Engineering Encyclopedia

Transcription

1 Saudi Aramco DeskTop Standards GENERATOR FUNDAMENTALS 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. Chapter : Electrical For additional information on this subject, contact File Reference: EEX PEDD Coordinator on

2 CONTENT PAGE INTRODUCTION... 8 ELEMENTS OF A POWER GENERATION SYSTEM Purpose and Types of Prime Movers Purpose Types of Prime Movers Prime Mover Governors/Speed Control Purpose of Governors/Speed Control Methods of Speed Control Purpose and Types of /Alternators Purpose Types of /Alternators Purpose and Types of Voltage Regulators Purpose Types of Voltage Regulators Purposes of Switchgear Isolation Protection Addition/Removal of Generating Capacity ELEMENTS OF A POWER GENERATOR Basic Generator Principles Mechanical to Electrical Conversion Sinusoidal Voltage Output Motor Versus Generator Comparison Single-Phase Saudi Aramco DeskTop Standards i

3 Components Operation Three-Phase Components Operation MAJOR AC GENERATOR COMPONENTS Stator Mechanical Components Wye Configuration Delta Configuration Types of Rotors Salient Pole Cylindrical Pole Types of Cooling Systems Air-Cooled Air-To-Water Heat Exchanger Gas-To-Water Heat Exchanger Types of Bearings and Lubrication Systems Types of Bearings Types of Lubrication Systems GENERATOR EXCITATION Purposes of Generator Excitation Power to the Rotating Electromagnetic Field Locking Rotor To Stator Means of Regulating Voltage Types of Generator Excitation Saudi Aramco DeskTop Standards ii

4 DC Exciters Static Excitation Brushless Excitation Concept of Response Time Versus Voltage Levels GENERATOR GROUNDING Introduction Purposes of Generator Grounding Personnel Safety Equipment Protection Methods of Generator Grounding Solidly-Grounded Resistance-Grounded Reactance-Grounded Source System Ground Versus Plant Generator Ground SAES-P-114 Grounding Requirements ELEMENTS OF GENERATOR PROTECTION Temperature Protection Rotor Stator Bearing Lubrication Electrical Protection Overcurrent Differential Current (Fault) Ground Fault Loss of Field Saudi Aramco DeskTop Standards iii

5 Phase Unbalance Frequency/Overspeed Voltage Sync Verification Reverse Power (Motoring) Solid-State Protective Devices Zone Protection Concepts Overlap Backup GLOSSARY LIST OF FIGURES Figure 1. Elements of a Power Generation System... 9 Figure 2. Power Generation System Efficiencies Figure 3. Power Generation System Components Figure 4. Fuel Consumption Curves Figure 5. Generation and Delivery of Mechanical Energy vs. Generation and Delivery of Kinetic Energy Figure 6. Generation and Delivery of Mechanical Energy Figure 7. Simple (Open) Cycle Gas Turbine Engine Figure 8. Energy Flow Diagrams for Gas Turbine Engine Cycles Figure 9. Combined Cycle Schematic Figure 10. Diesel Engine Generator Figure 11. Fuel/Load Efficiency Figure 12. Example of Speed/ Load Regulation Figure 13. Example of Electrical Speed Control Method Saudi Aramco DeskTop Standards iv

6 Figure 14. Example of Mechanical-Hydraulic Speed Control Method Figure 15. Example of Electrohydraulic Speed Control Method Figure 16. Generator Winding Figure 17. Torque - Slip Curves of Squirrel Cage Induction Motor Figure 18. Rotating Armature Figure 19. Stationary Armature Figure 20. Typical Generator Nameplate Data Figure 21. Electro-Mechanical Voltage Regulator Figure 22. Rotating Regulator Figure 23. Electronic Regulator Figure 24. Direct-Connected Generator Figure 25. Unit-Transformer Connected Generator Figure 26. Sinusoidal Voltage Output Figure 27. Rotating Flux Fields Figure 28. Single-Phase Generator Figure 29. Elementary Three-Phase Generator Figure 30. One Cycle of Three-Phase Sinusoidal Waveshape Figure 31. Three-Phase Armature (Stator) Winding in Wye Connection Figure 32. Torque Angle at No-Load Figure 33. Torque Angle at Rated Load Figure 34. Equivalent Circuit Figure 35. Vector Diagram Figure 36. Cross-Section View of Stator Windings or Two-Pole and Four-Pole Figure 37. Three-Phase Generator and Sine Wave Relationship Figure 38. Stator Frame (Housing) Figure 39. Stator Core Laminations - Large Generator Saudi Aramco DeskTop Standards v

7 Figure 40. Flexibly Supported Stator Core Figure 41. Conventionally Cooled Stator Winding Figure 42. Temperature Detector Lead Location Figure 43. Stator Winding End Turn Support Figure 44. Lead Box Figure 45. Wye Configuration Figure 46. Rotor Figure 47. Salient Pole Rotor Figure 48. Cylindrical Pole Rotor Figure 49. Laminated Pole Construction Figure 50. Cylindrical Pole Rotor Figure 51. Rotor Coil Ends Figure 52. Collector Rings Figure 53. Brushholders Figure 54. Natural Cooling Figure 55. Cross-Section of Typical Sleeve Bearing Assembly Figure 56. Induced EMF In A Rotating Coil Figure 57. Rectification By Two Segment Commutator Figure 58. Current Flow Through Commutator Figure 59. Current Flow Through Commutator (Cont d) Figure 60. Static Excitation Figure 61. Brushless Excitation Figure 62. Brushless Excitation System Schematic Figure 63. Brushless Exciter General Assembly Figure 64. Brushless Exciter With Permanent Magnet Pilot Exciter Figure 65. Typical Response Time Versus Voltage for Sudden Application of Load Saudi Aramco DeskTop Standards vi

8 Figure 66. Ungrounded System Figure 67. Grounded System Figure 68. Typical Types of Grounded Systems Figure 69. Solidly-Grounded Generator Figure 70. Low Resistance-Grounded Generator Figure 71. High Resistance Grounded Generator Figure 72. Reactance-Grounded Generator Figure 73. System Ground Versus Generator Ground Figure 74. Grounding of Large Direct-Connected Synchronous Figure 75. Grounding of Large Unit-Transformer Connected Figure 76. Grounding of Medium Direct-Connected Figure 77. Grounding of Low Voltage - Separately Derived System Figure 78. Grounding of Low Voltage -Not Separately Derived System Figure 79. Field Ground Detection Relay Figure 80. Stator RTD Protection Figure 81. Large Direct-Connected Generator Protection Scheme Figure 82. Protection Device Legend Figure 83. GE Type Voltage Restraint Relay Figure 84. ABB Type COV Voltage Controlled Relay Figure 85. ABB Type CA Generator Differential Relay Figure 86. ABB Type CWC Ground Fault Relay Figure 87. Generator Current Locus Figure 88. Generator k-values Figure 89. ABB Type COQ Unbalance Current Relay Figure 90. Zones of Protection Saudi Aramco DeskTop Standards vii

9 INTRODUCTION The 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. The mechanical energy is changed into magnetic energy, and finally from magnetic into electrical power. A power generation system includes not only the generation process and components, but also the equipment and the facilities needed to effectively, efficiently, and safely transmit the generated power to a location where it is needed to perform some useful work and from which it must be appropriately and safely distributed to the various work elements. Although the power generation system can be defined as ending at the points where the generated power has been transmitted to the specific work elements, the transformation process does not end because the electrical power must be converted back into a usable form of energy such as mechanical or thermal energy. These final transformations affect the efficiency and performance of the power generation system. The block diagram in Figure 1 illustrates the multiple transformations that take place in a power generation system, names the major component blocks, and identifies the most significant variables that must be regulated and controlled in the process. Saudi Aramco DeskTop Standards 8

10 Title: EPS from CorelDRAW! Creator: CorelDRAW! CreationDate: Fri Mar 31 14:04: Figure 1. Elements of a Power Generation System Saudi Aramco DeskTop Standards 9

11 The efficiency for different transformation processes and components varies substantially. A large, fully loaded, low-loss power transformer may have an efficiency as high as 99.4 percent, while the efficiency for a prime mover can be as low as 30 percent. A typical gasoline engine operates at 26 to 28 percent efficiency compared to a diesel engine that operates at 36 to 38 percent efficiency. Efficiency (power output divided by power input) is measured in BTUs of heat input, versus BTUs of horsepower output. Some useful measures of energy are: One horsepower is the amount of energy required to lift 33,000 pounds one foot in one minute. One horsepower = watts. One watt is the amount of energy required to lift pounds one foot in one minute. One BTU is the amount of heat required to raise one pound of water 1 degree Fahrenheit. One BTU = horsepower-hour The fuel required to deliver useful work must be sufficient to accommodate the multiples of all the efficiency factors, the losses that occur in transmission such as I 2 R and friction, and the actual work itself. The prime mover must also be of sufficient size to provide enough torque (mechanical energy) to accommodate the demands of the system it feeds. Although an individual customer or user of electrical energy may consider his use as efficient, it is not hard to visualize the actual use of the energy from the fuel stage to the work performed stage as being less than 30 percent efficient. Figure 2 illustrates this point. Saudi Aramco DeskTop Standards 10

12 Title: EPS from CorelDRAW! Creator: CorelDRAW! CreationDate: Fri Jul 29 14:43: Figure 2. Power Generation System Efficiencies Saudi Aramco DeskTop Standards 11

13 The actual energy that comes out of a prime mover and generator is determined by the load demand and the various efficiencies and loss factors associated with the system. The overwhelming majority of losses in a power generation system are manifested by heat from sources such as I 2 R, mechanical friction, and eddy currents, whereas typical inefficiencies (unused potential energy) are unburned fuel, power factor and stray magnetic fields. The available energy out of a prime mover and generator to perform useful work can therefore be increased by: Reducing losses in the system. Improving efficiencies in the system. Increasing energy into the prime mover. Combinations of the above. In this course, the power generation system components covered will include the prime mover, the generator and the equipment that provides generator regulation and protection. See Figure 3. Figure 3. Power Generation System Components Saudi Aramco DeskTop Standards 12

14 ELEMENTS OF A POWER GENERATION SYSTEM This section will describe the following five elements of a power generation system: Prime Mover Prime Mover Governors (Speed Control) Generator/Alternator Voltage Regulator Switchgear 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 finally from 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 in a very tight 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. Since the maximum speed for a generator to produce 60 hertz (a two pole machine) is 3600 RPM (to be discussed in the generator section), a prime mover that is operated above 3600 RPM must have reduction gearing between it and the power generator. Saudi Aramco DeskTop Standards 13

15 Most diesels are directly coupled and operate at 1800 rpm. Instead of speed torque curves, the manufacturer provides fuel consumption curves. The fuel consumption curves (Figure 4) communicate fuel consumed per hour as a function of gross engine power output in brake horsepower (BHP). Note that curves are given for both short term and continuous duty, with axis for 1800 and 1500 rpm. Saudi Aramco DeskTop Standards 14

16 Title: EPS from CorelDRAW! Creator: CorelDRAW! CreationDate: Fri Jul 29 14:05: Figure 4. Fuel Consumption Curves Saudi Aramco DeskTop Standards 15

17 Types of Prime Movers There are four basic types of prime movers: Steam Turbines Gas Turbines Diesel Engines Spark Ignition Gas Engines Steam Turbine - 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). Since steam is the energy source used to produce mechanical energy, a steam turbine is flexible with regard to the types of fuels used. 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 majority of today's electrical generating power stations, which operate on fossil fuel (coal, oil or natural gas), are designed to produce main steam at 1800, 2400 or 3500 psig and at 950, 1000 or 1050 F. Depending on the electrical frequency of the power station's output, the turbine's rotational speed could be 3000 rpm and/or 1500 rpm for 50 Hz or 3600 rpm and/or 1800 rpm for 60 Hz. However, in a nuclear-powered generating station, steam conditions to the turbine are in the range of 800 to 1000 psig, and temperatures are in the range of 500 to 600 F, and the turbines rotate at either 1500 rpm for 50 Hz operation or 1800 rpm for 60 Hz operation. Overall plant efficiencies for today's electrical generating power stations can range from 34% to 40 %. Saudi Aramco DeskTop Standards 16

18 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 stationary nozzle vanes and of rotating blades. The geometry of both 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 to 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 5a, 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 5b and 5c, 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. Saudi Aramco DeskTop Standards 17

19 Title: EPS from CorelDRAW! Creator: CorelDRAW! CreationDate: Wed Apr 05 12:01: Figure 5. Generation and Delivery of Mechanical Energy vs. Generation and Delivery of Kinetic Energy Also note that in Figures 5b and 5c, the reaction pressure, P r, on the wall area opposite the nozzle is not balanced by the escaping steam. If the nozzle chamber is fixed in place (Figure 5b), steam exits through the nozzle at its highest possible absolute velocity, and it exerts pressure P1 on anything in its path. However, if the chamber is free to move (Figure 5c), P r does work on it by speeding it in a direction opposite to the jet's motion. In this case, P 2 is lower than P 1. Turbine nozzles direct the steam so that it flows in uniform highspeed jets that impinge upon the surfaces of the moving blades (see Figure 6). 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 6a), the steam jet enters and leaves the boundaries of the blade surface with equal speed, and it develops maximum force F, but no mechanical work is done. As the blade is allowed to speed up (Figure 6b) and moves 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 6c), the force drops to half that of the locked condition. Steam now leaves the blade with zero speed, and does maximum work. Figure 6d shows how both force and work vary with blade speed. Saudi Aramco DeskTop Standards 18

20 Figure 6. Generation and Delivery of Mechanical Energy Saudi Aramco DeskTop Standards 19

21 Gas Turbine Engine - The gas turbine engine like the steam turbine engine, is a rotating engine that produces mechanical work from heat energy and that 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, and it is, therefore, unlike the steam turbine, which burns its fuel in an external boiler. The gas turbine manufactures its own working medium (a supply of pressured, high temperature gas) by compressing air and burning fuel in the compressed air. The gas turbine uses a different working fluid. Like the steam turbine, the name gas turbine refers to the working fluid, which is some type of gaseous substance, usually atmospheric air and products of combustion. (A common misconception with gas turbines is that the 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 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 what is required both to drive the compressor and to compensate for the overall inefficiencies. This surplus work then becomes available as a net plant output. Saudi Aramco DeskTop Standards 20

22 Although there exists a number of gas turbine cycle variations, the most common of these is the "simple" (or "open") cycle. Figure 7 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. Figure 7. Simple (Open) Cycle Gas Turbine Engine The energy flow in a simple cycle gas turbine engine is shown in Figure 8(a). 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 twothirds is recirculated to drive the compressor to sustain the cycle, with the remainder available to do external work. Saudi Aramco DeskTop Standards 21

23 Figure 8. Energy Flow Diagrams for Gas Turbine Engine Cycles Saudi Aramco DeskTop Standards 22

24 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. Gas turbine engines are found in every field requiring shaft-power and heat, separately or in combination. They are used to run aircraft, marine vessels, automotive road vehicles, locomotives, road-building equipment, and other equipment. Their exhaust heat is used for steam generation, process energy, drying, space heating, and air conditioning. In the energy systems area, gas turbines power compressors, pumps, fans, and electric generators. Combined with steam turbines, they form electric generating plants that work at higher efficiencies than either type of turbine is able to do when alone. It is in the electrical power generation industry where gas turbine engines of the highest power capacity are found. Designed and built as the simple (open) cycle units, they generate from 50 MW to almost 230 MW. These units incorporate compressors with pressure ratios that reach nearly 15:1, turbines that operate with inlet gas temperatures that range from nearly 2100 F to 2400 F, and exhaust gas temperatures that range from 950 F to 1100 F. Plant efficiencies achieved in the largest of these simple cycle engines is almost 36%. However, plant efficiency can be dramatically increased through the application of the gas turbine cycle known as the combined cycle by utilizing the wasted energy that simple cycle gas turbine engines exhaust to the atmosphere. The name combined cycle derives from the combination of the simple cycle gas turbine with the operating cycle of the steam turbine. As shown in Figure 9, exhaust gases from the gas turbine pass through a heat recovery steam generator. In this steam generator, much of the heat in the exhaust is utilized in the production of steam. The steam can then be used to drive a conventional steam turbine. The combined cycle very significantly improves the plant cycle efficiency over that of simple cycle gas turbine engines by 50%. Electrical generating stations that employ a combined cycle can achieve plant efficiencies of over 52%. The energy flow diagram for the combined cycle is shown in Figure 8(b). Saudi Aramco DeskTop Standards 23

25 Figure 9. Combined Cycle Schematic 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. Since 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 that is used. The diesel fuel ordinarily employed is a low cost product from a good-grade crude petroleum. The fuel oil is the residue left when distillation has removed the more expensive and highly refined gasoline, kerosene, and other light distillates from the crude. Saudi Aramco DeskTop Standards 24

26 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; diesels 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, which results in 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. With regard to its high uniform efficiency, the diesel generation unit, which usually operates at 1800 rpm for 60 hertz operation, can extract more work out of each heat unit than it can out of any other type of engine. The diesel engine has an efficiency of around 40% and no-load losses of 20 to 30 percent of full load. For that reason, it becomes an attractive prime mover wherever first cost is written off slowly enough so that operating costs are influential. Saudi Aramco DeskTop Standards 25

27 As for 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. Figure 10 shows a typical diesel engine generator used for emergency/ back-up power. 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". See Color Plate 1 Figure 10. Diesel Engine Generator Spark Ignition Gas Engines are four-cycle, internal combustion reciprocating engines that operate with a downward intake stroke, an upward compression stroke, a downward power stroke, and an upward exhaust stroke. The importance of these engines is based on their widespread use for both stationary and motive service. Some of their advantages include a high average effective pressure, high rotating speed, easy starting, and adaptability to production methods of manufacture. Saudi Aramco DeskTop Standards 26

28 Spark ignition gas engines vary from their reciprocating counterpart, the diesel engine, in several important areas. One difference is that spark ignition gas engines use spark plugs to ignite their gas-air fuel mixture. (This ignition is referred to as "spark-ignition".) Another difference is that, for fuel, these engines use gasoline, which is a more expensive and highly refined distillate of crude oil, as opposed to the low cost diesel fuel oil, which is a residue of that distillation process. Spark ignition gas engines also have comparatively lower compression ratios of about 9:1, as opposed to 20:1 in the diesel engine. With regard to comparative efficiency, gas engines, with efficiency values close to 30%, are less efficient than diesel engines. Figure 11 illustrates a comparison of fuel/load efficiency versus load for diesel and gas reciprocating engines. Figure 11. Fuel/Load Efficiency Saudi Aramco DeskTop Standards 27

29 Prime Mover Governors/Speed Control Purpose of Governors/Speed Control When prime movers are used to drive generators for the purpose of generating electric power, it is very important that the speed of the prime mover be maintained at a constant. Constant speed is required to ensure uniform voltage and frequency output from the generator. The purpose of a prime mover governor and its speed control system is to regulate and keep constant, to within a given tolerance, the speed of the prime mover. To better understand the need for regulation by the governor, it should be understood that during operation, the speed of the prime mover is a result of the amount of fuel delivered to the prime mover and the amount of work that the prime mover is performing, i.e., the electrical load. If the electrical load is increased, but the amount of fuel is not increased, the prime mover's speed will drop. As a consequence, the frequency of the electrical power being delivered by the generator will also drop. Conversely, if the electrical load is decreased, but the fuel delivery remains constant, the speed of the prime mover will increase, causing an increase in the electrical power's frequency. Thus, frequency of the generated power is obtained by regulating the speed of the prime mover. The relationship between speed and frequency can be expressed by the equation: where: f = np 120 f = frequency (Hz) n = speed (rpm) p = number of generator poles 120 = constant Saudi Aramco DeskTop Standards 28

30 The governor functions to regulate the speed of the prime mover by first sensing any change in actual speed from the preset reference speed. On sensing a difference between actual and reference speed, the governor sends a signal through its speed control system to the fuel actuator valve and causes the quantity of fuel being fed to the prime mover to be changed. The change in fuel supply attempts to restore the prime mover speed to its preset reference speed. In effect, the governor is a feedback control system in which the difference between a reference input (e.g., the reference 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. Constant speed over the entire range of the prime mover s load capacity is desirable, but some variation must be tolerated to obtain satisfactory governor operation. A governor mechanism must possess sensitivity, promptness of response, and stability. Sensitivity is the ability of a governor to detect, or be influenced by, small changes in prime mover speed. Response is the ability of the governor mechanism to make changes in the fuel supply to the prime mover to compensate for small variations in speed. Stability is the characteristic of a governor that permits rapid changes in load on the prime mover without fluctuations in speed. Speed regulation is one measure of governor performance and is defined as the change in speed necessary to cause the governor to operate the prime mover from no load to full load. This change in speed is expressed in percent of full-load speed. For example, assume that the speed of a prime mover is being controlled by a governor, and that the no-load speed of the prime mover is 105% of its full-load speed. The speed regulation for this unit is calculated as: Speed Regulation = = 5% Saudi Aramco DeskTop Standards 29

31 The decrease in speed as load is applied (speed regulation) for a governor-controlled unit is also referred to as speed droop. When generators are operated individually or in small systems, a small percentage of speed droop (depending on application) is acceptable. Governor systems typically provide a means of adjusting the steady-state speed regulation from 2.5% to 7%. Experience indicates that it is not practical to have a regulation of less than 4% on a public utility type unit. Regulation as low as 1% to 2% can be obtained on industrial units that do not operate in parallel with public utility systems by adding a compensator to the governor system. Figure 12 shows an example of the steady-state speed/load regulation for a governor-controlled unit with a given speedreference setting. The speed regulation (or speed droop) for the system shown in Figure 12 is seen to be 5%; the speed droops 5% for 100% change in load. This type of governor is sometimes referred to as a droop governor. If the unit shown in Figure 12 were operated independently, the speed/load characteristic would be as shown by the solid-line plot. With reference to the solid-line plot, it is seen that unit speed is 100% at zero load (point A). When load is increased to 50%, the speed decreases to 97.5% (point B). Because speed accuracy is very important for individually operated units, the speed droop experienced by the load increase to 50% (point B) must be corrected by changing the speed-reference setting for the governor. This correction is illustrated in Figure 12 by the dotted line from point B to point C. This correction restores the unit to 100% speed at 50% load (point C) and establishes a new dotted-plot that describes the speed/load regulation for the unit. Saudi Aramco DeskTop Standards 30

32 Title: EPS from CorelDRAW! Creator: CorelDRAW! CreationDate: Wed Jun 21 16:02: Figure 12. Example of Speed/ Load Regulation Saudi Aramco DeskTop Standards 31

33 For the speed/ load regulation example just described in Figure 12, manual resetting of the speed-reference point was used to reset the prime mover speed, and, thus, the generator speed to 100% after load was increased to 50%. Manual resetting of the speed-reference point permits a unit, whether isolated or tied to a system with other units, to be set to hold speed or carry load as the operator desires. However, if such a unit is to operate for long periods of time under varying load conditions, manual setting of the speed-reference point is not a satisfactory solution to the problem of maintaining speed accuracy. In such cases, the use of an automatic reset device or speed corrector is used to provide isochronous control. Isochronous speed governing is defined as governing with steady-state speed regulation of essentially zero. A governor that automatically corrects for droop and keeps the speed of a prime mover constant at all loads is referred to as an isochronous governor. If the unit shown in Figure 12 were tied to a system much larger than itself, and the same 50% load change occurred, the effect on the unit would be much less, and its speed would not vary as much. For this case, the system is said to be stiff compared to the individual unit. If the system were large enough, there would be no change to the speed of the individual unit with change in load. For large systems where several generators are operated in parallel, the speed of an individual generator is fixed by the system. When this is the condition, there are two points to remember. First, the speed governors do not control the speed. The load dispatcher controls system speed by asking operators to pick up or drop generation to keep power output essentially equal to power demand. Second, because of regulation, the governors do not restore speed to 60 Hz operation if there is a change in system speed due to a change in load or generation. The control to maintain frequency for large systems is typically accomplished by automatic load and frequency control equipment. Saudi Aramco DeskTop Standards 32

34 Methods of Speed Control The methods used for speed control include various combinations of mechanical, hydraulic, and electrical systems. Regardless of the type of system used, every method has the same control principle. A speed signal is taken from the shaft of the prime mover, converted to a usable form, and compared with a speed/load reference point, which may be manually or automatically set. The resultant error or command signal that is generated as a result of this comparison is usually amplified and then applied to move the fuel-control valve to the desired position. Included in this section are representative examples for electrical, mechanical-hydraulic, and electrohydraulic speed control methods. Figure 13 shows an example of a speed control method that uses electric controls to sense and control the speed of a diesel-driven generator. In this example, a magnetic pickup is used to sense the speed of the diesel prime mover. The tip of the magnetic pickup is placed in close proximity to the prime mover's rotor where evenly spaced gear-type teeth are present around the periphery of the surface. As the teeth rotate past the pickup, a periodic disturbance is imposed on the magnetic field surrounding the tip of the pickup, generating a series of pulses. The pickup is wired to a measuring circuit in the electronic control unit that counts the pulses and produces a signal proportional to the actual speed of the prime mover. At the same time, a load sensor senses the magnitude of the load and sends a signal proportionate to the load to the control unit. The control unit compares the input signals against a reference and sends an amplified command signal to operate the fuel-valve actuator linkage to correctly adjust the rate of fuel feed. For this example, the control circuit uses an isochronous-type governing system to correct for speed/load droop and, thus, control the speed with essentially zero percent speed regulation. Saudi Aramco DeskTop Standards 33

35 Figure 13. Example of Electrical Speed Control Method Saudi Aramco DeskTop Standards 34

36 Figure 14 shows an example of a simple mechanical-hydraulic speed control method used to control the position of a steam supply valve for a steam turbine prime mover. The mechanical portion of this system uses two weights connected by straps to a sliding collar on a shaft. The shaft is geared to the prime mover shaft to monitor actual speed. As the shaft turns in response to the prime mover speed, the rotating weights move outward due to centrifugal force and pull the sliding collar up the shaft. The upward motion of the sliding collar causes the connecting linkage to move upward, actuating the pilot piston in the hydraulic servo motor and allowing high pressure oil to flow into the main cylinder of the servo motor. The action of the servo motor in the hydraulic portion of this system amplifies the force of the connecting linkage to a magnitude that is sufficient to move the actuator stem of the steam supply valve and correctly adjust the flow of steam. A separate spring fastened to the connecting linkage and applying a downward force, opposes the upward force caused by the rotating weights and provides the reference for operating speed and speed adjustment. This method of speed control, typically used for individual units, provides droop-type governor characteristics similar to those shown in Figure 12 and requires manual resetting of the operating-speed reference as load is changed. Saudi Aramco DeskTop Standards 35

37 Figure 14. Example of Mechanical-Hydraulic Speed Control Method Saudi Aramco DeskTop Standards 36

38 Figure 15 shows an example of an electrohydraulic speed control method. This method uses: a permanent magnet generator or digital-type reluctance pickup to provide a prime mover shaftspeed signal; electronic circuitry for comparing the speed signal with a reference signal; a high-gain servo valve to convert the resulting electric signal to a hydraulic signal; a valve-gear poweractuator assembly capable of operating on high-pressure hydraulics upon receipt of the servo-valve signal to operate the main valve; a feedback transducer on the power actuator to restore the servo valve to a stable condition when the desired main-valve position is reached; and a high-pressure hydraulic system to provide the force required. As a result of using electronic circuitry to monitor not only speed but also load, this speed control method operates as an isochronous-type governing system and automatically corrects for speed droop as a result of load change. Figure 15. Example of Electrohydraulic Speed Control Method Saudi Aramco DeskTop Standards 37

39 Purpose and Types of /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 16, 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 16, one cycle of voltage is generated for each 360 mechanical revolution of the rotor. Figure 16. Generator Winding Saudi Aramco DeskTop Standards 38

40 Types of /Alternators 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 the armature windings are intersected during rotation by the magnetic field from the rotor in any one second of time. Induction are induction motors that are operated (run) with mechanical power 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. Since 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 17 illustrates, rotor torque increases with slip. Saudi Aramco DeskTop Standards 39

41 Figure 17. Torque - Slip Curves of Squirrel Cage Induction Motor When an induction motor is operated as a motor, it is run at fullload 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 is seldom used except in special applications such as dynamic braking (for example, elevator systems) because regulation is extremely poor. Synchronous are 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. Saudi Aramco DeskTop Standards 40

42 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 18, 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 limitation, most synchronous generators are built with stationary armature windings as illustrated in the simplified circuit of Figure 19. 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 capacitors to act as power correction devices. 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, so, it must receive a definite amount of lagging VARs for a given voltage and 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. Saudi Aramco DeskTop Standards 41

43 Figure 18. Rotating Armature Figure 19. Stationary Armature Saudi Aramco DeskTop Standards 42

44 Generator Ratings - The capacity of a generator is given in kilovolt amperes (kva) rather than kilowatts (kw). This practice is followed because the machine may be required to supply a load with a power factor other than unity. Information supplied on a typical nameplate on an alternator is shown in Figure 20. Figure 20. Typical Generator Nameplate Data Saudi Aramco DeskTop Standards 43

45 Purpose and Types of Voltage Regulators Purpose Types of Voltage Regulators An alternator will experience large changes in its terminal voltage. Also, changes in load current and 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. The following types of voltage regulators are described below: Electro-mechanical regulator Rotating regulator Electronic regulator Electro-Mechanical Regulators provide a DC supply for the generator field that comes from outside of the generator. Figure 21 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. Saudi Aramco DeskTop Standards 44

46 Title: EPS from CorelDRAW! Creator: CorelDRAW! CreationDate: Wed Apr 05 12:02: Figure 21. Electro-Mechanical Voltage Regulator Saudi Aramco DeskTop Standards 45

47 Rotating Regulators consist of a DC generator which is mechanically coupled to the main generator and rotates with it. Figure 22 shows a simplified schematic of an automatic voltage regulator for a three-phase alternator that is supplied by the rotating generator. A relay coil is wired across the terminals of two phases of the alternator, and a normally closed contact on the relay is wired into the field circuit paralleling a field resistor. Opening of the contact reduces the field current by inserting resistance into the field circuit with a consequential reduction of the generator output voltage. Closing the contact shorts out the resistor, allowing more current to flow in the field circuit with a consequential increase of the output of the generator voltage. By selecting a relay for a desired voltage pickup and dropout, the relay will alternately pickup and dropout, resulting in a generator output that is regulated within the band of the pickup and dropout values. It should be noted that increasing the alternator voltage will produce more energy output only if the prime mover can deliver more torque. Otherwise the increased voltage will be accompanied by a change in power factor that limits the output power. This will be discussed later in more detail. Figure 22. Rotating Regulator Saudi Aramco DeskTop Standards 46