the turbine, and it also directs and turns the steam jets so that the forces on the blades develop a torque on the shaft. The principle parts of a steam turbine are:

_ Stationary nozzle vanes to change the thermal energy to kinetic energy and direct the course of steam onto rotating blades.

_ Rotating blades, which change the kinetic energy of the steam into shaft horsepower.

_ Rotating shaft, to which the blades are affixed.

_ A casing, which encloses the steam path and supports the fixed parts. _ Governor, bearings, lubrication, and other auxiliary devices and systems.

To better understand how nozzle vanes change thermal energy into kinetic energy, first consider the operation of the simple reciprocating engine shown in Figure 12a, wherein the incoming steam applies pressure equally on stationary cylinder walls as well as on the movable piston. As the piston moves due to the pressure of the steam on its surface, the steam does work, and it uses some of its internal energy in the process. However, note that in the case of the nozzle chambers, shown in Figures 12b and 12c, although the steam enters the nozzle chamber, applying pressure equally on all walls, it escapes through the nozzle opening to form a high-speed jet that has considerable kinetic energy.

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Figure 12. Generation and Delivery of Mechanical Energy vs. Generation and Delivery of Kinetic Energy

Also, note that, in Figures 12b and 12c, the reaction pressure, Pr , on the wall area opposite the nozzle is not balanced by the escaping steam. If the nozzle chamber is fixed in place (Figure 12b), steam exits through the nozzle at its highest possible absolute velocity and exerts pressure P1 on anything in its path. However, if the chamber is free to move (Figure 12c), Pr does work on it by speeding it in a direction opposite to the jet's motion. In this case, P2 is lower than P1.

Turbine nozzles direct the steam so that it flows in uniform high-speed jets that impinge upon the surfaces of the moving blades (see Figure 13). The moving blades absorb the kinetic energy of the jet, converting it to mechanical energy in a rotating shaft. If the blade is fixed in place (Figure 13a), the steam jet enters and leaves the boundaries of the blade surface with equal speed and develops maximum force F, but no mechanical work is done. As the blade is allowed to speed up (Figure 13b) and move with 1/4 the speed of the steam jet, the force on the blade diminishes, but work is being done. When the blade speed equals 1/2 that of the steam jet (Figure 13c), the force drops to half that of the locked condition. Steam now leaves the blade with zero speed and does maximum work. Figure 13d shows how both force and work vary with blade speed.

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Gas Turbines- The gas turbine engine, like the steam turbine engine, is a rotating engine that produces mechanical work from heat energy and uses gas as the working medium. However, the gas turbine engine is distinguished from the steam turbine by three major differences:

_ The gas turbine is an internal combustion engine, unlike the steam turbine in which fuel is burned in an external boiler. The gas turbine engine manufactures its own working medium (a supply of pressured, high temperature gas) by compressing air and by burning fuel in it.

_ The gas turbine uses a different working fluid. Like the steam turbine, the gas turbine title refers to its working fluid, which is some type of gaseous substance, usually atmospheric air and products of combustion. (A common misconception with gas turbines is that its name refers to the fuel that the engine uses, [for example, natural gas]. Because of this misconception, the name "combustion turbine" is sometimes preferred.)

_ The gas turbine operates at high temperatures and low pressures, while the steam turbine generally operates at high pressures and moderate temperatures.

The gas turbine engine consists of an air compressor, a combustion chamber, and a gas turbine (generally referred to simply as the turbine). The air compressor is driven by the turbine, and its high pressure discharge flows into the combustion chamber. Fuel is injected into the combustion chamber and is burned at a pressure equivalent to that of the compressor discharge. The resulting products of combustion (high temperature gases) form the working medium of the turbine. The expansion of these gases through the turbine enable it to produce more work than the total of both what is required to drive the compressor, and what is spent as a result of the overall inefficiencies of the engine. This surplus work then becomes available as a net plant output. Although there exists a number of gas turbine cycle variations, the most common of these is the "simple" (or "open") cycle. Figure 14 is a general schematic drawing of the simple (open) cycle gas turbine engine, and it shows the relative positions of the major components, along with direction of flow for (1) atmospheric air into and through the compressor, (2) high pressure air and fuel into the combustion chamber , (3) high pressure-high temperature gas from the

combustion chamber into and through the turbine, and (4) turbine exhaust gas returning to the atmosphere.

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Figure 14. Simple (Open) Cycle Gas Turbine Engine

The energy flow in a simple cycle gas turbine engine is shown in Figure 15. It starts at the compressor inlet, where incoming air is arbitrarily assigned an internal energy level of zero.

During compression, the work expended in turning the compressor is transferred to the air, raising its energy level. In the combustor, the thermal energy of the burning fuel is released, increasing the internal energy of the air to the maximum of the cycle. This highly energized air is introduced to the turbine, where a portion of its energy is converted into mechanical work for turning the compressor and the output shaft. The rest of the energy, approximately half, is dissipated to the atmosphere through the exhaust. Of the useful work done by the expanding air, about two-thirds is recirculated to drive the compressor to sustain the cycle, with the remainder available to do external work.

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Figure 15. Energy Flow Simple (Open) Cycle

The advantages of gas turbine engines lie in their versatility of application, a variety of fuel

sources (including natural gas and distillate oils), and the wide range of power outputs from under 50 horsepower, in smaller industrial applications, to over 150 MW, in larger industrial

applications.

Diesel Engines have been the work horse of portable and emergency power supplies for years, and in the past decade, the generating power of the diesel generator set has been extended to 7.5 MW. Because the diesel is a reciprocating engine, it is standard practice for diesel power generator sets to have a heavy flywheel attached to the shaft to dampen pulsation.

Like its reciprocating counterpart, the spark ignition gas engine, the diesel engine, in its larger power generating applications, is a four-cycle, internal combustion engine with a downward intake stroke, an upward compression stroke, a downward power stroke and an upward exhaust stroke. However, the diesel engine differs from its gasoline counterpart in several areas, one of which is the type of fuel used. The diesel fuel ordinarily employed is a low cost product from a good-grade crude petroleum. The fuel oil is the residue that is left when distillation has removed the more expensive and highly refined gasoline, kerosene, and other light distillates from the crude.

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The principal and important difference between the diesel and the spark ignition gas engine is in the method of ignition. Compression of the air trapped in the cylinder of a diesel engine is employed as its means of ignition. (There are no spark plugs in a diesel.) The compression is carried to much higher pressures in the diesel engine than in the spark ignition gas engine; diesel's have compression ratios of more than 20:1, while a typical gasoline engine has a compression ration of 9:1. As a result of this difference, the temperature at the end of compression is higher in the diesel cycle. In fact, compression is high enough so that the temperature of the compressed gas exceeds the ignition temperature of the fuel. This is compression ignition.

The advantages of the diesel engine are: _ Low fuel cost.

_ No long warming up period. _ No standby losses.

_ Reliability and durability.

_ Uniformly high efficiency of all sizes. _ Simple plant layout.

_ No large water supply needed.

Because of its operating advantage of not requiring a long warming up period, the diesel engine is an excellent prime mover choice for emergency, or back-up power generation. The need for emergency power is very important for generating stations that are a sole source of power with no available incoming power. In the event that a complete plant shutdown occurs for this type of station, the diesel type standby generation unit can provide the subsequent required "black start".

Prime Mover Governors/Speed Control Purpose of Governors/Speed Control

The purpose of a power generating system is to provide, as a product, usable quality power. The quality of power is measured by its consistency in voltage, in electrical frequency, and in wave form. The purpose of governors and speed control equipment is to maintain consistent voltage, frequency, and waveform of generated electric power.

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The voltage of an electric power generator is basically determined by the field strength of its rotating magnetic field, whereas the electrical frequency (hereafter referred to, simply, as frequency) of the generated power is determined by the rotational speed (hereafter referred to, simply, as speed) of the generator rotor, which in turn is determined by the speed of the prime mover. The speed at which a prime mover is designed to operate is based on the type of prime mover selected and the frequency of the electrical power to be generated. Because power is usually generated at a specified frequency (e.g., 50Hz or 60 Hz), the speed of the prime mover is regulated to the speed required for the specified frequency, this speed is called synchronous speed. Turbine type prime movers can be operated above 10,000 rpm, while diesel or gas engines usually operate below 4000 rpm. The speed of a prime mover is therefore determined by the optimum fuel-to-torque characteristic of the selected prime mover. A reduction gear is required for speeds above 3000 rpm for 50 Hz electrical power generation and for speeds above 3600 rpm for 60 Hz electrical power generation.

The relationship between speed and frequency is expressed by: f = (n x p) / 120 or n = (f x 120) / p

where: f = frequency (Hz) n = speed (rpm)

120 = constant p = number of generator poles

For example, to obtain 60 cycles from a two-pole generator, the generator rotor must operate at 3600 rpm (i.e., n = 3600 = (60 x 120) / 2).

The means by which regulation of a prime mover's speed is achieved is a governor. Basically, the governor functions by sensing a change in the prime mover's speed from some equilibrium speed, and causing, in turn, a change in the quantity of fuel being delivered to the prime mover. Thus, the resulting differential in fuel supply restores the prime mover speed to its equilibrium point. In effect, the governor is a feedback control system in which the difference between a reference

input (e.g., the equilibrium speed) and some function of the controlled variable (e.g., the prime

mover's speed) is used to supply an actuating signal to the control elements (e.g., a command to a fuel valve to alter the fuel flow to the prime mover). The actuating signal endeavors to reduce to zero the difference between the reference input and the controlled variable.

A key aspect to the feedback phase of a governor's performance is an accurate measurement of the prime mover's speed. On prime movers with mechanical type or mechanical-hydraulic type governors, the governor system senses speed changes by sensing changes in the position of a spring-loaded centrifugal device within the governor assembly. In electric governing systems, a common speed measuring device is a magnetic speed pickup whose tip is placed in close

proximity to that section of the prime mover's rotor surface on which evenly spaced gear-type teeth have been machined around the periphery of the surface. As the teeth rotate past the pickup, a periodic disturbance is imposed on the magnetic field that surrounds the tip of the pickup. The pickup is wired to a measuring circuit, which counts the number of perturbations to the magnetic field in a given unit of time.

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Methods of Speed Control

The method (and the degree of sophistication) of speed control used on a power generation unit is determined by the application of the power produced. The method of speed regulation used on a unit that is paralleled with other power generation units is different from the method of speed regulation used on a unit that acts as a single power source and that is not to be paralleled with other units. Listed below are four applications of produced power, each requiring speed control. While all four applications involve control of the amount of fuel that is delivered to the prime mover, the first three are semiautomatic in that some operator actions are required in establishing the unit's reference, while the fourth is fully automatic control.

• The power generating unit is a sole load source.

• The power generating unit is to be paralleled with another source.

• The power generating unit is to be paralleled with another source, and load sharing is to be regulated.

• The power generating unit is to be paralleled with another source with fully automated regulation of load sharing .

On power generating units operating as a sole load source, speed controllers referred to as

“isochronous governors” are used. While, on generating units that are paralleled with other units, speed controllers referred to as “droop governors” are used. An isochronous governor is one that keeps the speed of the prime mover constant at all loads. A droop governor is one whose

equilibrium speed decreases as the load on the prime mover controlled by the governor increases. Conversely, with the droop governor, the equilibrium speed increases as the load on the prime mover controlled by the governor decreases.

As mentioned, when the power generating unit is the sole source for a load, an isochronous governor is used to obtain speed regulation. Figure 16 shows the control components of a Woodward Electric Speed Control System as applied to a diesel driven power generating unit. This arrangement of components provides for control of a single unit generator. Note that in this case a simple speed feedback system is used that employs the type of magnetic speed pickup that was described earlier.

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Figure 16. Electric Controls for Single Unit Generators

When the power generating unit being controlled is to be paralleled with other generators, the electrical frequency and voltage of the unit's generator and that of the source to be paralleled must be reasonably matched. In this case, feedbacks consist of the rotational speed of the prime mover and the voltage of the generator. As stated previously, the control term used to describe the action required (i.e., variation in speed and voltage) on the unit to be paralleled to effect the required match is "droop." As mentioned, this parallel application requires a semiautomatic form of droop governor. Figure 17 illustrates the control components required for a Woodward Control System to perform a simple paralleling operation.

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When the generator being controlled is to be paralleled and the portion of the load it supplies is to be regulated (load sharing), current feedback must be applied in addition to the rotational speed feedback from the prime mover and the generator voltage feedback. Figure 18 illustrates the control components of a Woodward Control System for this load sharing application. As in the previous application (refer to Figure 17), the application illustrated by Figure 18 employs a semiautomatic form of droop governor.

Figure 18. Electric Load Sharing System

As mentioned, the two parallel application systems previously described (Figures 17 and 18) employ droop-type governors, and they are semiautomatic in that some operator actions are required in establishing the unit's reference. The Woodward control system illustrated in Figure 19 is like the two previous units described in that it employs a droop-type governor. However, this system differs from the previous two examples in that it provides the power generating unit with the capability for fully automatic paralleling and load sharing operation.

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Figure 19. Fully Automated Electric Load Sharing System Purpose and Types of Generators/Alternators

Purpose

The general purpose of a generator is to transform mechanical energy into electrical energy by rotating a magnetic field inside of the generator armature winding. As shown in Figure 20, the magnetic flux of the rotating field passes from the North pole of the rotor, through the air gap and laminated steel shell of the stator, and back into the rotor’s South pole. As this flux intersects the armature conductors (a and -a), voltage is generated in the armature winding, thus completing the transformation of mechanical energy to electrical energy. For the two-pole generator shown in Figure 20, one cycle of voltage is generated for each 360° mechanical revolution of the rotor.

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Figure 20. Generator Winding

The amount of energy that is transformed is determined by the load requirement up to the point of the limitations of the generating system. The voltage is determined by the strength of the magnetic field, and the frequency is determined by the number of times that the armature windings are intersected during rotation by the magnetic field from the rotor in any one second of time.

Types of Generators/Alternators

Induction Generators are induction motors that are operated (run) by means of mechanical power being applied to the shaft. An induction motor works by applying a rotating magnetic field to a stator winding that is magnetically coupled via an air gap to windings on the rotor. The windings on the rotor are closed (short circuited), and current flows in the rotor windings, creating a magnetic field in the rotor that tries to align with the stator field. Because the stator field is revolving, the rotor revolves.

For induction motors to provide torque, the rotor must revolve slower than the synchronous speed of the machine, which is, by definition, the speed of the revolving field on the stator. The difference between the synchronous speed and the actual rotor speed is called slip. As Figure 21 illustrates, rotor torque increases with slip.

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Figure 21. Torque - Slip Curves of Squirrel Cage Induction Motor

When an induction motor is operated as a motor, it is run at full-load speed (minimum slip); however, when an induction motor operates as a generator, it runs at synchronous speed (zero slip). Overspeeding of an induction motor (via mechanical input to the shaft) in which the stator is excited through application of a fixed frequency rotating field will cause the motor to have zero slip. Such slip causes the machine to become a generator. However, this type of generation, because regulation of it is extremely poor, is seldom used except in special applications such as dynamic braking (for example, elevator systems).

Synchronous Generatorsare the standard type of generators used to produce electric power. This type of generator is made to be driven at a definite, constant speed, normally referred to as the synchronous speed of the generator. The frequency of the generated voltage is determined by this speed. Varying the excitation voltage of a synchronous generator will raise or lower the system voltage if the machine is operated as the only voltage source on a power system, or it will vary the reactive power (VARs), either leading or lagging, on a power system where the generator is operating in parallel with other synchronous generators.

In a synchronous generator, the generation of alternating current occurs as a result of a magnetic field intersecting a winding. The winding that produces the alternating current is called the armature winding. In theory, this winding may be placed on either the rotor or the stator. When the armature winding is placed on the rotor, as illustrated in the simplified circuit of Figure 22, collector rings and carbon brushes are required to conduct the generator power to the load. The need for rings and brushes introduces practical limits to the amount of power that can be

conducted using this method. Because of these limitations, most synchronous generators are built with stationary armature windings as illustrated in the simplified circuit of Figure 23.

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When comparing the differences between a synchronous generator and an induction generator, it is seen that the major difference between the two types is that the induction generator cannot be used alone to supply a power system. The induction generator must always be operated in parallel with synchronous machines or with capacitors to provide a means of correcting power factor. The output power factor of an induction generator is a fixed value determined by the generator characteristics, and it is always a leading value that is independent of the external circuit. The reason for this characteristic is because an induction generator draws all of its excitation from the power system, and, therefore, it must receive a definite amount of lagging VARs for a given voltage and a given load current.

With regard to the application of a generator as a standby power system, the inability of an induction generator to be self-excited, to supply voltage at a constant frequency, or to supply power at unity power factor makes the synchronous generator the preferred machine.

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Figure 23. Stationary Armature Purpose and Types of Voltage Regulators

Purpose

An alternator will experience large changes in its terminal voltage. Also, changes in load current and in the load power factor will occur because of the combined effects of the armature reactance and the armature reaction. However, a relatively constant terminal voltage can be maintained under changing load conditions by the use of an automatic voltage regulator.

Types of Voltage Regulators

The following types of voltage regulators are described below:

• Electro-mechanical regulator

• Electronic regulator

Electro-Mechanical Regulatorsprovide a DC supply for the generator field that comes from outside of the generator. Figure 24 shows an example of a DC supply provided by a common bus. In this example, three separate DC exciters supply voltage to the common bus, and four AC generators use the bus to supply DC voltage to their field windings. The amount of current that flows to each of the field windings is regulated by individual field rheostats Adjusting the rheostats to increase current flow will strengthen the individual fields and increase their generator voltages, but it will not necessarily increase power output from the generators.

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Figure 24. Electro-Mechanical Voltage Regulator

Electronic Regulatorsprovide a variable DC supply to the generator field from an AC source. The AC source is converted to variable DC by use of ignitron tubes in older equipment, and by use of

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MAJOR AC GENERATOR COMPONENTS

An AC generator has the following major components:

• Stator

• Rotor

• Cooling System

• Bearings and Lubrication System

Stator

This section will discuss the following pertinent to the stator component of an AC generator:

• Mechanical components

• Wye configuration

• Delta configuration

Mechanical Components

An AC generator has the following major mechanical components:

• The frame

• The core

• The winding (coils)

• The lead box (terminal box)

Figure 26 shows a cross-sectional view of a typical generator that illustrates the location of these components. The sections that follow discuss each of the components.

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Figure 26. Generator Mechanical Components

Frame (Housing)- The stator frame for a generator of medium size is typically fabricated from steel plates and bars electrically welded into a rigid box section. A short length of duct work is

normally provided on the bottom of the frame through which ventilating air is discharged. Holes drilled and tapped around the edges of the duct provide means for attaching customer’s duct work. Port holes with removable glass serve both as lifting holes for handling during erection and windows for the inspection of the end windings during operation. The generator feet rest directly on the sole plate.

Larger size generators are fabricated with a rolled steel shell supported by frame feet and reinforcing webs along its length. The frame is aligned using shims under the feet and then secured to the foundation with tie-down bolts.

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Core - The stator core for a generator is built with low-loss segmental silicon steel laminations that are assembled on bars (or building bolts) that span the length of the core. Figure 27 illustrates the assembly of the laminated segments on their building bolts and insulated “through” bolts for a large generator. Both sides of the laminations are treated with an insulating material to prevent short-circuiting the laminations. Vent spacers are built in with the laminations at intervals of approximately two inches to provide radial passages through the core for the ventilating air. Adequate pressure is applied at intervals during the stacking operation to produce a tight core. Heavy end plates and non-magnetic finger plates are used at the ends of the core to maintain adequate pressure at all times. The larger generators (4000 kW and above) have insulated through-bolts, which extend axially through the punchings in back of the slots. These provide additional clamping force on the end plates. On small generators, after being assembled, the entire core is coated with varnish and baked to protect it from rust and to further insulate between punchings.

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Winding (Coils) - Interchangeable closed-type coils are used on AC generators. When electrical conditions require it, groups of strands of the coils are transposed at the ends to equalize the induced voltage and heating. Figure 28 shows a typical “form wound” coil, which is the type most often used for stator windings. This type of coil is insulated and oven cured prior to its placement in the stator core. The stator windings of generators that have ratings of 5000 kVA or less and 6900 volts or less are typically provided with Class B insulation. Generators with ratings over 5000 kVA or voltages above 6900 volts normally have Class F insulation on their stator windings. When the generator voltage is 6600 volts or more, the slot sections of the coil are treated with semi-conducting compounds to eliminate corona and its harmful affects in that portion of the coil. Note: Saudi Aramco 17-SAMSS-510 requires all generators rated 125 kVA

through 1250 kVA to be provided with Class F insulation.

Figure 28. Typical Generator Stator Coil

The end turns of stator windings are normally braced against movement by placing spacer-strain blocks between the coil ends and fastened the coil-block assembly to an insulated frame brace with glass cord. For generators with higher ratings, the coil ends are further supported by

insulated steel support rings fixed on steel frame brackets. Figure 29 shows a side-elevation view of a typical stator winding end-turn support assembly.

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Figure 29. Stator Winding End-Turn Support

Temperature detectors, calibrated for 10 ohms resistance at 25oC, are typically embedded between coils in various slots in the stator winding of the machine to measure the operating winding temperature. The temperature detector leads are connected to a terminal board bolted to the frame of the generator as illustrated in Figure 30. Customer detector leads may then be connected to the external points on the terminal board. With reference to Figure 30, temperature detectors are positioned axially in the stator winding at the following locations:

• Coil 1, 4, and 7 at the collector end of the stator core.

• Coil 2, 5, and 8 at the center of the stator core.

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Lead Box (Terminal Box)- This component of the generator frame is found either on the bottom or the top of the collector end of the frame. Figure 26 shows a cross-section view of a generator with the lead box mounted to the bottom of the frame. Contained in the lead box are the six main lead bushings. The bushings mounted in the lead box serve two purposes. The first purpose that the bushings serve is to provide a gas tight penetration in the generator frame for the three line leads and the three neutral leads that make up both ends of the three phases of the stator winding. The second purpose of the bushings is to insulate the high voltage leads from the generator frame, which is at ground potential. Most lead boxes contain a manway cover to provide access to the lead box for purposes of inspection and clean out.

Wye Configuration

Generator stator windings are typically connected in a wye configuration as illustrated in Figure 31. T1, T2 and T3 are the line leads and T4, T5 and T6 are the neutral leads, which are tied together. This is the only configuration allowed by Saudi Aramco standards.

Figure 31. Wye Configuration Delta Configuration

Although not allowed by Saudi Aramco, there are some applications where a delta-connected stator winding is appropriate. With reference to Figure 31, the delta connection is made by connecting terminals 1 to 6, 2 to 4, and 3 to 5. Line leads are then connected to terminal 1, 2 and 3 accordingly.

Reconnecting the generator windings in a delta connection reduces the line-to-line voltage, but increases the available line current.

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Types of Rotors

To place the magnetic field on the rotor, poles are used that consist of stacked magnetic iron laminations (to reduce eddy currents) with copper conductors wrapped around the iron and excited by a DC current. The rotor poles must be arranged in pairs with a minimum arrangement of one pair of poles. The pairs are located 180 electrical degrees apart. As the North pole of the magnetic field of the rotor intersects one phase group of the stator winding, the South pole of the rotor is intersecting the diametrically opposite portion of the same phase winding.

There are two common rotor constructions for generators: salient pole (projecting poles) with concentrated windings, and cylindrical pole with distributed windings. Figures 32 and 33 show schematic illustrations of these two types of rotor construction.

The choice of construction of the rotor is primarily determined by the speed of operation. The two different types of construction have different magnetic coupling characteristics. The salient-pole rotor is used for very large low-speed machines, and the cylindrical rotor is used for high-speed machines that operate at 1800 or 3600 RPM.

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Figure 33. Cylindrical Pole Rotor Salient Pole

Salient pole rotors can be constructed with either laminated poles or solid poles.

Laminated Pole construction is more efficient since the magnetic lines of flux will travel through the laminated core in a perpendicular direction to the field winding. Causing the flux to travel in this manner reduces iron losses and provides more efficient magnetic coupling to the laminated core of the stator winding.

Solid Poleconstruction is utilized where the rating of the generator requires less concentrated flux density.

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Cylindrical Pole

Cylindrical pole rotors are utilized in high speed steam turbine generators where frictional losses from windage make the salient or projecting pole design unfeasible. Figure 34 illustrates a typical cylindrical pole rotor with end-turn retaining rings and collector rings shown in place. These rotors generally carry higher field current ratings and are more rigid, allowing for the longer core lengths common in horizontal turbine-generators. The surface of these rotor bodies is grooved to reduce surface currents and to increase heat transfer to the cooling medium, air or hydrogen. Radial slots for the field windings are machined in the rotor body.

Figure 34. Cylindrical Pole Rotor

The field coils of a cylindrical pole rotor are imbedded in the machined slots of the rotor. The DC supply that feeds the field coils to produce the desired magnetic field strength can be brought into the rotor through collector rings from an outside supply, or can be supplied by a DC generator that is attached directly to the field winding leads on the rotor shaft. The latter is more desirable because it eliminates the collector rings and the carbon brushes required to interface with the slip rings, thus eliminating a maintenance requirement.

The insulation used in a cylindrical pole rotor is normally Class “B” with molded glass insulation placed between the winding and ground, and flat mica insulation placed between turns. The top turns are mica taped for extra insulation. The rotor coils are baked while under high radial pressure until the winding becomes a solid mass. Finally, the coils are held in their slots by sturdy non-magnetic wedges.

The rotor end turns are supported radially by forged steel retaining rings that are lined with Micarta. The retaining rings are shrunk onto the ends of the rotor body as shown in the cut away view of rotor end turns in Figure 35.

Cooling air, or hydrogen, passes under the ends of the retaining rings, over the rotor winding end turns, and discharges through the radial holes located in the peripheral surfaces of the retaining rings. Axial support is provided to the coil end turns by means of fitted Micarta or glass epoxy blocks that also serve to direct and control the flow of ventilating air over the end turns.

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Figure 35. Rotor End-Turn Conductors

When collector rings are included on a cylindrical pole rotor, as shown in Figure 34, the rings are made of tool steel. These rings are then placed on a steel bushing, insulated with mica, and mounted outside of the collector-end bearing. By mounting the collector on the outboard side of the bearing, easy accessibility is maintained to the brush rigging. The leads from the collector rings to the field winding are made of semi-circular copper bars separated by Micarta plates and are located in a hole in the center of the shaft. The collector rings are ventilated by a stream of cool air, which is bled from the exciter air supply. Holes drilled in the collector ring flanges circulate air and provide additional cooling of the ring surfaces. Spiral grooves on the surface of the rings prevent hot spots from occurring and thus insure reliable current collection. The brushholders are fitted with adjustable coil springs to maintain desired brush pressure on the collector rings. Graduations on the brushholders indicate the force in pounds exerted on the back of each brush. The brushes are provided with pigtail connections to prevent corrosion of the holder. Corrosion can occur in the brushholder as a result of arcing between the holder and the brush. If allowed to occur, this corrosion will impede the motion of the brush in the holder and thus reduce the pressure of the brush against the ring surface.

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Types of Cooling Systems

The following types of cooling systems are used for AC generators:

• Air-cooled

• Air-to-water heat exchanger

• Gas-to-water heat exchanger

Air-Cooled

Air-cooled cooling systems include the following configurations:

• Natural cooling

• Air-to-air heat exchanger

Natural Cooling- A natural cooled generator, commonly referred to as an “open” cooling system, uses outside air at ambient temperature as a cooling medium. The air is circulated through the stator and rotor by propeller-type blowers mounted on both ends of the rotor. The cooling air that is taken into the generator is allowed to make only one pass through the rotor and stator and is then exhausted from the generator. This system is economical because it requires no other cooling system components. However, this type of system often requires air filters at the intake ports to minimize the contaminants passing into the machine. When filters are used, they can become dirty, and if not changed frequently enough, will restrict air flow, which can lead to overheating.

Air-To-Air Heat Exchanger - A generator with an air-to-air heat exchanger differs from the natural cooled configuration in that the machine with the heat exchanger is constantly recirculating the same air through the stator and rotor. This type of circulation keeps the generator windings cleaner than a system that does not recirculate the same air. The warm air exiting the stator is circulated through a plenum type heat exchanger that removes the heat from the generator cooling air by passing it over fins, or chevron-type baffles, that transfer the heat to outside air. This circulation eliminates the need for a filter system, but requires the need for added secondary air cooling equipment.

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Air-To-Water Heat Exchanger

This type of system uses a contained reservoir of air to cool the generator in the same manner as described above for the air-to-air heat exchanger. However, the difference with this system is the method of heat exchange used. With the air-to-water heat exchanger, the warm air coming out of the stator and rotor is circulated across a cooler that consists of a number of copper tubes with circular fins soldered around the outside diameter of the tubes. Water is circulated through the tubes and removes the heat from the air being passed over the outside of the tubes. This type of system helps to prevent contaminants from getting into the generator by constantly recirculating the same air through the machine. However, this method of cooling requires that a source of cooling water to be pumped through the coolers.

Gas-To-Water Heat Exchanger

The gas-to-water type of heat exchanger is utilized in most of the large (>200 MVA) turbine generators in service today. The type of gas used for the cooling is normally hydrogen. Hydrogen has the following distinct advantages over air:

• Hydrogen has a lower density than air (14 times lighter) and thus reduces windage and ventilation losses.

• Hydrogen has a better thermal conductivity than air (seven times greater) and a better heat transfer coefficient than air (40% higher) and thus increases output per unit volume.

• Insulation life is increased when hydrogen is used because oxidation ceases to be a problem.

• The closed gas system used by hydrogen systems reduces dirt and moisture contamination in the machine and achieves quieter operation.

When hydrogen systems were first used to cool generators, the pressure of the hydrogen inside of the machine was maintained to a pressure only slightly greater than atmospheric pressure

(approximately 0.5 psig). Using only 0.5 psig of hydrogen pressure, the output of hydrogen cooled generators were increased by as much as 20% over similar air cooled units. Gradually, the hydrogen pressure used inside of generators was increased to achieve more effective heat transfer. By 1948, hydrogen pressure had been increased to as much as 30 psig for conventionally cooled machines, achieving operating capabilities 25% greater than the earlier 0.5 psig units.

Types of Bearings and Lubrication Systems Types of Bearings

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• Anti-friction

• Sleeve

Anti-Frictionbearings come in two styles: ball bearing and roller bearing. Anti-friction bearings, used mainly for small generators, consist of a number of steel balls or roller shaped cylinders tightly sandwiched between an inner race and an outer race. Therace on the inside diameter of the bearing is pressed onto the rotor shaft, and the race on the outside diameter of the bearing is pressed into the stator end plate or bearing bracket. This type of bearing, when properly

lubricated with oil for high speed operation, or lubricated with grease for low speed operation, provides an extremely low resistance to the rotation of the rotor.

A Sleeve bearing consist of a soft porous metal lining, called babbitt, fabricated to the inside diameter of a steel housing or shell. Figure 36 shows two cross-sectional views of a typical style of sleeve bearing used on medium and large size generators. The bearing is lubricated by oil rings that rotate with the shaft. The oil rings dip into a bath of oil in the bottom of the housing as they rotate, and, thus, carry oil up and over the journal. Seals at both ends of the bearing help to prevent oil from escaping and keep contaminates from entering. Openings in the top of the housing provide for inspection of the oil rings. Normally a sight glass is provided on the reservoir to determine the oil level. For some generators, the oil is circulated into and out of the bearing pedestal to provide for cooling and filtering of the oil.

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