GENERATOR CLOSING UNTO DEAD BUS BASIC TUTORIALS

Closing onto a Dead Bus with Leading PF Load

It is possible to have a power system configuration where a bus might have capacitive loading.

• Static capacitors connected to it.

• Energizing a long high voltage transmission line. Note HV lines inherently appear like capacitors, which are able to supply MVARs.

In the capacitive loading situations the generator would have to absorb these MVARs. If the Automatic Voltage Regulator is in the Auto mode, the generator excitation is automatically decreased to cause the generator to take in the required MVARs and to hold the terminal voltage.

If the Automatic Voltage Regulator is in the manual mode, the excitation is constant and the leading power factor current which is required for the generator to take in MVARs could cause the generator terminal voltage to go very high.

Closing onto a Dead Bus with Lagging PF Load

Inductive loading can take the form of:

• Connected power transformers

• Motor Loads

Inductive loading will cause a significant voltage drop when the generator breaker is closed, due to the load absorbing MVARS.

Closing onto a Faulted Bus

Closing the generator output breakers onto a bus, which has a short circuit fault, can cause generator damage because of high winding currents, stresses and possible pole slipping.

Closing onto a Dead Bus with no Connected Loads

This should not present a problem as long as the bus has been proven to be free of faults or working grounds.

GENERATOR LOADING

Closing onto a Finite vs Infinite System

When we enter into the topic of generator loading we must consider whether or not the connected electrical system is very large and hence strong or smaller and weaker. The first is classed as infinite and the second finite.

A generator connected to a very large (infinite bus) electrical system will have little or no effect on its voltage or frequency. In contrast, a generator connected to a finite bus does have a substantial effect on voltage and frequency.

It is normally assumed that when a generator has a capacity of greater than 5% of the system size, then with respect to this generator, the system does not behave as an infinite bus. For example, when an 800 MW generator is loaded onto a grid having a capacity of l0,000 MW, the system voltage and frequency can vary and the system will behave

THERMAL GENERATING PLANTS CONSTRUCTION PROJECT BASIC INFORMATION AND TUTORIALS

Thermal generating plants are designed and constructed to convert energy from fuel (coal, oil, gas, or radiation) into electric power. The actual conversion is accomplished by a turbine-driven generator.

Thermal generating plants differ from industrial plants in that the nature of the product never changes. The plant will always produce electric energy. The things that may change are the fuel used (coal, oil, or gas) and environmental requirements.

Many plants that were originally designed for coal were later converted to oil, converted back to coal, and then converted to gas. Environmental requirements have changed, which has required the construction of air and water emissions control systems.

Plant electrical systems should be designed to allow for further growth. Sizing of transformers and buses is at best a matter of guesswork. The plant electrical system should be sized at 5 to 10% the size of the generating unit depending on the plant configuration and number of units at the plant site.

Plant Auxiliary System

Selection of Auxiliary System Voltages

The most common plant auxiliary system voltages are 13,800 V, 6900 V, 4160 V, 2400 V, and 480 V. The highest voltage is determined by the largest motor. If motors of 4000 hp or larger are required, one should consider using 13,800 V. If the largest motor required is less than 4000 hp, then 4160 V should be satisfactory.

Auxiliary System Loads

Auxiliary load consists of motors and transformers. Transformers supply lower level buses which supply smaller motors and transformers which supply lower voltage buses. Generation plants built before 1950 may have an auxiliary generator that is connected to the main generator shaft. The auxiliary generator will supply plant loads when the plant is up and running.

Auxiliary System Power Sources

The power sources for a generating plant consist of one or more off-site sources and one or more onsite sources. The on-site sources are the generator and, in some cases, a black start diesel generator or a gas turbine generator which may be used as a peaker.

Auxiliary System Voltage Regulation Requirements

Most plants will not require voltage regulation. A load flow study will indicate if voltage regulation is required. Transformers with tap changers, static var compensators, or induction regulators may be used to keep plant bus voltages within acceptable limits. Switched capacitor banks and overexcited synchronous motors may also be used to regulate bus voltage.

Plant One-Line Diagram

The one-line diagram is the most important document you will use. Start with a conceptual one-line and add detail as it becomes available. The one-line diagram will help you think about your design and make it easier to discuss with others.

Do not be afraid to get something on paper very early and modify as you get more information about the design. Consider how the plant will be operated. Will there be a start-up source and a running source? Are there on-site power sources?

Plant Equipment Voltage Ratings

Establish at least one bus for each voltage rating in the plant. Two or more buses may be required depending on how the plant will be operated.

Grounded vs. Ungrounded Systems

A method of grounding must be determined for each voltage level in the plant.

Ungrounded

Most systems will be grounded in some manner with the exception for special cases of 120-V control systems which may be operated ungrounded for reliability reasons. An ungrounded system may be allowed to continue to operate with a single ground on the system. Ungrounded systems are undesirable because ground faults are difficult to locate. Also, ground faults can result in system overvoltage, which can damage equipment that is connected to the ungrounded system.

Grounded

Most systems 480 V and lower will be solidly grounded.

Low-Resistance Grounding

Low-resistance grounding systems are used at 2400 V and above. This system provides enough ground fault current to allow relay coordination and limits ground fault current to a value low enough to prevent equipment damage.

High-Resistance Grounding

High-resistance grounding systems limit ground fault current to a very low value but make relay coordination for ground faults difficult.

HYDROELECTRIC POWER GENERATION BASIC INFORMATION AND TUTORIALS

Hydroelectric power generation involves the storage of a hydraulic fluid, normally water, conversion of the hydraulic energy of the fluid into mechanical energy in a hydraulic turbine, and conversion of the mechanical energy to electrical energy in an electric generator.

The first hydroelectric power plants came into service in the 1880s and now comprise approximately 22% (660 GW) of the world’s installed generation capacity of 3000 GW (Electric Power Research Institute, 1999).

Hydroelectricity is an important source of renewable energy and provides significant flexibility in base loading, peaking, and energy storage applications.

While initial capital costs are high, the inherent simplicity of hydroelectric plants, coupled with their low operating and maintenance costs, long service life, and high reliability, make them a very cost effective and flexible source of electricity generation.

Especially valuable is their operating characteristic of fast response for start-up, loading, unloading, and following of system load variations. Other useful features include their ability to start without the availability of power system voltage (“black start capability”), ability to transfer rapidly from generation mode to synchronous condenser mode, and pumped storage application.

Hydroelectric units have been installed in capacities ranging from a few kilowatts to nearly 1 GW. Multi-unit plant sizes range from a few kilowatts to a maximum of 18 GW.

Planning of Hydroelectric Facilities

Siting

Hydroelectric plants are located in geographic areas where they will make economic use of hydraulic energy sources. Hydraulic energy is available wherever there is a flow of liquid and head. Head represents potential energy and is the vertical distance through which the fluid falls in the energy conversion process.

The majority of sites utilize the head developed by fresh water; however, other liquids such as salt water and treated sewage have been utilized. The siting of a prospective hydroelectric plant requires careful evaluation of technical, economic, environmental, and social factors.

A significant portion of the project cost may be required for mitigation of environmental effects on fish and wildlife and re location of infrastructure and population from flood plains.

Hydroelectric Plant Schemes

There are three main types of hydroelectric plant arrangements, classified according to the method of controlling the hydraulic flow at the site:

1. Run-of-the-river plants, having small amounts of water storage and thus little control of the flow through the plant.

2. Storage plants, having the ability to store water and thus control the flow through the plant on a daily or seasonal basis.

3. Pumped storage plants, in which the direction of rotation of the turbines is reversed during offpeak hours, pumping water from a lower reservoir to an upper reservoir, thus “storing energy” for later production of electricity during peak hours.

STEAM TURBINE LOSSES BASIC INFORMATION

Clearance leakage. A 100% efficiency cannot be obtained because of friction in the blading and clearance between the stationary and rotating parts, and because the nozzle angle cannot be zero degrees.

Axial clearance increases in the stages further from the thrust bearing to satisfy the need to maintain a minimum clearance at extreme operating conditions when the differential expansion between the light rotor and heavy casing is at its worst.

To reduce this leakage, radial spillbands are used. These thin, metal-strip seals may be attached to the diaphragm or casing and extend close to the shroud bands covering the rotating blades.

This clearance can be kept quite close (0.020 to 0.060 in), and axial changes in the rotor position do not affect the clearance since the important on reaction stages with small blade heights because of the pressure drop across the moving blades.

Nozzle leakage. Leakage around the nozzles between the bore of the blade ring or nozzle diaphragm and the drum or rotor must be kept to a minimum.

This leakage is controlled through the use of a metallic labyrinth packing which consists of a single ring with multiple teeth arranged to change the direction of the steam as well as to minimize the leakage area.

Labyrinth packings are also used at the shaft ends to step the pressure down at the high-pressure end and to seal the shaft at the vacuum end.

Rotation loss. Rotation of the rotor consist of losses due to the rotation of the disks, the blades, and shrouds. Partial-arc impulse stages have a greater windage loss within the idle buckets. Rotation losses vary directly with the steam density, the fifth power of the pitch diameter, and the third power of the rpm.

In general, the windage loss amounts to less than 1% of stage output at normal rated output. At no-load conditions, windage loss for noncondensing turbines approximates 1.5% of the rating per 100 lb/in2 exhaust pressure, and on condensing units approximates from 0.4% to 1.0% of the rating at 1.5 inHg (abs) exhaust pressure.

Carryover loss. A carryover loss (about 3%) occurs on certain stages when the kinetic energy of the steam leaving the rotating blades cannot be recovered by the following stage because of a difference in stage diameters or a large axial space between adjacent stages.

Typically, this happens in control stages and in the last stages of noncondensing sections. The last stages of condensing turbines have the largest carryover losses (normally referred to as exhaust loss) because of the large variations in exhaust volumetric flow with exhaust pressure and the large variation of stage pressure ratio with load.

Stages preceding the last operate with essentially a constant pressure ratio down to very low loads and consequently can be designed for peak efficiency at a wide range of loads.

Leaving loss. Condensing turbines are frequently “frame sized” by last-stage blade height. It is sometimes economical to size the unit with exhaust loss equal to 5% deterioration in overall turbine performance at the design point (valves wide-open throttle flow and 1.5 inHg [abs] exhaust pressure) when the normal expected exhaust pressure will be higher or the unit will be operating at part load for a large part of the time.

Nozzle end loss, partial arc. Control stages and partial-arc impulse stages are subject to end losses at the interface of the active and inactive portions of the blading as the stagnant steam within the idle bucket passages enters the active arc of nozzles and must be accelerated.

There is also a greater turbulence in the steam jet at both ends of the active arc. In partial-arc impulse stages, the increase in efficiency due to larger blade heights (aspect ratio) is partially offset by increased rotation and end losses, and there is an optimum to this proportioning beyond which there is an overall loss.

Supersaturation and moisture loss. Moisture in the steam causes supersaturation and moisture losses in the stage. The acceleration of the moisture particles is less than that of the steam, causing a momentum loss as the steam strikes the particles.

The moisture particles enter the moving blades (buckets) at a negative velocity relative to the blades, resulting in a braking force on the back of the blades. Supersaturation is a temporary state of supercooling as the steam is rapidly expanded from a superheated state to the wet region before any condensation has begun.

The density is greater than when in equilibrium, resulting in a lower velocity as the steam leaves the nozzle. As soon as some condensation occurs at approximately 3.5% moisture, according to Yellot, a state of equilibrium is almost instantly achieved and supersaturation ceases.

NUCLEAR POWER PLANT EMERGENCY POWER SYSTEMS BASIC INFORMATION

Because of the need for protective action to be available at all times when the reactor is operating and the need for continued cooling and monitoring when the reactor is shut down, systems must be provided to assure high availability of electric power.

Primary Coolant Circulators

The largest single plant load is the drives for primary coolant circulation. Since it is important to maintain coolant circulation and since these drives are generally too large to be supplied by engine driven sources, provisions should be made to supply the coolant circulator drives from two or more sources.

Frequently, arrangements are made for the main generator to supply two or more power lines. Provisions in the switchyard enable the plant distribution system to be supplied from the plant generator or from one or more of the outside lines.

In spite of possible connection of plant loads to multiple external power sources, it is possible to lose all external lines, for instance, by a tornado. In this event, a local source of power to supply critical ac loads is required.

For these purposes, engine (diesel)-driven generators are usually used. Credit can sometimes be taken for local hydro generators or gas-turbine generators if these sources can meet the requirements. These power systems must be designed so that they provide power to the station following a design-basis event.

An ac power system (generation and distribution), a dc power system, and a vital instrumentation and control power system are provided. An example of a safety grade power system is shown in Fig. 5-21.

In the ac system, each of the redundant load groups must have access to both a preferred and a standby power supply. The units of the standby supply must have sufficient independence from the preferred supply and from one another to preclude a common failure mode.

Load assignment must be such that the safety actions of each group are redundant and independent. Protective devices must be provided to limit the degradation of the system and maintain the power quality (voltage and frequency) within acceptable limits.

Following a demand for the standby power supply, it must be available within a time consistent with the requirements of the engineered safeguards features and the shutdown systems. In the dc system, batteries, distribution equipment, and load groups are arranged to supply critical dc loads and switching and control power.

Redundant load groups, and corresponding battery sections, must be sufficiently independent to preclude common failure modes. Each of the redundant load groups must have access to one or more battery chargers; the batteries are to be kept charged.

The battery supplies must be sized to be able to start and operate their assigned loads in the expected loading sequence for a length of time commensurate with the protection provided.

Battery chargers supplying the redundant load groups must have sufficient capacity to restore the battery from its design minimum charge to its fully charged state while supplying normal and post accident loads.

Each charger supply must have a disconnecting device in its ac feeder and one in its dc output line. The dc system must be equipped with surveillance equipment to monitor its status and to indicate actions.

The vital instrument system is provided to power the instrumentation needed for reactor protection and engineered safety features. Since there may be considerable variation in the instrumentation in various plants, the vital system may be required to supply ac or dc or both.

To preserve freedom from common-mode failure, the vital supply must be divided into redundant and independent systems with adequate status indication. Provisions for testing, adjustment, and repair should be included in the parts of the emergency power systems to improve reliability and availability.

FLUE-GAS DESULFURIZATION SYSTEMS BASIC INFORMATION

Flue-gas desulfurization (FGD) began in England in 1935. The technology remained dormant until the mid-1960s when it became active primarily in the United States and Japan. Since then, over 50 FGD processes have been developed, differing in the chemical reagents and the resultant end products.

The most common FGD system is a lime/limestone wet scrubber. After the flue gas has been treated in the precipitation (or baghouse), it passes through the induced fans and enters the SO2 scrubber. If the required SO2 removal efficiency is less than 85%, a fraction of the flue gas can be treated while bypassing the rest to mix with and reheat the saturated flue gas leaving the scrubber.

For higher-sulfur fuels requiring SO2 removal efficiencies of 90% or greater, the entire flue-gas stream must be treated. Upon leaving the SO2 absorption section, the flue gas is passed through entrainment separators to remove any slurry droplets mixed with the gas.

The saturated flue gas is then reheated approximately 25 to 50°F above the water dewpoint before it is vented to the stack. For low- to medium-sulfur fuels, an alternate scrubbing technology is dry scrubbing.

This process minimizes water consumption and eliminates the requirement for flue-gas reheating but requires more expensive additives than the wet limestone systems. The typical dry SO2 absorber is a cocurrent classifying spray dryer.

Flue gas enters the top of the absorber through inlet assemblies containing swirl vanes. The absorbent is injected pneumatically into the center of each swirler assembly by ultrasonic atomizing nozzles that require an air pressure of about 60 lb/in2 (gage).

Slurry feed pressures are 10 to 15 lb/in2 (gage). The compressed air induces primary dispersion of the absorbent slurry by mechanical shear forces produced by the two fluid streams.

Final dispersion is accomplished by shattering the droplets with ultrasonic energy produced by the compressed air used with a proprietary nozzle design. Then ultrasonic nozzles generate extremely fine droplets, which have diameters that range from 10 to 50 #m, as shown by photographic studies.

The flue-gas outlet design requires that effluent gases make a 180° turn before leaving the absorber. Besides eliminating product accumulation in the outlet duct, the abrupt directional change also allows the larger particles to drop out in the absorber product hopper.

This design curtails the particulate loading to the fabric filter. Consequently, the number of cleaning cycles as well as abrasion of the filter medium are reduced.

As compared with ordinary fly-ash collection applications, fabric filters together with dry scrubbing offer a broader choice of design options. In conventional fly-ash collection applications, the fabric filter experiences flue-gas temperatures about 100 to 150°F higher than encountered in dry scrubbing.

Filter media unsuitable at the higher temperatures can be used when the fabric filter follows a dry absorber. In particular, acrylic fibers become attractive because of their strength and flex characteristics, as well as their ability to support more vigorous cleaning methods like mechanical shaking.

FURNACE SIZING OF COAL POWER PLANT BASIC INFORMATION

The most important step in coal-fired unit design is to properly size the furnace. Furnace size has a first-order influence on the size of the structural-steel framing, the boiler building and its foundations, as well as on the sootblowers, platforms, stairways, steam piping, and duct work.

The fuel-ash properties that are particularly important when designing and establishing the size of coal-fired furnaces include

The ash fusibility temperatures (both in terms of their absolute values and the spread or difference between initial deformation temperature and fluid temperature)

The ratio of basic to acidic ash constituents

The iron/calcium ratio

The fuel-ash content in terms of pounds of ash per million British thermal units

The ash friability

These characteristics and others translate into the furnace sizes in Fig. 5-5, which are based on the six coal ranks shown in Table 5-2. This size comparison illustrates the philosophy of increasing the furnace plan area, volume, and the fuel burnout zone (the distance from the top fuel nozzle to the furnace arch), as lower-grade coals with poorer ash characteristics are fired.

 Figure 5-5 is a simplified characterization of actual furnaces built to burn the fuels listed in Table 5-2.

Wide variations exist in fuel properties within coal ranks, as well as within several subclassifications (e.g., subbituminous A, B, C), each of which may require a different size furnace.

Among the most important design criteria in large pulverized-fuel furnaces are net heat input in British thermal units per hour per square foot of furnace plan area (NHI/PA) and the vertical distance from the top fuel nozzle to the furnace arch.

Furnace dimensions must be adequate to establish the necessary furnace retention time to properly burn the fuel as well as to cool the gaseous combustion products. This is to ensure that the gas temperature at the entrance to the closely spaced convection surface is well below the ash-softening temperature of the lowest-quality coal burned.

Heat-absorption characteristics of the walls are maintained using properly placed wall blowers to control the furnace outlet gas temperature by removing ash deposited on the furnace walls below the furnace outlet plane.

CLASSIFICATION OF COALS USED IN POWER PLANTS BASIC AND TUTORIALS

Coals are grouped according to rank. For the purposes of the power-plant operator, there are several suitable ranks of coal:

Anthracite

Bituminous

Subbituminous

Lignite

The following description of coals by rank gives some of their physical characteristics.

Anthracite.

Hard and very brittle, anthracite is dense, shiny black, and homogeneous with no marks or layers. Unlike the lower-rank coals, it has a high percentage of fixed carbon and a low percentage of

volatile matter.

Anthracites include a variety of slow-burning fuels merging into graphite at one end and into bituminous coal at the other. They are the hardest coals on the market, consisting almost entirely of fixed carbon, with the little volatile matter present in them chiefly as methane, CH4.

Anthracite is usually graded into small sizes before being burned on stokers. The “metaanthracites” burn so slowly as to require mixing with other coals, while the “semianthracites,” which have more volatile matter, are burned with relative ease if properly fired.

Most anthracites have a lower heating value than the highestgrade bituminous coals. Anthracite is used principally for heating homes and in gas production. Some semianthracites are dense, but softer than anthracite, shiny gray, and somewhat granular in structure.

The grains have a tendency to break off in handling the lump, and produce a coarse, sandlike slack. Other semianthracites are dark gray and distinctly granular. The grains break off easily in  handling and produce a coarse slack.

The granular structure has been produced by small vertical cracks in horizontal layers of comparatively pure coal separated by very thin partings. The cracks are the result of heavy downward pressure, and probably shrinkage of the pure coal because of a drop in temperature.

Bituminous.

By far the largest group, bituminous coals derive their name from the fact that on being heated they are often reduced to a cohesive, binding, sticky mass. Their carbon content is less than that of anthracites, but they have more volatile matter. The character of their volatile matter is more complex than that of anthracites, and they are higher in calorific value.

They burn easily, especially in pulverized form, and their high volatile content makes them good for producing gas. Their binding nature enables them to be used in the manufacture of coke, while the nitrogen in them is utilized in processing ammonia.

The low-volatile bituminous coals are grayish-black and distinctly granular in structure. The grain breaks off very easily, and handling reduces the coal to slack. Any lumps that remain are held

together by thin partings. Because the grains consist of comparatively pure coal, the slack is usually lower in ash content than are the lumps.

Medium-volatile bituminous coals are the transition from high-volatile to low-volatile coal and, as such, have the characteristics of both. Many have a granular structure, are soft, and crumble easily. Some are homogeneous with very faint indications of grains or layers. Others are of more distinct laminar structure, are hard, and stand handling well.

High-volatile A bituminous coals are mostly homogeneous with no indication of grains, but some show distinct layers. They are hard and stand handling with little breakage. The moisture, ash, and sulfur contents are low, and the heating value is high.

High-volatile B bituminous coals are of distinct laminar structure; the layers of black, shiny coal alternate with dull, charcoal-like layers. They are hard and stand handling well. Breakage occurs generally at right angles and parallel to the layers, so that the lumps generally have a cubical shape.

High-volatile C bituminous coals are of distinct laminar structure, are hard, and stand handling well. They generally have high moisture, ash, and sulfur contents and are considered to be free-burning coals.

Subbituminous.

These coals are brownish black or black. Most are homogeneous with smooth surfaces, and with no indication of layers. They have high moisture content, as much as 15% to 30%, although appearing dry.

When exposed to air they lose part of the moisture and crack with an audible noise. On long exposure to air, they disintegrate. They are free-burning, entirely noncoking, coals.

Lignite.

Lignites are brown and of a laminar structure in which the remnants of woody fibers may be quite apparent. The word lignite comes from the Latin word lignum meaning wood. Their origin is mostly from plants rich in resin, so they are high in volatile matter.

Freshly mined lignite is tough, although not hard, and it requires a heavy blow with a hammer to break the large lumps. But on exposure to air, it loses moisture rapidly and disintegrates. Even when it appears quite dry, the moisture content may be as high as 30%.

Owing to the high moisture and low heating value, it is not economical to transport it long distances.

Unconsolidated lignite is also known as “brown coal.” Brown coals are generally found close to the surface, contain more than 45% moisture, and are readily won by strip mining.

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.

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.

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.”

THERMOELECTRICS AND THERMIONICS BASIC AND TUTORIALS

Thermoelectrics
Thermal energy can be directly converted to electrical energy by using the thermoelectric effects in materials. Semiconductors offer the best option as thermocouples since thermojunctions can be constructed using a p-type and an n-type material to cumulate the effects around a thermoelectric circuit.

Moreover, by using solid solutions of tellurides and selenides doped to result in a low density of charge carriers, relatively moderate thermal conductivities and reasonably good electrical conductivities can be achieved.

In a thermoelectric generator, the Seebeck voltage generated under a temperature difference drives a dc current through the load circuit. Even though there is no mechanical conversion, the process is still Carnot limited since it operates over a temperature difference.

In practice, several couples are assembled in a seriesparallel configuration to provide dc output power at the required voltage. Typical thermoelectric generators employ radioisotope or nuclear reactor or hydrocarbon burner as the heat source.

They are custom-made for space missions as exemplified by the SNAP (systems for nuclear auxiliary power) series and the RTG (radioisotope thermoelectric generator) used by the Apollo astronauts.

Maximum performance over a large temperature range is achieved by cascading stages. Each stage consists of thermocouples electrically in series and thermally in parallel. The stages themselves are thermally in series and electrically in parallel.

Tellurides and selenides are used for power generation up to 600° C. Silicon germanium alloys turn out better performance above this up to 1000° C. With the materials available at present, conversion efficiencies in the 5 to 10% range can be expected.

Whenever small amounts of silent reliable power is needed for long periods of time, thermoelectrics offer a viable option. Space, underwater, biomedical, and remote terrestrial power such as cathodic protection of pipelines fall into this category.

Thermionics
Direct conversion of thermal energy into electrical energy can be achieved by employing the Edison effect the release of electrons from a hot body, also known as thermionic emission. The thermal input imparts sufficient energy (≥ work function) to a few electrons in the emitter (cathode), which helps them escape.

If these electrons are collected using a collector (anode) and a closed path through a load is established for them to complete the circuit back to the cathode, then electrical output is obtained. Thermionic converters are heat engines with electrons as the working fluid and, as such, are subject to Carnot limitations.

Converters filled with ionizable gases such as cesium vapor in the interelectrode space yield higher power densities due to space charge neutralization. Barrier index is a parameter that signifies the closeness to ideal performance with no space charge effects. As this index is reduced, more applications become feasible.

A typical example of developments in thermionics is the TFE (thermionic fuel element) that integrates the converter and nuclear fuel for space nuclear power in the kW to MW level for very long (7 to 10 years) duration missions.

Another niche is the thermionic cogeneration burner module, a high-temperature burner equipped with thermionic converters. Electrical outputs of 50 kW/MW of thermal output have been achieved. High (600 to 650° C) heat rejection temperatures of thermionic converters are ideally suited for producing flue gas in the 500 to 550° C range for industrial processes. A long-range goal is to use thermionic converters as toppers for conventional power plants. Such concepts are not economical at present.

WIND - ELECTRIC ENERGY CONVERSION BASIC AND TUTORIALS

HOW TO TURN WIND INTO ELECTRICITY?

Wind energy is intermittent, highly variable, and site-specific, exists in three dimensions, and is the least dependent upon latitude among all renewable resources. The power density (in W/unit area) in moving air (wind) is a cubic function of wind speed and therefore even small increases in average wind speeds can lead to significant increases in the capturable energy. 

Wind sites are typically classified as good, excellent, or outstanding, with associated mean wind speeds of 13, 16, and 19 mph, respectively.

Aeroturbines employ lift and/or drag forces to convert wind energy to rotary mechanical energy, which is then converted to electrical energy by coupling a suitable generator. The power coefficient Cp of an aeroturbine is the fraction of the incident power converted to mechanical shaft power, and it is a function of the tip speedto- wind speed ratio λ as shown in Figure 60.3. For a given propeller configuration, at any given wind speed, there is an optimum tip speed that maximizes Cp.

Several types of aeroturbines are available. They can have horizontal or vertical axes, number of blades

ranging from one to several, mounted upwind or downwind, and fixed- or variable-pitch blades with full blade control or tip control. Vertical-axis (Darrieus) turbines are not self-starting and require a starting mechanism.

Today, horizontal-axis turbines with two or more blades are the most prevalent, and considerable work is underway to develop advanced versions of these. The electrical output Pe of a wind-electric conversion system (WECS) is given as

where ηg and ηm are the efficiencies of the electrical generator and mechanical interface, respectively, A is the swept area, K is a constant, and v is the wind speed incident on the aeroturbine.

There are two basic options for wind-electric conversion. With varying wind speeds, the aeroturbine can be operated at a constant speed by blade-pitch control, and a conventional synchronous machine is then employed to generate constant-frequency ac. 

More commonly, an induction generator is used with or without an adjustable var supply. In this case, the aeroturbine will operate at a nearly constant speed. Alternatively, the aeroturbine rotational speed can be allowed to vary with wind to maintain a constant and optimum tip speed ratio, and then a combination of special energy converters and power electronics is employed to obtain utility-grade ac.

The variable-speed option allows optimum efficiency operation of the turbine over a wide range of wind speeds, resulting in increased outputs with lower structural loads and stresses. All future utility-grade advanced turbines are expected to operate in the variable-speed mode and use power electronics to convert the variable-frequency output to constant frequency with minimal harmonic distortion.

Large-scale harnessing of wind energy will require hundreds or even thousands of WECS arranged in a wind farm with spacings of about 2 to 3 diameters crosswind and about 10 diameters apart downwind. The power output of an individual WECS will fluctuate over a wide range, and its statistics strongly depend on the wind statistics. 

When many WECS are used in a wind farm, some smoothing of the total power output will result, depending on the statistical independence of the outputs of individual WECS. This is desirable, especially with high (>20%) penetration of WECS in the generation mix. While the output of WECS is not dispatchable, with large wind farms the possibility of assigning some capacity credit to the overall output significantly improves.

Although wind-electric conversion has overall minimum environmental impacts, the large rotating structures involved do generate some noise and introduce visual aesthetics problems. By locating wind energy systems sufficiently far from centers of population, these effects can be minimized. 

The envisaged potential for bird kills turned out to be not a serious problem. Wind energy systems occupy only a very small fraction of the land. However, the area surrounding them can be used only for activities such as farming and livestock grazing.

Thus, there is some negative impact on land use. Today, the cost of energy delivered by wind plants rivals those obtained from some nonrenewable sources. By 1990, wind became the most utilized and competitive option among all the solar energy technologies for the bulk power market at a cost of generation of about 8¢/kWh (or roughly 7¢/kWh in 1987 dollars). 

Ongoing research and development work in new design tools, advanced airfoils, site tailoring, operating strategies, array spacing, and improved reliability and manufacturability is expected to bring the cost of energy further down by a factor of 2 to 3.

At around 1600 MW, nearly 90% of all the WECS installed in the world are in California. They are expected to generate nearly 3 billion kWh of electricity per year to the state’s utilities to which they are interconnected.

Although their lack of control and the intermittent nature of wind-derived energy are not embraced enthusiastically by electric utilities, this gap is expected to be bridged very soon with appropriate computer controls and operating strategies. Wind energy is already an economical option for remote areas endowed with good wind regimes. 

The modularity of WECS, coupled with the associated environmental benefits, potential for providing jobs, and economic viability point to a major role for wind energy in the generation mix of the world in the decades to come.

PHOTOVOLTAICS - SOLAR ENERGY POWER GENERATION BASIC AND TUTORIALS

PV refers to the direct conversion of insolation (incident solar radiation) to electricity. A PV cell (also known as a solar cell) is simply a large-area semiconductor pn junction diode with the junction positioned very close to the top surface.

Typically, a metallic grid structure on the top and a sheet structure in the bottom collect the minority carriers crossing the junction and serve as terminals. The minority carriers are generated by the incident photons with energies greater than or equal to the energy gap of the semiconductor material.

Since the output of an individual cell is rather low (1 or 2 W at a fraction of a volt), several (30 to 60) cells are combined to form a module. Typical module ratings range from 40 to 50 W at 15 to 17 V. PV modules are progressively put together to form panels, arrays (strings or trackers), groups, segments (subfields), and ultimately a PV plant consisting of several segments. Plants rated at several MW have been built and operated successfully.

Advantages of PV include demonstrated low operation and maintenance costs, no moving parts, silent and simple operation, almost unlimited lifetime if properly cared for, no recurring fuel costs, modularity, and minimal environmental effects. The disadvantages are its cost, need for large collector areas due to the diluteness of insolation, and the diurnal and seasonal variability of the output.

PV systems can be flat-plate or concentrating type. While flat-plate systems utilize the global (direct and diffuse) radiation, concentrator systems harness only the direct or beam radiation. As such, concentrating systems must track (one axis or two axis) the sun. Flat-plate systems may or may not be mounted on trackers.

By 1990, efficiencies of flat-plate crystalline and thin-film cells had reached 23 and 15%, respectively. Efficiencies as high as 34% were recorded for concentrator cells. Single-crystal and amorphous PV module efficiencies of 12 and 5% were achieved by the early ’90s. For an average module efficiency of 10% and an insolation of 1 kW/m2 on a clear afternoon, 10 m 2 of collector area is required for each kW of output.

The output of a PV system is dc and inversion is required for supplying ac loads or for utility-interactive operation. While the required fuel input to a conventional power plant depends on its output, the input to a PV system is determined by external factors such as cloud cover, time of day, season of the year, geographic location, orientation, and geometry of the collector.

Therefore, PV systems are operated, as far as possible, at or near their maximum outputs. Also, PV plants have inertialess generation and are subject to rapid changes in their outputs due to moving clouds. The current-voltage (IV) characteristic of an illuminated solar cell is shown in Figure 60.1. It is given as

where
Io and Is are the dark and source currents, respectively, k is the Boltzmann constant (1.38×10–23 J/K), T is the temperature in K, and e is the electronic charge. Under ideal conditions (identical cells), for a PV module with a series-parallel arrangement of cells, the IV characteristic will be similar, except that the current scale should be multiplied by the number of parallel branches and the voltage scale by the number of cells in series in the module.

The source current varies linearly with insolation. The dark current increases as the cell
 operating temperature increases. Also, the larger the energy gap of the material, the smaller the dark current. The ratio of source current to dark current should be made as large as possible for improved operation.

Single-crystal silicon is still the dominant technology for fabricating PV devices. Polycrystalline, semicrystalline, and amorphous silicon technologies are developing rapidly to challenge this. Highly innovative technologies such as spheral cells are being introduced to reduce costs. Concentrator systems typically employ gallium arsenide or multiple junction cells. Many other materials and thin-film technologies are under investigation as potential candidates.

PV applications range from milliwatts (consumer electronics) to megawatts (central station plants). They are suitable for portable, remote, stand-alone, and utility-interactive applications. PV systems should be considered as energy sources and their design should maximize the conversion of insolation into useable electrical form.

Power requirements of practical loads are met using an energy storage and reconversion system or utility interconnection. Concentrating systems have been designed and operated to provide both electrical and low grade thermal outputs with combined peak utilization efficiencies approaching 60%.


The vigorous growth of PV technology is manifested by a doubling of world PV module shipments in six years — from 42 MW in 1989 to 84 MW in 1995. Tens of thousands of small (<1 kW) systems are in operation around the world. Thousands of kilowatt-size systems (1 to 10s of kW) also have been installed and are in operation.

Many intermediate-scale systems (10 to 100s of kW) and large-scale systems (1 MW or larger) are being installed by utility- and government-sponsored programs as proof-of-concept experiments and to glean valuable operational data.

By 1988, nearly 11 MW of PV was interconnected to the utility system in the United States alone. Most were in the 1- to 5-kW range. The two major exceptions are the 1-MW Hesperia-Lugo project installed in 1982 and the 6.5-MW Carrisa Plains project installed in 1984, both in California.

In Germany, a 340-kW system began operation in 1988 as part of a large program. Switzerland had a plan to install 1 MW of PV in 333 roof-mounted units of 3 kW each. By 1990, the installed capacity of PV in Italy exceeded 3 MW.

Many nations have recognized the vast potential of PV and have established their own PV programs within the past decade. A view of the 300 kW flat-plate grid-connected PV system installed and operated by the city of Austin electric utility department in Austin, Texas.

From a capital cost of $7000/kW in 1988 with an associated levelized energy cost of 32¢/kWh, even with a business-as-usual scenario, a twofold reduction to $3500/kW by 2000 and an additional 3-to-1 reduction to $1175/kW by 2030 are being projected. The corresponding energy costs are 15 and 5¢/kWh, respectively. These estimates put the cost of energy from PV in par with the cost of energy from conventional plants in the early part of the twenty-first century.

ELECTRIC POWER GENERATION - ENERGY CONVERSION BASIC AND TUTORIALS

Power generating plants, like other manufacturing plants, process raw materials into useful products, often accompanied by some waste products. For power plants the useful product is electrical energy. The waste products for fossil plants include ash and smoke visibly and heat invisibly.

Similarities include the use of equipment and materials that serve to expedite and improve efficiency of operations, although they may not be directly involved in the manufacture of the product. For example, water may be used to produce the steam and for cooling purposes, oil to lubricate moving parts, and fans and pumps to move gases and fluids.

Additional similarities include facilities for the reception of raw materials, disposal of waste, and for delivery of the finished product as well as trained personnel to operate the plant. Economic considerations, including capital investment and operating expenses which determine the unit costs of the product while meeting competition (oil, natural gas in this case) are common to most business enterprises.

There are some important dissimilarities. As a product electricity not only is invisible and hazardous in its handling but for the most part cannot be stored. Inventories cannot be accumulated and the ever changing customer demands must be met instantly.

All of this imposes greater standards of reliability in furnishing a continuing supply both in quantity and quality. This criteria assumes even greater importance as such generating plants are vital to national economy and contribute greatly to the standard of living.

In the larger central generating plants, fossil or nuclear energy (in the form of fuel) is fi rst converted into heat energy (in the form of steam), then into mechanical energy (in an engine or turbine), and fi nally
into electrical energy (in a generator) to be utilized by consumers. A
schematic arrangement is shown in Figure 2-1, below.

Most commonly, electricity is produced by burning a fossil fuel (coal, oil or natural gas) in the furnace of a steam boiler. Steam from the boiler drives a steam engine or turbine connected by a drive shaft to an electrical generator.

A nuclear power plant is a steam-electric plant in which a nuclear reactor takes the place of a furnace and the heat comes from the reaction within the nuclear fuel (called fi ssion) rather than from the burning of fossil fuel.

The equipment used to convert heat to power is essentially the same an ordinary steam-electric plant. The product, electrical energy is identical; see Figure 2-2.

The processes and the equipment to achieve these energy transformations will be described in fundamental terms, encompassing arrangements and modifi cations to meet specifi c conditions. Some may be recognized as belonging to older practices (for example burning lump coal on iron grates).

While serving purposes of illustration, it must be borne in mind that for a variety of reasons, some of the equipment and procedures continue in service and, hence, knowledge of their operation is still desirable.

Pertinent changes, developments and improvements, brought about by technological, economic and social considerations are included.

The four conversion processes in a typical steam generating plant may be conveniently separated into two physical entities, following accepted general practice. The fi rst two processes comprise operations known as the BOILER ROOM, while the latter two are included in those known as the TURBINE ROOM.