CORE LOSSES OF DC GENERATORS BASIC INFORMATION AND TUTORIALS


The flux in any portion of the armature passes through p/2 c/r (cycles per revolution) or through (p/2 [(r/min)/60] Hz. The iron losses consist of the hysteresis loss, which equals Kb1.6fw watts, and the eddy current loss, which equals Ke(Bft)^2w watts.

K is the hysteresis constant of the iron used, Ke is a constant inversely proportional to the electrical resistance of the iron, B is the maximum flux density in lines per square inch, f is the frequency in hertz, w is the weight in pounds, and t is the thickness of the core laminations in inches.

The eddy loss is reduced by using iron with as high an electrical resistance as is feasible. Very high resistance iron has a tendency to have low flux permeability and to be mechanically brittle and expensive. It is seldom justified in dc machines. The loss is kept to an acceptable value by the use of thin core laminations, 0.017 to 0.025 in thickness.

Another significant loss is the pole-face loss. These losses are included in the core loss.

Unfortunately, there are other losses in the core that may differ widely even on duplicate machines and that do not lend themselves to calculation. These include:

1. Loss due to filing of slots. When the laminations have been assembled, it will be found in some cases that the slots are rough and must be filed to avoid cutting the coil insulation. This burrs the laminations and tends to short circuit the interlaminar resistance.

2. Losses in the solid spider, core end plates, and coil supports from leakage fluxes may be appreciable.

3. Losses due to nonuniform distribution of flux in the rotor core are difficult to anticipate. In calculating core density, it is customary to assume uniform distribution over the core section.

However, flux takes the path of least resistance and crowds behind the teeth until saturation forces it into the less used, longer paths below. As a result of the concentration, the core loss, which is about proportional to the square of the density, is greater than calculated.

Thus, it is not possible to predetermine the total core loss by the use of fundamental formulas. Consequently, core-loss calculations for new designs are usually based on the results from tests on similar machines built under the same conditions.




Such test results are plotted in Fig. 8-54 for machines using ordinary laminations 0.017 in thick and a limited amount of filing. They do not include the pole-face losses, which would increase the values about 30%.

ELECTRICAL SPECIFICATIONS BASIC INFORMATION AND TUTORIALS

What Are Electrical Specifications?


Electrical specifications for buildings or projects are written legal descriptions of the work to be performed by the electrical contractor, subcontractors, and electric power utilities and the responsibilities and duties of the architect/engineer, general contractor, and owner. Electrical specifications and electrical drawings are integral parts of the contract requirements for the performance of electrical work.

Because specifications are a significant part of a legally binding contract, typically involving expenditures of thousands or even millions of dollars, it is important that they be mutually compatible with the drawings and as free as possible of errors or discrepancies.

It has long been known that even minor errors in wording or intent or the presentation of incorrect data or measurements can result in expensive repairs or replacements of hardware, lost time in the completion of the schedule, and serious project cost overruns due to delays and the need for additional labor and supervision.

In most engineering and architectural firms, regardless of size, specifications writers are skilled persons with technical backgrounds who report to a responsible project supervisor. The preparation of an error-free specification is a time-consuming task calling for the writer’s patience and the ability to deal effectively with complex technical details.

The process might call for many drafts and revisions following the review, comments, and corrections made by persons within the architect/engineering organization with specialized know ledge and experience in each of the trades involved in the project. As with drawings, all responsible reviewers are expected to sign the final version that is released for bid.

Nevertheless, this does not relieve specifications writers of their responsibilities, because they are expected to have sufficient knowledge of the project to make them capable of finding and resolving any discrepancies between the specifications and the drawings. Discrepancies are most likely to occur when

A generic master or prototype specification is used without making all of the modifications necessary to reflect what is actually shown on the working drawings. Revisions that should have been made in a previously prepared drawings are indicated only by a note in the revision block, leaving the drawing unchanged.


Revisions in items that are listed both in schedules on the drawings and in the written specifications are made on only one of these documents. For example, there is a discrepancy if the specification calls for one load center but the drawing has been revised to show two load centers and this change is not reflected back to the specifications.

Such a discrepancy could result in unnecessary costs, unless caught in time. For this reason, it is not good professional practice to duplicate the same information on both specifications and drawings. It is preferable that the required information be placed on the document on which it is most logically found to assure compliance, with perhaps a reference to its location on the other document.

If for some reason duplication of information occurs in both drawings and specifications, and it is not practical to delete it from one of the documents, the project supervisor should add a note to the contract before it is put out for bid stating whether the specifications or drawings take precedence.

ELECTRICAL PRODUCT AND WORK STANDARDS BASIC INFORMATION AND TUTORIALS


The generation, transmission, and distribution of electrical power are now deregulated, but many rules, regulations, and standards still apply to the manufacture of electrical equipment, the job site, and the installation of electrical systems. Many of these standards are focused on safety issues, such as the elimination or avoidance of hazards in working with or using electricity.

The installation of any inferior wiring devices or equipment, substandard workmanship, or inadequate test and maintenance procedures could be the cause of fires or explosions and result in the creation of electric shock hazards.

There is a need for standards that, when adopted, will serve as a basis for proper inspection and supervision. There are regulatory standards, national consensus standards, product standards, installation standards, and international standards.

The consensus standards include the National Electrical Code (NEC), the National Electrical Safety Code (NESC), National Fire Protection Association (NFPA) 70B and 70E, and other NFPA standards, as well as American National Standards Institute (ANSI) and the Institute of Electrical and Electronic Engineers (IEEE) standards.

Interested persons with requisite education, training, and background experience volunteer their time and expertise to develop these standards. Some might be employees of electrical product manufacturers, and others might be consultants or engineering professors.

Some standards were developed specifically for electrical applications. These include the National Electrical Code (NEC) (officially NFPA 70) and the National Electrical Installation Standards (NEIS).

In addition, the National Electrical Manufacturers Association (NEMA) standards cover equipment design and construction, while the Underwriters Laboratories Inc. (UL) standards cover safety provisions in the manufacture of electrical devices, products, and accessories.


The National Fire Protection Association (NFPA), publisher of both the NEC and NESC, has also developed standards related to electrical work including:  NFPA 79 Industrial Machinery  NFPA 780 Lightning Protection  NFPA Static Electricity

The NFPA has also developed standards for fire prevention, installation of sprinklers, stacking materials, and a standard building code. The following organizations have also developed standards that have a bearing on electrical work:

ACS: American Chemical Society
ACGIH: American Conference of Governmental Industrial Hygienists
AIChE: American Institute of Chemical Engineers
ASME: American Society of Mechanical Engineers
ASTM: American Society for Testing and Materials
ASSE: American Society of Safety Engineers
AWS: American Welding Society
CGA: Compressed Gas Association
CMA: Chemical Manufacturing Association
CMAA: Crane Manufacturer’s Association
GSA: General Services Administration Federal Supply Services
NSC: National Safety Council
OSHA: Occupational Safety and Health Administration

Individuals or organizations with a professional or business interest in these standards can join these organizations to help support them and gain access to their newsletters so that they can stay informed on any changes within the standards. Standards-making organizations may make changes to their standards between normal cycles that are not included in the printed text of the original issue of the standard.

The NFPA, for example, does this in the form of a Tentative Interim Amendment (TIA). Additionally, changes may take place without a formal notice of change, so it is important to stay current with any given standard. Interested parties can communicate with these standards organizations and suggest changes or revisions in standards.

It is the responsibility of all electrical contractors, electricians, and installers in the field to know which standards apply to any project taking place within any given location or job site. It is also important to remember that not all standards that might apply to every job site or location actually apply. A hazard assessment by the project supervisor or licensed electrician must determine which standards apply at each workplace and that they are followed.


The contractor has the responsibility for assuring that a workplace is free from recognized hazards and is a safe place for electricians and installers to work. This could apply to such factors as the quality of ladders or scaffolding at the site or the need for safety goggles or face masks when performing certain kinds of work.

For example, eye protection should be worn during any grinding or cutting operations that could result in flying chips of metal, and proper face masks should be worn by anyone performing burning or welding that could result in the release of toxic gases.

Designers, engineers, contractors, electricians, or equipment installers and all other persons whose work is governed by one or more standards should be familiar with and know how to apply the rules found in all of the applicable standards. These are the rules that relate to design, including safety considerations, for a particular project or task.

ELECTRICAL WIRING DIAGRAM GRAPHIC SYMBOLS BASIC INFORMATION AND TUTORIALS


Electrical engineers and designers generally follow accepted standards for the basic electrical and electronic symbols. These electrical symbols can be classified as those used on connection and interconnection diagrams and those used on elementary or schematic diagrams.

Connection and interconnection symbols represent complete electrical devices such as switch outlets, receptacle outlets, lighting fixtures or luminaires, and auxiliary systems. These symbols take the form of relatively simple geometric shapes modified with lines and letters inside or outside of them. The intent was to create a kind of technical shorhand that could be easily learned.

They were kept simple to reduce the time and expense of preparing drawings, particularly those used in the field for installation of common off-the-shelf electrical components.

Figure below includes a selection of electrical connection and interconnection symbols recommended by the American National Standards Institute (ANSI) for use on architectural drawings. These symbols, or modified versions of them, are widely used on electrical drawings in North America. Appendix A also includes a page of these symbols.


CAD electrical drafting software has eliminated the chore of reproducing these symbols. The software contains a library of symbols that can be accessed from a menu, downloaded, and dragged into position on the face of the screen as needed.

The basic symbols can be modified to fulfill special requirements or identify devices not listed in the standard symbol list. In the past, symbols were usually drawn by the drafts person tracing around the inside of geometric cutouts in templates made of sheet plastic.

As with line conventions, the motivation for using standardized symbols is to eliminate the time involved in trying to interpret drawings that include unfamiliar proprietary symbols. It is important that the symbols be easily recognized by all parties involved in an electrical project, from the designer to the electricians doing the work.

As a result, the chances of making costly mistakes in interpretation are lessened. Moreover, large architectural and consulting engineering firms with national and international clients approve of symbol standardization because of the many people of different backgrounds, languages, and cultures who could be using the drawings. This is especially true of large-scale new construction projects such as hospitals, power stations, and industrial plants involving many different contractors.

EXCITATION SYSTEM CEILING VOLTAGE OF SYNCHRONOUS GENERATORS


Fast control of excitation current is needed to preserve SG transient stability and control its voltage. Higher ceiling excitation voltage, corroborated with low electrical time constants in the excitation system, provides for fast excitation current control.

Today’s ceiling voltages are in the range of 1.6 to 3.0 P.U. There is a limit here dictated by the effect of magnetic saturation, which makes ceiling voltages above 1.6 to 2.0 P.U. hardly practical.

This is more so as higher ceiling voltage means sizing the insulation system of the exciter or the rating of the static exciter voltage for maximum ceiling voltage at notably larger exciter costs.

The debate over which is best — the alternating current (AC) brushless exciter or static exciter (which is specified also with a negative ceiling voltage of –1.2 to 1.5 P.U.) is still not over.

A response time of 50 msec in “producing” the maximum ceiling voltage is today fulfilled by the AC brushless exciters, but faster response times are feasible with static exciters.

However, during system faults, the AC brushless exciter is not notably disturbed, as it draws its input from the kinetic energy of the turbine-generator unit.

In contrast, the static exciter is fed from the exciter transformer which is connected, in general, at SG terminals, and seldom to a fully independent power source.

Consequently, during faults, when the generator terminal voltage decreases, to secure fast, undisturbed excitation current response, a higher voltage ceiling ratio is required.

Also, existing static exciters transmit all power through the brush slipring mechanical system, with all the limitations and maintenance incumbent problems.

THE ROLE OF THE ENERGY MANAGER

One very important part of an energy management program is to have top management support. More important, however, is the selection of the energy manager, who can among other things secure this support.

The person selected for this position should be one with a vision of what managing energy can do for the company. Every successful program has had this one thing in common—one person who is a shaker and mover that makes things happen. The program is then built around this person.

There is a great tendency for the energy manager to become an energy engineer, or a prima donna, and attempt to conduct the whole effort alone. Much has been accomplished in the past with such individuals working alone, but for the long haul, managing the program by involving everyone at the facility is much more productive and permanent.

Developing a working organizational structure may be the most important thing an energy manager can do. The role and qualifications of the energy manager have changed substantially in the past few years, caused mostly by EPAC92 requiring certification of federal energy managers, deregulation of the electric utility industry bringing both opportunity and uncertainty, and by performance contracting requiring more business skills than engineering. In her book titled “Performance Contracting:

Expanded Horizons,” Shirley Hansen give the following requirements for an energy management:

• Set up an Energy Management Plan

• Establish energy records

• Identify outside assistance

• Assess future energy needs

• Identify financing sources

• Make energy recommendations

• Implement recommendations

• Provide liaison for the energy committee

• Plan communication strategies

• Evaluate program effectiveness

Tidal Lagoon Power Generation Scheme in Swansea Bay - White Paper PDF Download Link

Tidal Lagoon Power Generation Scheme in Swansea Bay

A report on behalf of the Department of Trade and Industry and the Welsh Development Agency

Executive Summary
Tidal Electric Limited (TEL) has proposed the construction of a 60MW “tidal lagoon” power generation scheme in Swansea Bay. TEL has estimated that the lagoon could be constructed for £81.5M and generate power at a cost of ~3.5p/kWh.

Renewable energy generated at this cost would be commercially very attractive and both the Department of Trade and Industry (DTI) and the Welsh Development Agency (WDA) have separately undertaken reviews to assess the claims made. The two reviews, undertaken independently, arrived at the same main conclusion.

This was that the generation costs of the proposed tidal lagoon would be substantially higher than TEL’s estimate. The most significant differences between TEL’s proposal and the Reviewers’ assessment are as summarised in the following table:

Thus, the total construction cost for the proposed tidal lagoon scheme is likely to be around £234M, compared with the TEL estimate of £81.5M (i.e. an increase by a factor of 3.6), and the lagoon is only likely to generate around 66% of the energy projected by TEL.

The cost of energy from the proposed lagoon is therefore estimated to be more than 4 times greater than that presented by TEL.

On this basis the cost of energy from the proposed lagoon would be at least 17p/kWh at 8% discount rate which conforms with results from previous studies of tidal lagoons undertaken by others.

The Reviewers see little prospect for reducing the cost of energy from this concept through replication, innovation or experience as it would uses standard engineering principles and mature technologies. The cost of the lagoon is dominated by the materials cost where there is little prospect for cost reduction and much scope for cost increase.

There are also proposals to construct tidal lagoons in other locations, such as off the coast of North Wales. Whilst this review has not studied these other proposed schemes, the issues surrounding the Swansea Bay scheme are sufficient to cast doubt on the economic viability of the tidal lagoon concept for the foreseeable future.

The few possible locations with a higher tidal range offer a greater annual power output, and larger lagoons may offer some economy of scale. Studies of tidal barrages, however, have shown that the economies of scale are relatively minor whereas the potential for unacceptable impacts tends to increase with size.

Should TEL’s engineering solutions prove deliverable, then the economics of all the potential tidal barrage schemes around the UK would also be significantly improved.


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Modeling Distributed Electricity Generation in the NEMS Buildings Models - White Paper PDF Download Link

Modeling Distributed Electricity Generation in the NEMS Buildings Models
by Erin Boedecker, John Cymbalsky, and Steven Wade

Distributed generation refers to the production of electricity in a decentralized facility—in the present context, a building. This “nontraditional” electricity source has the advantage of allowing the capture of the “waste” heat from generation, thereby offsetting the energy requirements of other end uses and potentially lowering total energy requirements across multiple end uses (i.e., the combined requirements for electric energy, space heating energy, and water heating energy).

This paradigm contrasts with central generation, where waste heat is often a negative externality that is emitted directly into the biosphere. In addition to utilizing heat energy that would otherwise be wasted, on-site generation has the additional efficiency benefit of avoiding the transmission and distribution losses associated with centralized generation and, possibly, the need for upgrades to transmission and distribution grids.

Currently, the National Energy Modeling System (NEMS) buildings models characterize several distributed generation technologies: conventional oil or gas engine generation, combustion turbine technologies, and newer, still developing technologies such as solar photovoltaics (PV), fuel cells, and microturbines.

This paper describes the modeling techniques, assumptions, and results for the Annual Energy Outlook 2000 reference case. In addition, a series of alternative simulations are described, and key results for distributed generation are presented.

Introduction
Recently, distributed generation technologies have received much attention for the potential energy savings and reliability assurances that might be achieved as a result of their widespread adoption. Fueling the attention have been the possibilities of international agreements to reduce greenhouse gas emissions, electricity sector restructuring, high power reliability requirements for certain activities, and concern about easing transmission and distribution capacity bottlenecks and congestion.

This paper presents the modeling methodology, projected market penetration, and impact of distributed generation with respect to offsetting future electricity needs and carbon dioxide emissions in the residential and commercial buildings sector in the Annual Energy Outlook 2000 (AEO2000) reference case.

Also, a series of alternate simulations are presented with key distributed generation results. These alternatives include more optimistic assumptions regarding the cost of the newer distributed technologies, favorable compensation rates for grid sales (net metering), and aggressive tax incentives for selected technologies. Projections of future levels of distributed generation and estimated impacts on fuel consumption and carbon dioxide emissions are presented.

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Biomass for Electricity Generation PDF WHITE PAPER DOWNLOAD LINK

Biomass for Electricity Generation

by Zia Haq

This paper examines issues affecting the uses of biomass for electricity generation. The methodology used in the National Energy Modeling System to account for various types of biomass is discussed, and the underlying assumptions are explained. The Energy Information Administration’s estimation of biomass resources shows that there are 590 million wet tons of biomass available in the United States on an annual basis; 20 million wet tons (enough to supply about 3 gigawatts of capacity) are available today at prices of $1.25 per million Btu or less. The average price of coal to electric utilities in 2001 was $1.23 per million Btu.

Introduction
The U.S. economy uses biomass-based materials as a source of energy in many ways. Wood and agricultural residues are burned as a fuel for cogeneration of steam and electricity in the industrial sector. Biomass is used for power generation in the electricity sector and for space heating in residential and commercial buildings.

Biomass can be converted to a liquid form for use as a transportation fuel, and research is being conducted on the production of fuels and chemicals from biomass. Biomass materials can also be used directly in the manufacture of a variety of products.

In the electricity sector, biomass is used for power generation. The Energy Information Administration (EIA), in its Annual Energy Outlook 2002 (AEO2002) reference case,1 projects that biomass will generate 15.3 billion kilowatthours of electricity, or 0.3 percent of the projected 5,476 billion kilowatthours of total generation, in 2020.

In scenarios that reflect the impact of a 20-percent renewable portfolio standard (RPS)2 and in scenarios that assume carbon dioxide emission reduction requirements based on the Kyoto Protocol,3 electricity generation from biomass is projected to increase substantially.

Therefore, it is critical to evaluate the practical limits and challenges faced by the U.S. biomass industry. This paper examines the range of costs, resource availability, regional variations, and other issues pertaining to biomass use for electricity generation.

The methodology by which the National Energy Modeling System (NEMS) accounts for various types of biomass is discussed, and the underlying assumptions are explained. A major challenge in forecasting biomass energy growth is estimating resource potential. EIA has compiled available biomass resource estimates from Oak Ridge National Laboratory (ORNL),4 Antares Group, Inc.,5 and the U.S. Department of Agriculture (USDA).

This paper discusses how these data are used for forecasting purposes and the implications of the resulting forecasts, focusing on biomass used in grid-connected electricity generation applications.

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PERMANENT MAGNET SYNCHRONOUS GENERATORS BASIC AND TUTORIALS

The rapid development of high-energy PMs with a rather linear demagnetization curve led to widespread use of PM synchronous motors for variable speed drives. As electric machines are reversible by principle, the generator regime is available, and, for direct-driven wind generators in the hundreds of kilowatt or MW range, such solutions are being proposed.

Super-high-speed gas-turbine-driven PM synchronous generators in the 100 kW range at 60 to 80 krpm are also introduced. Finally, PM synchronous generators are being considered as starter generators for the cars of the near future.

There are two main types of rotors for PM synchronous generators:
• With rotor surface PMs (Figure 2.10) — nonsalient pole rotor (SPM)
• With interior PMs (Figure 2.11a through Figure 2.11c) — salient pole rotor (IPM)

The configuration in Figure 2.10 shows a PM rotor made with parallelepipedic PM pieces such that each pole is patched with quite a few of them, circumferentially and axially.


The PMs are held tight to the solid (or laminated) rotor iron core by special adhesives, and a highly resilient resin coating is added for mechanical rigidity. The stator contains a laminated core with uniform slots (in general) that house a three-phase winding with distributed (standard) coils or with concentrated (fractionary) coils.

The rotor is practically isotropic from the magnetic point of view. There is some minor difference between the d and the q axis magnetic permeances, because the PM recoil permeability (μrec = (1.04 – 1.07) μ0 at 20°C) increases somewhat with temperature for NeFeB and SmCo high-energy PMs.

So, the rotor may be considered as magnetically nonsalient (the magnetization inductances Ldm and Lqm are almost equal to each other).

To protect the PMs, mechanically, and to produce reluctance torque, the interior PM pole rotors were introduced. Two typical configurations are shown in Figure 2.11a through Figure 2.11c. Figure 2.11a shows a practical solution for two-pole interior PM (IPM) rotors. A practical 2p1 = 4,6,… IPM rotor as shown in Figure 2.11b has an inverse saliency: Ldm < Lqm, as is typical with IPM machines.


Finally, a high-saliency rotor (Ldm > Lqm), obtained with multiple flux barriers and PMs acting along axis q (rather than axis d), is presented in Figure 2.11c. It is a typical IPM machine but with large magnetic saliency.

In such a machine, the reluctance torque may be larger than the PM interactive torque. The PM field first saturates the rotor flux bridges and then overcompensates the stator-produced field in axis q.

This way, the stator flux along the q axis decreases with current in axis q. For flux weakening, the Id current component is reduced. A wide constant power (flux weakening) speed range of more than 5:1 was obtained this way. Starters/generators on cars are a typical application for this rotor.

As the PM’s role is limited, lower-grade (lower Br) PMs, at lower costs, may be used. It is also possible to use the variable reluctance rotor with high magnetic saliency (Figure 2.11a) without permanent magnets. With the reluctance generator, either power grid or stand-alone mode operation is feasible.


For stand-alone operation, capacitor self-excitation is needed. The performance is moderate, but the rotor cost is also moderate. Standby power sources would be a good application for reluctance synchronous generators with high saliency Ldm/Lqm > 4.

PM synchronous generators are characterized by high torque (power) density and high efficiency (excitation losses are zero). However, the costs of high-energy PMs are still up to $100 per kilogram.

Also, to control the output, full-power electronics are needed in the stator (Figure 2.12). A bidirectional power flow pulse-width modulator (PWM) converter, with adequate filtering and control, may run the PM machine either as a motor (for starting the gas turbine) or as a generator, with controlled output at variable speed.


The generator may work in the power-grid mode or in stand-alone mode. These flexibility features, together with fast power-active and power-reactive decoupled control at variable speed, may make such solutions a way of the future, at least in the tens and hundreds of kilowatts range.

Many other PM synchronous generator configurations were introduced, such as those with axial airgap. Among them, we will mention one that is typical in the sense that it uses the IPM reluctance rotor (Figure 2.11c), but it adds an electrical excitation. (Figure 2.13).


In addition to the reluctance and PM interaction torque, there will be an excitation interaction torque. The excitation current may be positive or negative to add or subtract from Id current component in the stator.

This way, at low speeds, the controlled positive field current will increase and control the output voltage, while at high speeds, a negative field current will suppress the electromagnetic torque, when needed, to keep the voltage constant.

For DC-controlled output only a diode rectifier is necessary, as the output voltage is regulated via DC current control in four quadrants. A low-power four-quadrant chopper is needed.

For wide speed range applications such a hybrid excitation rotor may be a competitive solution. The rotor is not very rugged mechanically, but it can easily handle peripheral speeds of up to 50 m/sec (10,000 rpm for 0.1 m diameter rotor).

DIRECT CURRENT (DC) GENERATORS BASIC AND TUTORIALS

Applications. The most important role played by the dc generator is the power supply for the important dc motor. It supplies essentially ripple-free power and precisely held voltage at any desired value from zero to rated.

This is truly dc power, and it permits the best possible commutation on the motor because it is free of the severe waveshapes of dc power from rectifiers. It has excellent response and is particularly suitable for precise output control by feedback control regulators.

It is also well suited for supplying accurately controlled and responsive excitation power for both ac and dc machines.

The dc motor plays an ever-increasing vital part in modern industry, because it can operate at and maintain accurately any speed from zero to its top rating. For example, high-speed multistand steel mills for thin steel would not be possible without dc motors.

Each stand must be held precisely at an exact speed which is higher than that of the preceding stand to suit the reduction in thickness of the steel in that stand and to maintain the proper tension in the steel between stands.

General Construction Of DC Generator.
Figure 8-1 shows the parts of a medium or large dc generator. All sizes differ
from ac machines in having a commutator and the armature on the rotor. They also have salient poles
on the stator, and, except for a few small ones, they have commutating poles between the main poles.


Construction and Size.
Small dc machines have large surface-to-volume ratios and short paths for heat to reach dissipating surfaces. Cooling requires little more than means to blow air over the rotor and between the poles. Rotor punchings are mounted solidly on the shaft, with no air passages through them.

Larger units, with longer, deeper cores, use the same construction, but with longitudinal holes through the core punchings for cooling air. Medium and large machines must have large heat-dissipation surfaces and effectively placed cooling air, or “hot spots” will develop.

Their core punchings are mounted on arms to permit large volumes of cool air to reach the many core ventilation ducts and also the ventilation spaces between the coil end extensions.

Design Components.
Armature-core punchings are usually of high-permeability electrical sheet steel, 0.017 to 0.025 in thick, and have an insulating film between them. Small and medium units use “doughnut” circular punchings, but large units, above about 45 inches in diameter, use segmental punchings shaped as shown in Fig. 8-2, which also shows the fingers used to form the ventilating ducts.


Main- and commutating-pole punchings are usually thicker than rotor punchings because only the pole faces are subjected to highfrequency flux changes. These range from 0.062 to 0.125 in thick, and they are normally riveted.

The frame yoke is usually made from rolled mild steel plate, but, on high-demand large generators for rapidly changing loads, laminations may be used. The solid frame has a magnetic time constant of 1/2 s or more, depending on the frame thickness. The laminated frame ranges from 0.05 to 0.005 s.

The commutator is truly the heart of the dc machine. It must operate with temperature variations of at least 55 DEG C and with peripheral speeds that may reach 7000 ft/min. Yet it must remain smooth concentrically within 0.002 to 0.003 in and true, bar to bar, within about 0.0001 in.

The commutator is made up of hard copper bars drawn accurately in a wedge shape. These are separated from each other by mica plate segments, whose thicknesses must be held accurately for nearly perfect indexing of the bars and for no skew.

This thickness is 0.020 to 0.050 in, depending on the size of the generator and on the maximum voltage that can be expected between bars during operation. The mica segments and bars are clamped between two metal V-rings and insulated from them by cones of mica.

On very high speed commutators of about 10,000 ft/min, shrink rings of steel are used to hold the bars. Mica is used under the rings.

Carbon brushes ride on the commutator bars and carry the load current from the rotor coils to the external circuit. The brush holders hold the brushes against the commutator surface by springs to maintain a fairly constant pressure and smooth riding.

AC GENERATORS COOLING SYSTEM BASIC AND TUTORIALS

Dissipation in generators appears as heat which must be removed. This heat appears in the armature conductors, field-winding conductors, stator core, rotor surface, and other structural elements of the machine. Cooling of armature and field conductors may be direct or indirect; the difference is direct contact of the cooling medium with the conductor or contact through electrical insulation.

Cooling Media
Alternating-current generators may be cooled by air, hydrogen, water, or (very infrequently) oil. In large machines, no matter what the cooling medium, heat is transferred to water in heat exchangers that are located within the machine case.

Smaller machines are cooled by air. Recently, there has been a trend toward air-cooling larger machines.

The upper limit in size for air-cooled machines is, as of this writing, about 350 MVA, and may increase further. The advantage of air cooling is simplicity. The disadvantage is machine size.

Hydrogen has had wide application in cooling of larger generators. It has a high specific heat and thermal conductivity and low density, so it provides better heat transfer with lower windage losses than does air.

Hydrogen also does not support oxidation, with some advantage to insulation systems. Cooling a generator with hydrogen requires additional systems to maintain hydrogen purity and to remove hydrogen from lubricating oil and shaft seals.

Since the “explosive” range of hydrogen/ oxygen mixtures is about 5% to 75% hydrogen, if the purity of hydrogen is kept above about 95%, the cooling medium will be nonexplosive. Water is used in armature winding cooling in very large machines.

Ventilation Paths
Fans used in electric machines may be of either radial flow or axial flow, and a wide variety of cooling paths are used. Figures 7-22 and 7-23 show two possible schemes.



by punching holes in the laminations. Radial passages are formed by spacers that hold the core packets apart.

Radial passages might be about 1 cm in axial length with spacing of about 5 cm. In some cases, as shown in Fig. 7-22, axial and radial cooling passages are mixed in one machine. In some cases, the ventilating gas passes first radially inward and then radially outward.

Rotor Ventilation
As with the stator core, rotor cooling takes on a variety of forms. In some cases cooling gas passes axially from the ends of the rotor and then exits through holes in the rotor surface into the air gap, and then passes through the stator core.

In other cases, gas passes radially inward from the air gap, diagonally through the rotor, and then radially outward to the air gap. This scheme can be coordinated with cooling of the stator core.

Direct and Indirect Cooling
Direct cooling, the norm for rotor windings and widely used in stator windings, exposes the cooling medium directly to the conductors. Figure 7-24 shows hydrogen and water directly cooled conductors for both stator and rotor. In a directly gas-cooled stator, relatively large passages are built into the conductor bar.



The conductor strands are transposed around the gas passages. There is strand insulation between the conductor strands and gas passage (which is often made of stainless steel), but the gas is within the ground wall.

In a directly gas-cooled rotor the gas flow may be radial, axial, or diagonal, or some combination of all three. In a directly water-cooled stator winding, the water flow may be in direct contact with the conductors.

In some cases some or all of the conducting strands are made of hollow copper tubing. In others, stainless-steel tubes are used. Typically, water flows through the machine only one or two axial passes before being returned to the cooler.

If water cooling is used, then
1. The water is maintained at very high purity so that it has low conductivity.

2. Water is carried to the armature conductors through specially made hoses, since the conductor bars are at high potential and the water header is at ground.

3. Generally, hydrogen pressure in the machine is maintained above water pressure so that any leak will be of hydrogen into the water system, rather than water into the electrical insulation. Water-cooled field windings are relatively rare, although many have been in highly reliable service for decades in some of the world’s most powerful nuclear turbine generators.

EXCITATION SYSTEM OF HYDRO POWER GENERATOR BASIC INFORMATION


The excitation system fulfills two main functions:

1. It produces DC voltage (and power) to force current to flow in the field windings of the generator. There is a direct relationship between the generator terminal voltage and the quantity of current flowing in the field windings.

2. It provides a means for regulating the terminal voltage of the generator to match a desired set point and to provide damping for power system oscillations.

Prior to the 1960s, generators were generally provided with rotating exciters that fed the generator field through a slip ring arrangement, a rotating pilot exciter feeding the main exciter field, and a regulator controlling the pilot exciter output.

Since the 1960s, the most common arrangement is thyristor bridge rectifiers fed from a transformer connected to the generator terminals, referred to as a “potential source controlled rectifier high initial response exciter” or “bus-fed static exciter” (IEEE, 421.1-1986; 421.2-1990; 421.4-1990; 421.5-1992).

Another system used for smaller high-speed units is a brushless exciter with a rotating AC generator and rotating rectifiers.

Modern static exciters have the advantage of providing extremely fast response times and high field ceiling voltages for forcing rapid changes in the generator terminal voltage during system faults. This is necessary to overcome the inherent large time constant in the response between terminal voltage and field voltage (referred to as T do , typically in the range of 5 to 10 sec).

Rapid terminal voltage forcing is necessary to maintain transient stability of the power system during and immediately after system faults.

Power system stabilizers are also applied to static exciters to cause the generator terminal voltage to vary in phase with the speed deviations of the machine, for damping power system dynamic oscillations.

Various auxiliary devices are applied to the static exciter to allow remote setting of the generator voltage and to limit the field current within rotor thermal and under excited limits. Field flashing equipment is provided to build up generator terminal voltage during starting to the point at which the thyristors can begin gating. Power  for field flashing is provided either from the station battery or alternating current station service.

GENERATOR TYPE SELECTION FOR HYDROELECTRIC POWER PLANTS BASIC AND TUTORIALS


Synchronous generators and induction generators are used to convert the mechanical energy output of the turbine to electrical energy. Induction generators are used in small hydroelectric applications (less than 5 MVA) due to their lower cost which results from elimination of the exciter, voltage regulator, and synchronizer associated with synchronous generators.

The induction generator draws its excitation current from the electrical system and thus cannot be used in an isolated power system. Also, it cannot provide controllable reactive power or voltage control and thus its application is relatively limited.

The majority of hydroelectric installations utilize salient pole synchronous generators. Salient pole machines are used because the hydraulic turbine operates at low speeds, requiring a relatively large number of field poles to produce the rated frequency.

A rotor with salient poles is mechanically better suited for low-speed operation, compared to round rotor machines which are applied in horizontal axis high-speed turbo-generators.

Generally, hydroelectric generators are rated on a continuous-duty basis to deliver net kVA output at a rated speed, frequency, voltage, and power factor and under specified service conditions including the temperature of the cooling medium (air or direct water). Industry standards specify the allowable temperature rise of generator components (above the coolant temperature) that are dependent on the voltage rating and class of insulation of the windings (ANSI, C50.12-1982; IEC, 60034-1).

The generator capability curve, describes the maximum real and reactive power output limits at rated voltage within which the generator rating will not be exceeded with respect to stator and rotor heating and other limits. Standards also provide guidance on short circuit capabilities and continuous and short-time current unbalance requirements (ANSI, C50.12-1982; IEEE, 492-1999).

Synchronous generators require direct current field excitation to the rotor, provided by the excitation system described in Section entitled “Excitation System”. The generator saturation curve, describes the relationship of terminal voltage, stator current, and field current.

While the generator may be vertical or horizontal, the majority of new installations are vertical. The basic components of a vertical generator are the stator (frame, magnetic core, and windings), rotor (shaft, thrust block, spider, rim, and field poles with windings), thrust bearing, one or two guide bearings, upper and lower brackets for the support of bearings and other components, and sole plates which are bolted to the foundation.

Other components may include a direct connected exciter, speed signal generator, rotor brakes, rotor jacks, and ventilation systems with surface air coolers (IEEE, 1095-1989).

The stator core is composed of stacked steel laminations attached to the stator frame. The stator winding may consist of single turn or multi-turn coils or half-turn bars, connected in series to form a three phase circuit.

Double layer windings, consisting of two coils per slot, are most common. One or more circuits are connected in parallel to form a complete phase winding. The stator winding is normally connected in wye configuration, with the neutral grounded through one of a number of alternative methods which depend on the amount of phase-to-ground fault current that is permitted to flow (IEEE, C62.92.2-1989; C37.101-1993).

Generator output voltages range from approximately 480 VAC to 22 kVAC line-to-line, depending on the MVA rating of the unit. Temperature detectors are installed between coils in a number of stator slots.

The rotor is normally comprised of a spider attached to the shaft, a rim constructed of solid steel or laminated rings, and field poles attached to the rim. The rotor construction will vary significantly depending on the shaft and bearing system, unit speed, ventilation type, rotor dimensions, and characteristics of the driving hydraulic turbine.

Damper windings or amortisseurs in the form of copper or brass rods are embedded in the pole faces, for damping rotor speed oscillations. The thrust bearing supports the mass of both the generator and turbine plus the hydraulic thrust imposed on the turbine runner and is located either above the rotor (“suspended unit”) or below the rotor (“umbrella unit”).

Thrust bearings are constructed of oil-lubricated, segmented, babbit-lined shoes. One or two oil lubricated generator guide bearings are used to restrain the radial movement of the shaft.

Fire protection systems are normally installed to detect combustion products in the generator enclosure, initiate rapid de-energization of the generator and release extinguishing material. Carbon dioxide and water are commonly used as the fire quenching medium.

Excessive unit vibrations may result from mechanical or magnetic unbalance. Vibration monitoring devices such as proximity probes to detect shaft run-out are provided to initiate alarms and unit shutdown.

The choice of generator inertia is an important consideration in the design of a hydroelectric plant. The speed rise of the turbine-generator unit under load rejection conditions, caused by the instantaneous disconnection of electrical load, is inversely proportional to the combined inertia of the generator and turbine.

Turbine inertia is normally about 5% of the generator inertia. During design of the plant, unit inertia, effective wicket gate or nozzle closing and opening times, and penstock dimensions are optimized to control the pressure fluctuations in the penstock and speed variations of the turbine-generator during load rejection and load acceptance.

Speed variations may be reduced by increasing the generator inertia at added cost. Inertia can be added by increasing the mass of the generator, adjusting the rotor diameter, or by adding a flywheel. The unit inertia also has a significant effect on the transient stability of the electrical system, as this factor influences the rate at which energy can be moved in or out of the generator to control the rotor angle acceleration during system fault conditions.

TESTING OF AC GENERATORS BASIC AND TUTORIALS


Tests are performed on generators to establish conformance with projected performance and dynamic performance parameters. Details of such tests are contained in IEEE Standard 115, IEC 60034-2 and IEC 60034-4 standards.

Resistance
Field and armature resistances are typically small, so measurements should be made using a 4-wire technique. It is important that resistance be measured at a known temperature so that correction can be made to actual operating temperature.

Open-Circuit Saturation Curve
The generator is driven by a motor to rated speed and excitation varied to produce terminal voltage over a range, typically from perhaps 30% to 120% that of rated. Some caution is required here, particularly for large machines in which excessive flux can damage the core. Open-circuit losses may be established by this test if the drive motor is well characterized and input power is measured.

Short-Circuit Saturation Curve
This test is similar to the open-circuit test, except the armature terminals are short-circuited and excitation varied to produce armature current over some convenient range. Windage and friction losses may be inferred from power input at zero excitation.

Stray load loss may be estimated as the difference between input power at rated armature current and the sum of friction and windage and armature I2R.

Zero Power Factor Saturation Curve
For a relatively small generator, the zero power factor saturation curve can be determined by running the machine with its shaft unloaded, driven by a second generator. By adjusting the excitation on the ac generator under test and excitation on the second generator, it is possible to measure the zero power factor saturation curve.

Rather extensive discussion of this method is described in IEEE 115. For large generators for which this “back-to-back” method is not practical, the zero power factor curve is usually determined by numerical methods. Often those methods employ finite elements.

Deceleration
Deceleration may be used for determining losses if the inertia of the machine is known. Since, if the shaft of a machine is unloaded, power dissipated is


where wm is mechanical speed, deceleration through synchronous speed can give a good measure of dissipation. The test may be run with the machine operating either at open-circuit or short-circuit conditions, or at zero excitation. It is usually run from a slight overspeed.

This test can be used to determine an unknown inertia from known losses and observed deceleration.

Heat Runs
These are tests performed by operating the generator at some condition until the temperature stabilizes. Heat runs at open-circuit, short-circuit, and zero power factor may be combined to estimate temperature rise in actual operation.

In large machines, good estimates of dissipation may be made by measuring the temperature rise of coolant (e.g., water). This is an alternative or supplement to measuring input power to the drive motor or machine deceleration. wm

STATOR AND ROTOR CONSTRUCTION OF AC GENERATORS


Stator Construction


Armature cores are built up of thin laminations, produced as either segments or complete rings, depending on the size of the generator. Successive layers or groups of layers of the segmented laminations are staggered to minimize the effect of the joints in the magnetic circuit.

The core is clamped between pressure plates and fingers to support it with sufficient pressure to prevent undue vibration of the laminations. Especially in long cores, the clamping arrangement may include some provision to compensate for compacting of the core after initial assembly.

The armature windings are fitted tightly in the slots and secured radially by wedges driven into suitable notches at the air gap end of the slots. It is necessary that the stator coil ends be able to resist the abnormal forces associated with short circuits.

A supporting structure may be employed for this purpose. There are many variations of support design; most of them provide filler blocks between the coil sides, strategically located to transmit the circumferential forces from coil to coil, and additional structure to counteract the radial forces.

Coil supports ordinarily are designed to suit the need of a particular machine. Large 2-pole machines require a quite elaborate structure; the combination of large short-circuit currents and coil ends inherently flexible because of their long length makes these machines particularly susceptible to coil-end movement.

Low-speed machines with stiffer coil ends require less support; in the smallest ratings the coils may be capable of withstanding the short-circuit sources without any additional support.

Stator frames, sometimes called casings, are commonly fabricated from structural steel, designed to support the core in alignment with the rotor and to suit the ventilating scheme used. In large machines with 2-pole or sometimes 4-pole construction, the stator core is mounted on springs to isolate core vibration from the machine frame.

Rotor Construction


The pole pieces of salient-pole alternators may be built up of steel laminations, both as manufacturing convenience and a means of limiting the loss in their air gap surfaces due to pulsations in air gap flux. The field coils, wound directly on the poles or preformed and then mounted on the poles, are suitably insulated from the poles for the voltages associated with normal and transient operation.

The pole-and-coil assembly is bolted, dovetailed, or otherwise attached to the rotor body. It is the limitation of this attachment which usually dictates when round-rotor construction must be used rather than salient-pole construction.

The rotor body for a salient-pole machine may be a solid forging or assembly of heavy steel plates, for high speed designs, or a spider-and-rim assembly for low-speed designs. The shaft may be integral with the body, as in the case of a forging, or may be bolted to or inserted into the body.

When the spider-and-rim construction is used, the entire assembly may be an integral weldment or casting, or the rim may be separate from the spider, as in the case of large waterwheel-driven generators.

A common construction for this latter case is a rim built up of thin steel laminations, assembled around a cast or fabricated spider, bolted together between steel end plates and keyed to the spider.

The rotor of a round-rotor machine is cylindrical in shape with axial slots provided in its body for the field coils. The body is usually a steel forging with integral shaft ends. In special applications, other constructions may be used, with this same general configuration.

The field coils are wound in axial slots in the rotor body, held in place by heavy slot wedges and by retaining rings over the coil ends.

Rotors are designed for operation at overspeeds, which depend on the characteristics of the prime mover. The overspeed limit (the speed above which the unit is no longer capable of safe operation) may be as low as 20% for a steam-turbine-driven unit or as high as 125% for some adjustable-blade, axial-flow hydraulic turbine-driven units.

STRANDING AND TRANSPOSITION OF AC GENERATORS ARMATURE WINDINGS BASIC AND TUTORIALS

A wide variety of winding types may be used to produce a desired voltage with the desired number of phases and a suitable waveshape. In small generators, “scramble wound” armature windings may be used.

However, in most alternator applications, double-layer, form-wound coils in open slots with 60° phase belts are used. In such a winding, each slot has two conductor bars (often called halfcoils), not necessarily from the same phase winding.

These bars are insulated from ground and secured in the slot, usually by wedges. It is usually necessary for the bar to have the ability to slide axially in the slot to accommodate thermal expansion, but it must not be loose in either the radial or azimuthal directions. This has led to a number of proprietary techniques for armature construction.

Winding Forms
Figure 7-20 shows an example winding diagram. For the purposes of this figure, the machine is shown “rolled out flat,” with the dotted lines on either side representing the same azimuthal location. In this case, the machine has 24 slots, each with two half-coils, as shown in the slot allocation section of the drawing, at the bottom of the figure.

FIGURE 7-20 Armature in 24 slots, 5/16 pitch.

The upper part of the figure shows how one phase of the winding would be laid out. This drawing shows a lap type winding (the most commonly used) with a 5/6 pitch. In a 24-slot, 2-pole winding a full-pitch coil would span 12 slots, while in the 5/6 pitch winding the coils span 10 slots.

Fractional Slot Windings.
Fractional slot windings, in which the number of slots per pole per phase is not an integer, have coil groups that differ from one another. These can be arranged to produce balanced voltages under circumstances that are beyond the scope of this discussion.

Stranding and Transposition
At power frequencies (50 or 60 Hz), the skin depth in copper is on the order of 1 cm so that it is usually necessary to subdivide armature conductors into a number of parallel strands. In form-wound coils, these strands are usually rectangular to allow for good space factor.

To prevent circulating currents between parallel strands, it is necessary to employ transposition to ensure that voltages induced in each strand are approximately the same.

The simplest form of transposition, often used in transformers and sometimes in generators, is to twist the armature conductors at 180° in the end turns. Or sometimes, groups of conductors are connected together in the end turns with a progressive transposition that constitutes a “twist” of the winding from half-coil to half-coil.

Transposition of strands in the end turns is generally not satisfactory in large ac generators. A transposition scheme attributed to Roebel is usually used (see Fig. 7-21). The Roebel transposition is equivalent to a twist of the conductors in the slot. It is usual to carry out the Roebel transposition only within the slot part of the winding.


FIGURE 7-21 Illustration of Roebel transposition: (a) typical offset conductor strand; (b) group of conductor strands composing half the conductor; (c) complementary group; (d) assembly.

A variety of transpositions are used, including 180°, 360°, and 540°. The first two are effective only at eliminating circulating currents due to flux crossing the slot, but do not compensate for flux in the end windings.

The 540° transposition, attributed to Ringland and Rosenberg (1959) is often applied because it filters out most of the circulating currents in a bar. Other more complex Roebel transposition arrangements that more extensively filter out circulating currents are possible but are rarely used because of manufacturing complexity.

AC GENERATOR MAGNETIC CIRCUIT AND MATERIAL BASIC AND TUTORIALS


The magnetic circuit of an ac generator, as with other electric machines, is made up of the air gap, the stator teeth and backiron, the rotor poles, and the shaft section. Each of these elements has an effect on machine rating and operation. The function of the magnetic circuit is to carry flux that links the armature conductors to produce voltage.

Air gap.
The air gap constitutes the division between the rotating part of the machine—the rotor, which carries the field winding—and the stationary part of the machine—the stator, which carries the armature winding. In ac generators, the air-gap dimension is determined by the electrical characteristics of the machine.

There is a trade-off between excitation mmf (toward a small air-gap dimension) and armature reaction flux (toward a large air-gap dimension). This trade-off generally results in an air gap, which is substantially larger than mechanical considerations such as machining tolerances or windage loss would dictate.

Stator Teeth and Backiron.
The armature magnetic circuit carries alternating flux and is always laminated, either with complete ring laminations (for small machines) or with overlapping segmented laminations. The material most commonly used is sheet steel, of an alloy containing about 3.5% silicon, in sheets of thickness between about 0.35 and 0.65 mm.

Grain-oriented steel, with reduced losses and improved permeability in the direction of rolling, is often used in large turbogenerators. Orientation in the circumferential direction is advantageous in such machines because of the large proportion of steel and moderate flux densities in the backiron.

At high flux densities characteristic of the armature teeth, the advantage of grain orientation becomes less pronounced.

The active region of the armature constitutes the alternation of stator teeth and slots carrying the armature winding. The division between teeth and slots is a compromise between flux-carrying capability and current-carrying capability.

The trade-off generally results in a division that is about half slots and half teeth. Flux densities in the stator teeth are usually high enough to result in moderate saturation of the magnetic material.

Rotor Iron.
The magnetic flux in the rotor is nearly constant, varying in the main only slightly with changes in load and terminal voltage and with small higher frequency components due to time and space harmonics of armature flux.

This allows the rotor magnetic circuit to be made of solid steel. In turbine generators, the rotor is typically made of a single-piece forging of steel with slots for the field winding cut by machining.

The losses caused by harmonic driven eddy currents in the solid steel pole faces can be problematic, and are reduced by making the air gap larger, by increasing the number of stator slots and, by choosing a suitable (short-pitch) coil throw for the armature.

Salient-pole machines may have solid or laminated poles. In many cases, laminated poles are necessary to control eddy current losses. Pole laminations are commonly made of low carbon steel, 1.5 to 2 mm thick.

Thinner steel, sometimes with silicon content, may be used where further control of eddy current losses is required. The shaft, or inner portion of the rotor of salient-pole machines, is often a solid forging, or in large machines such as hydroelectric generators may be fabricated from structural steel pieces.

Magnetic Materials. 
Typical magnetization characteristics of steel materials used in the magnetic circuit of ac generators are shown in Fig. 7-14.

Fig 7-14 Magnetization curves of commonly used steels.

POLES AND FREQUENCY OF ALTERNATING CURRENT (AC) GENERATORS BASIC AND TUTORIALS


The rotor and stator (field and armature) of a synchronous machine must have the same number of poles, as the magnetic interaction is between a succession of north-south magnetic-field pole pairs. The number of pole pairs for a machine will be noted as p.

The relationship between electrical frequency fe and mechanical speed N is

fe = pN/60

where P is number of poles (not number of pole pairs).

Synchronous generators are built in two elementary forms:

• Round-rotor machines are constructed with a rotor consisting of a cylinder of magnetic steel. In modern generators, the cylinder is formed from a single forging of vacuum degassed steel. The field winding is contained in radial slots in the surface of the rotor.

Round-rotor machines usually have two or four poles as illustrated in Figs. 7-2 and 7-3 respectively. The diameter of the rotor of a typical 25-MW generator is about 700 mm. The diameter of a 2000-MW generator can approach 2 m.

Figure 7.2 Round-rotor generator with two poles.

• Salient-pole machines are constructed with a number of pole pieces mounted to a central rotor shaft. The rotor pole pieces can be solid steel or assemblies of steel plates that are bound together axially with bolts.

The diameter of the rotor can range from less than 1 m in smaller salient pole generators to nearly 20 m in the largest hydroelectric generators.

In both round-rotor and salient-pole generators, the magnetic flux passing through the rotors does not vary in time, and the magnetic flux passing through the stator core does vary periodically in time at the electrical line frequency.

Figure 7.3 4-pole generator (left is round rotor, right is salient pole.

Consequently, the rotors can be made of solid steel, but the stator cores must be made of thousands of thin layers of highly permeable electrical steel. Each layer of stator core steel is coated with a thin layer of electrical insulation.

For electric utility operation, in which generation takes place at 50 or 60 Hz, mechanical speed is inversely proportional to the number of poles. Thus, 2-pole machines, which turn at 3000 or 3600 r/min, are used for most fossil-(fuel)-fired steam turbine generators which require high shaft speeds.

Most nuclear steam turbine generators, which have a lower shaft speed requirement, employ 4-pole designs and therefore turn at 1500 or 1800 r/min. Turbine generators for both fossil and nuclear power plants are typically round-rotor designs.

Hydroelectric generators, which typically have much lower shaft speeds than turbine generators and consequently require a large number of poles, are generally built as salient-pole machines. This is true also for generators intended for operation with large reciprocating engines, such as medium-speed diesels.

PARALLEL OPERATION OF GENERATORS BASIC AND TUTORIALS

Parallel operation of shunt generators.
Shunt-wound generators will in general operate very well in parallel and will divide the load well if the machines have similar characteristics. If the machines do not have similar characteristics, one machine will take more than its share of the load and may tend to drive the other as a motor.

When this machine is running as a motor, its direction of rotation will be the same as when it was generating; hence the operator must watch the ammeters closely for an indication of this trouble. Shunt generators are now seldom installed. Figure below shows the connections for shunt generators that are to be operated in parallel.



Parallel operation of compound-wound generators.
It is readily effected if the machines are of the same make and voltage or are designed with similar electrical characteristics (Westinghouse Electric Corp.) The only change that is usually required is the addition of an equalizer connection between machines.

If the generators have different compounding ratios, it may be necessary to adjust the series-field shunts to obtain uniform conditions.

Operation of a shunt and a compound dynamo in parallel is not successful because the compound machine will take more than its share of the load unless the shunt-machine field rheostat is adjusted at each change in load.

Shutting down a shunt- or compound-wound generator operating in parallel with others.
(1) Reduce the load on the machine as much as possible by cutting resistance into the shunt-field circuit with the field rheostat.

(2) Throw off the load by opening the circuit breaker if one is used; otherwise open the main generator switches.

(3) Shut down the driving machine.

(4) Wipe off all oil and dirt, clean the machine, and put it in good order for the next run. Turn on all resistance in the field rheostat. Open the main switch.

Adjusting the division of load between two compound-wound generators.
First adjust the series shunts of both machines so that, as nearly as possible, the voltages of both will be the same at one-fourth, one-half, three-fourths, and full load. Then connect the machines in parallel, for trial.

If, upon loading, one machine takes more than its share of the load (amperes), increase the resistance of the path through its series-field coil path until the load divides between the machines proportionally to their capacities.

Only a small increase in resistance is usually needed. The increase can be provided by inserting a longer conductor between the generator and the busbar, or iron or nickel silver washers can be inserted under a connection lug.

Inasmuch as adjustment of the series-coil shunt affects both machines when the machines are connected in parallel, nothing can be accomplished through making such an adjustment.

INDUCTION GENERATORS - GENERAL CHARACTERISTICS BASIC AND TUTORIALS


An induction or asynchronous generator is one that operates without an independent source for its rotor field current, but in which the rotor field current appears by electromagnetic induction from the field of the armature current.

 The rotor field then interacts with the stator field to transmit mechanical torque just as it does in a synchronous generator, regardless of the fact that it was the stator field that created it (the rotor field) in the first place.

This may seem reminiscent of pulling yourself up by your own bootstraps, but it does actually work. The catch is that some armature current must be provided externally; thus, an induction generator cannot be started up without being connected to a live a.c. system. Another practical concern is that, as we show later in this chapter, induction generators can only operate at leading power factors. For both reasons, their use is quite limited.

Their one important application in power systems is in association with wind turbines. In this case, induction generators offer an advantage because they can readily absorb the erratic fluctuations of mechanical power delivered by the wind resource.

They also cost less than synchronous machines, especially in the size range up to one megawatt. In terms of mechanical operation, the most important characteristic of the induction generator is that the rate of rotation is not fixed, as in the case of the synchronous generator, but varies depending on the torque or power delivered.

The reference point is called the synchronous speed, which is the speed of rotation of the armature magnetic field (corresponding to the a.c. frequency) and also the speed at which a synchronous rotor would spin. The more power is being generated, the faster the induction rotor spins in relation to the synchronous speed; the difference is called the slip speed and typically amounts to several percent.

The rotor may also spin more slowly than the armature speed, but in this case, the machine is generating negative power: it is operating as a motor! While induction machines are usually optimized and marketed for only one purpose, either generating or motoring, they are all in principle reversible. (The same is true for synchronous machines, though their design tends to be even more specialized.)

Figure below shows a curve of torque versus slip speed for a generic induction machine. Zero slip corresponds to synchronous speed, and at this point, the machine delivers no power at all: neglecting friction, it spins freely in equilibrium.


This is called a no-load condition. If a forward torque is exerted on the rotor in this equilibrium state (say, by a connected turbine), it accelerates beyond synchronous speed and generates electric power by boosting the terminal voltage. If the rotor is instead restrained (by a mechanical load), it slows down below synchronous speed and the machine is operating as a motor.

Now we call the torque on the rotor negative, and it acts to push whatever is restraining it with power derived from the armature current and voltage.

The synchronous speed of a given induction machine may be equal to the a.c. frequency (3600 rpm for 60 Hz; 3000 rpm for 50 Hz) or some even fraction thereof (such as 900 or 1800 rpm), depending on the number of magnetic poles, which in this case are created by the armature conductor windings instead of the rotor.

Note that unlike the synchronous generator, where the stator magnetic field has two poles but the rotor field may have any even number of poles, an induction generator must have the same number of poles in the rotor and stator field (because there is no independent excitation).

GENERATOR REAL POWER PRODUCTION BASIC AND TUTORIALS


HOW REAL POWER IS PRODUCED BY GENERATOR?

Real power output is controlled through the force or torque exerted by the prime mover, for example, the steam turbine driving the generator rotor. Intuitively, this is straightforward: if more electrical power is to be provided, then something must push harder.

The rotor’s rate of rotation has to be understood as an equilibrium between two opposing forces: the torque exerted by the turbine, which tends to speed up the rotor, and the torque exerted in the opposite direction by the magnetic field inside the generator, which tends to slow it down. The slowing down is directly related to the electric power being supplied by the generator to the grid.

This is because the magnetic field that provides the retarding effect (the armature reaction) is directly proportional to the current in the armature windings, while the same current also determines the amount of power transmitted.

For example, if the load on the generator suddenly increases (someone is turning on another appliance), this means a reduction in the load’s impedance, resulting in an increased current in the armature windings, and the magnetic field associated with this increased current would slow down the generator. In order to maintain a constant rotational frequency of the generator, the turbine must now supply an additional torque to match.

Conversely, if the load is suddenly reduced, the armature current and thus its magnetic field decreases, and the generator would speed up. To maintain equilibrium, the turbine must now push less hard so that the torques are equal and the rotational frequency stabilizes.

The torque supplied by the prime mover is adjusted by a governor valve (Figure 4.10). In the case of a steam turbine, this increases or decreases the steam flow; for a hydro turbine, it adjusts the water flow. This main valve can be operated manually (i.e., by deliberate operator action) or, as is general practice, by an automated control system.


In any situation where a generator must respond to load fluctuations, either because it is the only one in a small system or because it is designated as a load-following generator in a large power system, automatic governor control will be used; in this case, the generator is said to operate “on the governor.”

The automatic governor system includes some device that continually monitors the generator frequency. Any departure from the set point (e.g., 3600 rpm) is translated into a signal to the main valve to open or close by an appropriate amount.

Alternatively, a generator may be operated at a fixed level of power output (i.e., a fixed amount of steam flow), which would typically correspond to its maximum load (as for a baseload plant); in this case, the generator is said to operate “on the load limit.”

Various designs for governor systems are in use. Older ones may rely on a simple mechanical feedback mechanism such as a flywheel that expands with increasing rotational speed due to centrifugal force, which is then mechanically connected to the valve operating components.

Newer designs are based on solid-state technology and digitally programmed, providing the ability to govern based on not just the frequency measured in real time but its time rate of change (i.e., the slope). This allows anticipation of changes and more rapid adjustment, so that the actual generator frequency ultimately undergoes much smaller excursions.

In any case, such a governor system allows the generator to follow loads within the range of the prime mover’s capability, and without direct need for operator intervention.

LORENTZ FORCE LAW BASIC DEFINITION AND TUTORIALS


The Lorentz Force Law


F = q(E + v × B) (3.1), gives the force F on a particle of charge q in the presence of electric and magnetic fields. In SI units, F is in newtons, q in coulombs, E in volts per meter, B in teslas, and v, which is the velocity of the particle relative to the magnetic field, in meters per second.

Thus, in a pure electric-field system, the force is determined simply by the charge on the particle and the electric field

F = qE (1)

The force acts in the direction of the electric field and is independent of any particle motion. In pure magnetic-field systems, the situation is somewhat more complex. Here the force

F = q(v × B) (2)

is determined by the magnitude of the charge on the particle and the magnitude of the B field as well as the velocity of the particle. In fact, the direction of the force is always perpendicular to the direction of both the particle motion and that of the magnetic field. Mathematically, this is indicated by the vector cross product v× B in Eq. 2.

The magnitude of this cross product is equal to the product of the magnitudes of v and B and the sine of the angle between them; its direction can be found from the right-hand rule, which states that when the thumb of the fight hand points in the direction of v and the index finger points in the direction of B, the force, which is perpendicular to the directions of both B and v, points in the direction normal to the palm of the hand, as shown in Fig below.


For situations where large numbers of charged particles are in motion, it is convenient to rewrite Eq. 1 in terms of the charge density p (measured in units of coulombs per cubic meter) as

Fv = p(E + v × B) (3)

where the subscript v indicates that Fv is a force density (force per unit volume) which in SI units is measured in newtons per cubic meter. The product p v is known as the current density

J = pv

which has the units of amperes per square meter. The magnetic-system force density corresponding to Eq. 3 can then be written as

Fv = J x B (3.6)

For currents flowing in conducting media, Eq. 3.6 can be used to find the force density acting on the material itself. Note that a considerable amount of physics is hidden in this seemingly simple statement, since the mechanism by which the force is transferred from the moving charges to the conducting medium is a complex one.

SHUNT WOUND GENERATOR BASIC AND TUTORIALS

WHAT ARE SHUNT WOUND GENERATORS?


The shunt-wound generator is shown diagrammatically in Figure below. A small part of the total current, the exciting current, is shunted through the fields. The exciting current varies from possibly 5 percent of the total current in small machines to 1 percent in large ones.


The exciting current is determined by the voltage at the brushes and the resistance of the field winding. Residual magnetism in the field cores permits a shunt generator to “build-up.” This small amount of magnetism that is retained in the field cores induces a voltage in the armature (William H. Timbie, Elements of Electricity).

This voltage sends a slight current through the field coils, which increases the magnetization. Thus, the induced voltage in the armature is increased. This in turn increases the current in the fields, which still further increases the magnetization, and so on, until the normal voltage of the machine is reached and conditions are stable. This “building-up” action is the same for any self-excited generator and often requires 20 to 30 s.

If a shunt generator (Timbie) runs at a constant speed, as more and more current is drawn from the generator, the voltage across the brushes fails slightly. This fall is due to the act that more and more of the generated voltage is required to force the increasing current through the windings of the armature; i.e., the armature IR drop increases.

This leaves a smaller part of the total emf for brush emf, and when the brush voltage falls, there is a slight decrease in the field current, which is determined by the brush voltage. This and armature reactions cause the total emf to drop a little, which still further lowers the brush potential. These causes combine to lower the voltage gradually, especially at heavy overloads.

The curve in Fig.III, shows these characteristics. For small loads the curves is nearly horizontal, but at heavy overloads it shows a decided drop. The point at which the voltage of a commercial machine drops off rapidly is beyond the operating range and is of importance only for short-circuit conditions.

The voltage of a shunt machine can be kept fairly constant by providing extra resistance in the field circuit, which may be cut out as the brush potential falls.


This will allow more current to flow through the field coils and increase the number of magnetic lines set up in the magnetic circuit. If the speed is kept constant, the armature conductors cut through the stronger magnetic field at the same speed and thus induce a greater emf and restore the brush potential to its former value. This resistance can be cut out either automatically or by hand.

Shunt-wound generators give a fairly constant voltage, even with varying loads, and can be used for any system which incorporates constant-potential loads. This will operate well in parallel because the voltage of the machines decreases as the load increases. Shunt generators running in parallel will divide the load well between themselves if the machines have similar characteristics.

The necessary change in connections when reversing the direction of rotation of a shunt wound machine is indicated in figure below. Rotation is clockwise when, facing the commutator end of a machine, the rotation is in the direction of the hands of a clock.


Counterclockwise rotation is the reverse. When changing the direction of rotation, do not reverse the direction of current through the field windings. If the direction is reversed, the magnetism developed by the windings on starting will oppose the residual magnetism and the machine will not “build up.”

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