Electrical Insulators

Material

High-voltage insulators used for high-voltage power transmission are made from glass, porcelain, or composite polymer materials. Porcelain insulators are made from clay, quartz or alumina and feldspar, and are covered with a smooth glaze to shed dirt. The design of insulators often includes deep grooves, or sheds, that provides increased arc-lengths. Insulators made from porcelain rich in alumina are used where high mechanical strength is a criterion. Glass insulators were (and in some places still are) used to suspend electrical power lines. Some insulator manufacturers stopped making glass insulators in the late 1960s, switching to various ceramic and, more recently, composite materials.

Recently, some electric utilities have begun converting to polymer composite materials for some types of insulators which consist of a central rod made of fibre reinforced plastic and an outer weathershed made of silicone rubber or EPDM. Composite insulators are less costly, lighter in weight, and have excellent hydrophobic capability. This combination makes them ideal for service in polluted areas. However, these materials do not yet have the long-term proven service life of glass and porcelain.

Insulation in electrical apparatus

The most important insulation material is air. A variety of solid, liquid, and gaseous insulators are also used in electrical apparatus. In smaller transformers, generators, and electric motors, insulation on the wire coils consists of up to four thin layers of polymer varnish film. Film insulated magnet wire permits a manufacturer to obtain the maximum number of turns within the available space. Windings that use thicker conductors are often wrapped with supplemental fiberglass insulating tape. Windings may also be impregnated with insulating varnishes to prevent electrical corona and reduce magnetically induced wire vibration. Large power transformer windings are still mostly insulated with paper, wood, varnish, and mineral oil; although these materials have been used for more than 100 years, they still provide a good balance of economy and adequate performance. Busbars and circuit breakers in switchgear may be insulated with glass-reinforced plastic insulation, treated to have low flame spread and to prevent tracking of current across the material.

In older apparatus made up to the early 1970s, boards made of compressed asbestos may be found; while this is an adequate insulator at power frequencies, handling or repairs to asbestos material will release dangerous fibers into the air and must be carried out with caution. Live-front switchboards up to the early part of the 20th century were made of slate or marble.

Some high voltage equipment is designed to operate within a high pressure insulating gas such as sulfur hexafluoride.

Insulation materials that perform well at power and low frequencies may be unsatisfactory at radio frequency, due to heating from excessive dielectric dissipation.

Electrical wires may be insulated with polyethylene, crosslinked polyethylene (either through electron beam processing or chemical crosslinking), PVC, rubber-like polymers, oil impregnated paper, Teflon, silicone, or modified ethylene tetrafluoroethylene (ETFE). Larger power cables may use compressed inorganic powder, depending on the application.

Flexible insulating materials such as PVC (polyvinyl chloride) are used to insulate the circuit and prevent human contact with a 'live' wire -- one having voltage of 600 volts or less. Alternative materials are likely to become increasingly used due to EU safety and environmental legislation making PVC less economic.

Class 1 and Class 2 insulation

All portable or hand-held electrical devices are insulated to protect their user from harmful shock.

Class 1 insulation requires that the metal body of the apparatus/equipment is solidly connected via a "grounding" wire which is earthed at the main Service Panel; but only basic insulation of the conductors is needed. This equipment is easily identified by a third pin for the grounding connection.

Class 2 insulation means that the equipment/apparatus is double insulated and is used on some appliances such as electric shavers, hair dryers and portable power tools. Double insulation requires that the devices have basic and supplementary insulation, each of which is sufficient to prevent electric shock. All internal electrically energized components are totally enclosed within insulated packaging that prevents any contact with "live" parts. They can be recognised because their leads have two pins, or on three pin plugs the third (earth) pin is made of plastic rather than metal. In the EU, double insulated appliances all are marked with a symbol of two squares, one inside the other.

cooling of generator or motor

Cooling of Rotating Machines :

Forced Air Cooling : For large machines which may require many tonnes of cooling air/hr. , forced ventilation permits the cleaning of the air by suitable filters , avoiding clogging of the ducts. The air is filtered or washed with a water spray , then baffled against flooded scrubbing surfaces to precipitate the dust. It is then dried by passing over a series of dry scrubbing plates.

Hydrogen Cooling : To build air cooled turbo-generators above 50 MW rating present serious ventilation difficulties, not only in circulating the requisite quantity of air through the machine but also the high fan power required to circulate the air. Therefore the use of air as a coolant for high rating machines is ruled out and hydrogen is used as the alternate cooling media..

Gas Cooling : Large stator cores for turbo-generators are provided with both axial & radial ducts. For machines of rating more than 100 MW, the temperature gradient over the conductor insulation is high enough to call for direct contact between the coolant & the material of the conductor themselves. The rotor conductors comprise rectangular tubes ,ventilated by the cooling circuit separate from that of the stator, the hydrogen gas being admitted to the tubes through insulating flexible connections at the ends from a centrifugal impeller mounted on the outboard end of the rotor shaft. The direct gas cooling of the stator & particularly of rotor winding permits of much higher electrical loading. With ratings of 1000 MW even this is not enough to cope with the very large flow rate & pressure, which might be 15 cu. m of hydrogen at 1.2 atm to absorb an excitation loss of 5 MW.

Water cooling : Turbo-generators of highest rating so far contemplated are likely to have hydrogen cooled stator cores & direct water cooled stator & rotor windings. In the direct water cooling of stator windings, one problem is to device flexible water tube connections with insulation against the high winding voltages and to preserve a low water conductivity. Water cooling of rotor winding, even more desirable because of it?s high electric loading, offers more mechanical difficulty. Water cooled field windings have been used for salient poles of large hydro-generators. The mass of cooling water required is only one quarter of that of air at atmospheric pressure for the same cooling effect.

Rotor of generator

The rotor shaft is forged from a vacuum cast steel ingot. The high mechanical stresses resulting from the centrifugal forces & short circuit torque call for high quality heat-treated steel. Comprehensive tests are done to ensure adherence to the specified mechanical & magnetic properties as well as homogeneous forging.

The rotor consists of an electrically active portion & two shaft ends. Approximately, 60% of the rotor body circumference has longitudinal slots, which hold the field winding. Slot pitch is selected so that the two poles are displaced by 180 degree. The rotor wedges act as damper winding within the range of the winding slots.

The rotor teeth at the end of the rotor body are provided with axial & radial holes, enabling the cooling gas to be discharged into the air gap after intensive cooling of the end windings.


Cooling of Rotor :

Each turn is subdivided into four parallel cooling zones. One cooling zone includes the slot from the centre to the end of the rotor body , while other covers the half the end winding to the centre of the rotor body.
The cooling of Air for the slot portion is admitted into the slot bottom ducts below the winding . The hot gases at the end of the rotor body is then discharged into the air gap between the rotor body and stator core through radial opening in the rotor slots wedges .

The cooling air for the end winding is drawn from below the rotor-retaining ring. It rises radially along the air gap via axial and radial slots in the end portions of the rotor teeth.


Types of Rotor :

There are two types of rotors used in generators.

Salient Pole Type : It is used in low & medium speed (engine driven) generators. It has a large number of projecting (salient) poles having their cores bolted or dovetailed onto a heavy magnetic wheel of cast iron, or steel of good magnetic quality

Smooth Cylindrical Type : It is used for steam turbine or gas turbine driven generators, which run at very high speed (3000rpm). The rotor consists of a smooth solid forged steel cylinder, having a number of slots milled out at intervals along the periphery & along the shaft for accommodating field coils. To avoid excessive peripheral velocity, such rotors have very small diameters. Hence, turbo generators are characterised by small diameters & long axial (or rotor) lengths. The cylindrical construction of the rotor gives better balance, quieter operation & also less windage loss.

armature windings of generator

There are two types of armature windings most commonly used for a three phase generator.

i)Single layer winding.
ii) Double layer winding.

Conductors Construction :

Each bar consists of a large number of separately insulated strands to reduce the skin effect losses. In the straight slot portion the strands are transposed by 360` .The transposition provides for a mutual neutralisation of the voltage induced in the individual strands due to the slot cross filled and ensures that no or only small circulating current exists in the bra interior. The current flowing through the bar is thus uniformly distributed over the entire cross section and losses will be reduced.


Insulation of Bars :

The high voltage insulation is provided ,with thermosetting system. This system is void free and has excellent electrical ,mechanical and thermal Properties .All bars are provided with an end corona protection from the slots to the end winding portion and to prevent the formation of the crepage sparks.


Corona Protection :

To prevent potential difference and possible corona discharges between the insulation and the slot wall ,the slot section of the bars are provided with the outer corona protection .This corona protection consists of a wear resistance, highly flexible coating of alkyd varnish containing graphite.

Electrical Connection of Bars & Phase Connectors :


Electrical Connection of Bars :

Electrical connection between the top and bottom bar is made by brazing, one top bar being brazed to the associated bottom bar. The coil connections are wrapped with tapes applied half over lap. The thickness of the wrapper depends upon machine voltage. After taping, an insulating gap between the individual coil connections being sufficiently large, no additional insulation is required.


Phase Connectors :

The phase connectors consist of flat copper sections, the cross section of which results in the low specific current loading. The connections to the stator winding are or riveted and soldered type. The phase connectors are wrapped with mica resin tape applied half overlap, which contain the full quantity of synthetic resin having good quantity of penetrating properties .Then a wrapper of shrinking tape is applied .


Output Lead :

The beginning and end of the 3-phase winding is solidly bolted to the output leads with flexible. The output leads consists of fat copper sections with mica insulation. To prevent eddy current losses and unwarranted temperature rise , the output leads are brought through insulating plates.

construction of generator

An alternating current generator consists principally of a magnetic circuit, DC field winding and mechanical structure, including cooling & lubricating system.
Broadly the alternator parts can be classified as

i) Stationary part - comprising the stator & stator winding.
ii) Rotating part - comprising the rotor , rotor winding & rotor fan.

Main parts of a generator are described below

Stator Frame :
The stator frame with core & stator winding is the heaviest component of the entire generator. The stator frame is used for holding the armature stampings and windings in position. It is built up of steel plates, which are electrically welded together, instead of the more expensive cast-steel frames formerly used. Ventilating ducts are provided in the radial ribs for passage of cooling medium. The stator end covers are attached to the end flanges of the stator frame & also rest on foundation frame. The end covers are aluminium alloy castings.

Stator Core :
The stator core is stacked from insulated electrical sheet steel lamination with a low loss index and suspended in the stator frame from insulated dove tailed guide bars. Axial compression of the stator core is obtained by clamping fingers, clamping plates, and non magnetic through type clamping bolts, which are insulated from the core . The clamping fingers ensure a uniform clamping pressure, especially within the range of the teeth, and provide for uniform , intensive cooling of the stator core ends.

In order to minimise hysteresis and eddy current losses of the rotating magnetic flux, which interacts with the core, the entire core is built up of thin laminations. Each lamination is made up from a number of individual segments. The segments are punched in one operation from electrical sheet steel lamination having a high silicon content and then carefully de-burred. The stator frame is turned on end while the core is stacked with lamination segments in individual layers . The clamping bolts running through the core are made of non-magnetic steel and are insulated from the core and the clamping plates to prevent the clamping bolts from short circuiting the laminations and allowing the flow of eddy currents. The pressure is transmitted from the clamping plates to the core by clamping finger.

Stator Winding :

The stator winding or the armature winding in generators have open circuit winding unlike DC machines which have closed circuit winding. There is no closed path for the armature current in the winding itself. One end of the winding is joined to the neutral point & the other end is brought out(for a star connected armature).Each conductor consists of large number of separately insulated strands to reduce the skin effect losses. The high voltage insulation is provided with thermosetting system to prevent corona discharge. A coat of conducting varnish is applied to the surface of all the Bars. A final wrapping of glass fabric impregnated with epoxy resin serves as surface protection.The beginnings & ends of the three phase windings are solidly bolted to the output leads with flexibles.

Basics of Power Generation

PRINCIPLE OF OPERATION:

Generators, motors form a part of an electro-mechanical energy network. They convert one form of energy to another as governed by some laws. A generator converts mechanical energy to electrical energy as defined by Faraday?s law of induction.

Faraday's Law of Induction :

According to Faraday's law, in any closed linear path in space, when the magnetic flux surrounded by the path, varies with time, a voltage is induced around the path equal to the negative of rate of change of flux in Webers/sec.

V = - dF / dT volts.

where, V - EMF in volts & F - Flux in webers

A generator achieves the relative motion by providing a rotating, rotor mounted field against the stationary armature coils.

When a DC field current If flows in the rotor field winding, a MMF (magneto motive force) is set up which causes the formation of a rotor based field flux. In a salient - pole generator the pole faces are tapered, resulting in a maximum flux density along the ?d? axis diminishing to zero in the ?q? direction. By proper tapering one creates, in actuality, a sinusoidal flux distribution along the air gap periphery.

In case of a turbogenerator the magnetic flux distribution has a staircase look, where each step is due to added MMF contributed by the field current in the discrete rotor slots. However the fundamental sinusoidal space wave is dominating over the harmonics.

If the rotor spins at a constant synchronous speed, the stator conductors will experience a travelling "flux wave".

When a magnetic flux of density B cuts a perpendicular conductor at a relative speed s an EMF (electromotive force) is induced in the conductor, the instantaneous magnitude E of which follows the formula.

E = B.s V/m
Where, B = magnetic flux in Tesla , S = relative speed in metres/sec.

As the flux distribution is assumed sinusoidal so will be the EMF distribution. The stator will thus experience an "EMF wave" or " E wave" of the same speed as the flux wave.
The two waves consist of P/2 full cycles around the full periphery. Thus if the rotor speed is n rev./min., each stator conductor will experience an AC EMF of frequency

F = (P / 2) . (n / 60)= (P. n / 120) Hz
where, P = no. of poles, n = speed, F=Frequency

For e.g. a two-pole machine, when rotating at 3000 rev./min. will generate a 50 Hz EMF. When the rotor turns one mechanical degree, the stator EMF completes P/2 electrical degrees. In a two-pole machine electrical & mechanical degrees are identical. Due to their winding locations the EMFs' induced in phase b & c will lag that in phase a by 120 & 240 electrical degrees respectively.

Power Transmission Line

AC power transmission

AC power transmission is the transmission of electric power by alternating current. Usually transmission lines use three phase AC current. Single phase AC current is sometimes used in a railway electrification system.

Overhead conductors are not covered by insulation. The conductor material is nearly always an aluminum alloy, made into several strands and possibly reinforced with steel strands. Overhead conductors are a commodity supplied by several companies worldwide. Improved conductor material and shapes are regularly used to allow increased capacity and modernize transmission circuits. Conductor sizes in overhead transmission work range in size from #6 American wire gauge (about 12 square millimetres) to 1,590,000 circular mils area (about 750 square millimetres), with varying resistance and current-carrying capacity. Thicker wires would lead to a relatively small increase in capacity due to the skin effect, that causes most of the current to flow close to the surface of the wire.

Today, transmission-level voltages are usually considered to be 110 kV and above. Lower voltages such as 66 kV and 33 kV are usually considered sub-transmission voltages but are occasionally used on long lines with light loads. Voltages less than 33 kV are usually used for distribution. Voltages above 230 kV are considered extra high voltage and require different designs compared to equipment used at lower voltages.

Overhead transmission lines are uninsulated wire, so design of these lines requires minimum clearances to be observed to maintain safety.

Electric power can also be transmitted by underground power cables instead of overhead power lines. This is a more expensive option, as the life-cycle cost of an underground power cable is a multiple of the overhead power line. However, they can assist the transmission of power across:

* Densely populated urban areas
* Areas where land is unavailable or planning consent is difficult
* Rivers and other natural obstacles
* Land with outstanding natural or environmental heritage
* Areas of significant or prestigious infrastructural development
* Land whose value must be maintained for future urban expansion and rural development

Compared to overhead lines, underground cables emit no electric field, can be engineered to emit no magnetic fields, have better power loss characteristics, and can absorb emergency power loads. They also need merely a narrower strip of about 10 metres to install, whereas the lack of cable insulation requires an overhead line to be installed on a strip of about 200 metres wide to be kept permanently clear for safety, maintenance and repair. Those advantages can in some cases justify the higher investment cost.

Most high-voltage underground cables for power transmission that are currently sold on the market are insulated by a sheath of cross linked polyethylene (XLPE). Some cable may have a lead jacket in conjunction with XLPE insulation to allow for fiber optics to be seamlessly integrated within the cable. In the past underground power cables used to be insulated with oil and paper and ran in a rigid steel pipe, or a semi-rigid aluminium or lead jacket or sheath. The oil was kept under pressure to prevent formation of voids that would allow partial discharges within the cable insulation. There are still many of those oil-and-paper insulated cables in use worldwide.ny particular renewable alternative is economically sensible. Costs can be prohibitive for transmission.


Grid input

At the generating plants the energy is produced at a relatively low voltage of up to 30 kV (Grigsby, 2001, p. 4-4), then stepped up by the power station transformer to a higher voltage (115 kV to 765 kV AC, ± 250-500 kV AC, varying by country) for transmission over long distances to grid exit points (substations).

Losses
Transmitting electricity at high voltage reduces the fraction of energy lost to Joule heating. For a given amount of power, a higher voltage reduces the current and thus the resistive losses in the conductor. For example, raising the voltage by a factor of 10 reduces the current by a corresponding factor of 10 and therefore the I^2R losses by a factor of 100, provided the same sized conductors are used in both cases. Even if the conductor size is reduced x10 to match the lower current the I^2R losses are still reduced x10. Long distance transmission is typically done with overhead lines at voltages of 115 to 1,200 kV. However, at extremely high voltages, more than 2,000 kV between conductor and ground, corona discharge losses are so large that they can offset the lower resistance loss in the line conductors.

Transmission and distribution losses in the USA were estimated at 7.2% in 1995 , and in the UK at 7.4% in 1998.

As of 1980, the longest cost-effective distance for electricity was 4,000 miles (7,000 km), although all present transmission lines are considerably shorter. (see Present Limits of High-Voltage Transmission)

In an alternating current transmission line, the inductance and capacitance of the line conductors can be significant. The currents that flow in these components of transmission line impedance constitute reactive power, which transmits no energy to the load. Reactive current flow causes extra losses in the transmission circuit. The ratio of real power (transmitted to the load) to apparent power is the power factor. As reactive current increases, the reactive power increases and the power factor decreases. For systems with low power factors, losses are higher than for systems with high power factors. Utilities add capacitor banks and other components throughout the system — such as phase-shifting transformers, static VAR compensators, and flexible AC transmission systems (FACTS) — to control reactive power flow for reduction of losses and stabilization of system voltage.

Electrical power is always partially lost by transmission. This applies to short distances such as between components on a printed circuit board as well as to cross country high voltage lines

Electrical Power Station

Elements of a substation

Substations generally contain one or more transformers, and have switching, protection and control equipment. In a large substation, circuit breakers are used to interrupt any short-circuits or overload currents that may occur on the network. Smaller distribution stations may use recloser circuit breakers or fuses for protection of branch circuits. Substations do not (usually) have generators, although a power plant may have a substation nearby. A typical substation will contain line termination structures, high-voltage switchgear, one or more power transformers, low voltage switchgear, surge protection, controls, grounding (earthing) system, and metering. Other devices such as power factor correction capacitors and voltage regulators may also be located at a substation.

Substations may be on the surface in fenced enclosures, underground, or located in special-purpose buildings. High-rise buildings may have indoor substations. Indoor substations are usually found in urban areas to reduce the noise from the transformers, for reasons of appearance, or to protect switchgear from extreme climate or pollution conditions.

Where a substation has a metallic fence, it must be properly grounded (UK: earthed) to protect people from high voltages that may occur during a fault in the transmission system. Earth faults at a substation can cause ground potential rise at the fault location. Currents flowing in the earth's surface during a fault can cause metal objects to have a significantly different voltage than the ground under a person's feet; this touch potential presents a hazard of electrocution.

Transmission substation

A transmission substation connects two or more transmission lines. The simplest case is where all transmission lines have the same voltage. In such cases, the substation contains high-voltage switches that allow lines to be connected or isolated for maintenance. A transmission station may have transformers to convert between two transmission voltages, or equipment such as phase angle regulators to control power flow between two adjacent power systems.

Transmission substations can range from simple to complex. A small "switching station" may be little more than a bus plus some circuit breakers. The largest transmission substations can cover a large area (several acres/hectares) with multiple voltage levels, and a large amount of protection and control equipment (capacitors, relays, switches, breakers, voltage and current transformers)


Distribution substation

system of an area. It is uneconomical to directly connect electricity consumers to the high-voltage main transmission network, unless they use large amounts of energy; so the distribution station reduces voltage to a value suitable for local distribution.

The input for a distribution substation is typically at least two transmission or subtransmission lines. Input voltage may be, for example, 115 kV, or whatever is common in the area. The output is a number of feeders. Distribution voltages are typically medium voltage, between 2.4 and 33 kV depending on the size of the area served and the practices of the local utility.

The feeders will then run overhead, along streets (or under streets, in a city) and eventually power the distribution transformers at or near the customer premises.

Besides changing the voltage, the job of the distribution substation is to isolate faults in either the transmission or distribution systems. Distribution substations may also be the points of voltage regulation, although on long distribution circuits (several km/miles), voltage regulation equipment may also be installed along the line.

Complicated distribution substations can be found in the downtown areas of large cities, with high-voltage switching, and switching and backup systems on the low-voltage side. More typical distribution substations have a switch, one transformer, and minimal facilities on the low-voltage side.

Switching function
An important function performed by a substation is switching, which is the connecting and disconnecting of transmission lines or other components to and from the system. Switching events may be "planned" or "unplanned".

A transmission line or other component may need to be deenergized for maintenance or for new construction; for example, adding or removing a transmission line or a transformer.

To maintain reliability of supply, no company ever brings down its whole system for maintenance. All work to be performed, from routine testing to adding entirely new substations, must be done while keeping the whole system running.

Perhaps more importantly, a fault may develop in a transmission line or any other component. Some examples of this: a line is hit by lightning and develops an arc, or a tower is blown down by a high wind. The function of the substation is to isolate the faulted portion of the system in the shortest possible time.

There are two main reasons: a fault tends to cause equipment damage; and it tends to destabilize the whole system. For example, a transmission line left in a faulted condition will eventually burn down, and similarly, a transformer left in a faulted condition will eventually blow up. While these are happening, the power drain makes the system more unstable. Disconnecting the faulted component, quickly, tends to minimize both of these problems.

Steam electric generator

An alternating-current (ac) synchronous generator driven by a steam turbine for 50- or 60-Hz electrical generating systems.

The synchronous generator is a relatively simple machine made of two basic parts: a stator (stationary) and a rotor (rotating). The stator consists of a cylindrical steel frame. Inside the frame, a cylindrical iron core made of thin insulated laminations is mounted on a support system. The iron core has equally spaced axial slots on its inside diameter, and wound within the core slots is a stator winding. The stator winding copper is electrically insulated from the core. The rotor consists of a forged solid steel shaft. Wound into axial slots on the outside diameter of the shaft is a copper rotor winding that is held in the slots with wedges. Retaining rings support the winding at the rotor body ends. The rotor winding, commonly called the field, is electrically insulated from the shaft and is arranged in pole pairs (always an even number) to form the magnetic field which produces the flux. The rotor shaft (supported by bearings) is coupled to a steam turbine, and rotates inside the stator core.

The stator winding (armature) is connected to the ac electrical transmission system through the bushings and output terminals. The rotor winding (field) is connected to the generator's excitation system. The excitation system provides the direct-current (dc) field power to the rotor winding via carbon brushes riding on a rotating collector ring mounted on the generator rotor. The synchronous generator's output voltage amplitude and frequency must remain constant for proper operation of electrical load devices. During operation, the excitation system's voltage regulator monitors the generator's output voltage and current. The voltage regulator controls the rotor winding dc voltage to maintain a constant generator stator output ac voltage, while allowing the stator current to vary with changes in load. Field windings typically operate at voltages between 125 and 575 V dc. The synchronous generator's output frequency is directly proportional to the speed of the rotor, and the speed of the generator rotor is held constant by a speed governor system associated with the steam turbine.

Synchronous generators range in size from a few kilovoltamperes to 1,650,000 kVA. 60-Hz steam-driven synchronous generators operate at speeds of either 3600 or 1800 rpm; for 50-Hz synchronous generators these speeds would be 3000 or 1500 rpm. These two- and four-pole generators are called cylindrical rotor units. For comparison, water (hydro)-driven and air-driven synchronous generators operate at lower speeds, some as low as 62 rpm (116 poles). The stator output voltage of large (generally greater than 100,000 kVA) units ranges 13,800–27,000 V.

There are five sources of heat loss in a synchronous generator: stator winding resistance, rotor winding resistance, core, windage and friction, and stray losses. Removing the heat associated with these losses is the major challenge to the machine designer. The cooling requirements for the stator windings, rotor windings, and core increase proportionally to the cube of the machine size. The early synchronous generators were air-cooled. Later, air-to-water coolers were required to remove the heat.

Synchronization of a generator

Synchronization of a generator to a power system is the act of matching, over an appreciable period of time, the instantaneous voltage of an alternating-current generator (incoming source) to the instantaneous voltage of a power system of one or more other generators (running source), then connecting them together. In order to accomplish this ideally the following conditions must be met:

1. The effective voltage of the incoming generator must be substantially the same as that of the system.

2. In relation to each other the generator voltage and the system voltage should be essentially 180° out of phase; however, in relation to the bus to which they are connected, their voltages should be in phase.

3. The frequency of the incoming machine must be near that of the running system.

4. The voltage wave shapes should be similar.

5. The phase sequence of the incoming polyphase machine must be the same as that of the system.

Synchronizing of ac generators can be done manually or automatically. In manual synchronizing an operator controls the incoming generator while observing synchronizing lamps or meters and a synchroscope, or both. The operator closes the connecting switch or circuit breaker as the synchroscope needle slowly approaches the in-phase position.

Automatic synchronizing provides for automatically closing the breaker to connect the incoming machine to the system, after the operator has properly adjusted voltage (field current), frequency (speed), and phasing (by lamps or synchroscope). A fully automatic synchronizer will initiate speed changes as required and may also balance voltages as required, then close the breaker at the proper time, all without attention of the operator. Automatic synchronizers can be used in unattended stations or in automatic control systems where units may be started, synchronized, and loaded on a single operator command

Generator

A machine in which mechanical energy is converted to electrical energy. Generators are made in a wide range of sizes, from very small machines with a few watts of power output to very large central-station generators providing 1000 MW or more. All electrical generators utilize a magnetic field to produce an output voltage which drives the current to the load. The electric current and magnetic field also interact to produce a mechanical torque opposing the motion supplied by the prime mover. The mechanical power input is equal to the electric power output plus the electrical and mechanical losses.



Generators can be divided into two groups, alternating current (ac) and direct current (dc). Each group can be subdivided into machines that use permanent magnets to produce the magnetic field (PM machines) and those using field windings. A further subdivision relates to the type of prime mover and the generator speed. Large generators are often driven by steam or hydraulic turbines, by diesel engines, and sometimes by electric motors. Generator speeds vary from several thousand rotations per minute for steam turbines to very low speeds for hydraulic or wind turbines.



Electric power generation

The production of bulk electric power for industrial, residential, and rural use. Although limited amounts of electricity can be generated by many means, including chemical reaction (as in batteries) and engine-driven generators (as in automobiles and airplanes), electric power generation generally implies large-scale production of electric power in stationary plants designed for that purpose. The generating units in these plants convert energy from falling water, coal, natural gas, oil, and nuclear fuels to electric energy. Most electric generators are driven either by hydraulic turbines, for conversion of falling water energy; or by steam or gas turbines, for conversion of fuel energy. Limited use is being made of geothermal energy, and developmental work is progressing in the use of solar energy in its various forms.



An electric load (or demand) is the power requirement of any device or equipment that converts electric energy into light, heat, or mechanical energy, or otherwise consumes electric energy as in aluminum reduction, or the power requirement of electronic and control devices. The total load on any power system is seldom constant; rather, it varies widely with hourly, weekly, monthly, or annual changes in the requirements of the area served. The minimum system load for a given period is termed the base load or the unity load-factor component. Maximum loads, resulting usually from temporary conditions, are called peak loads, and the operation of the generating plants must be closely coordinated with fluctuations in the load. The peaks, usually being of only a few hours' duration, are frequently served by gas or oil combustion-turbine or pumped-storage hydro generating units. The pumped-storage type utilizes the most economical off-peak (typically 10 P.M. to 7 A.M.) surplus generating capacity to pump and store water in elevated reservoirs to be released through hydraulic turbine generators during peak periods. This type of operation improves the capacity factors or relative energy outputs of base-load generating units and hence their economy of operation.


The size or capacity of electric utility generating units varies widely, depending upon type of unit; duty required, that is, base-, intermediate-, or peak-load service; and system size and degree of interconnection with neighboring systems. Base-load nuclear or coal-fired units may be as large as 1200 MW each, or more. Intermediate-duty generators, usually coal-, oil-, or gas-fueled steam units, are of 200 to 600 MW capacity each. Peaking units, combustion turbines or hydro, range from several tens of megawatts for the former to hundreds of megawatts for the latter. Hydro units, in both base-load and intermediate service, range in size up to 825 MW.
The total installed generating capacity of a system is typically 20 to 30% greater than the annual predicted peak load in order to provide reserves for maintenance and contingencies.



Voltage regulation

Voltage regulation is the change in voltage for specific change in load (usually from full load to no load) expressed as percentage of normal rated voltage. The voltage of an electric generator varies with the load and power factor; consequently, some form of regulating equipment is required to maintain a reasonably constant and predetermined potential at the distribution stations or load centers. Since the inherent regulation of most alternating-current (ac) generators is rather poor (that is, high percentagewise), it is necessary to provide automatic voltage control.



The rotating or magnetic amplifiers and voltage-sensitive circuits of the automatic regulators, together with the exciters, are all specially designed to respond quickly to changes in the alternator voltage and to make the necessary changes in the main exciter or excitation system output, thus providing the required adjustments in voltage. A properly designed automatic regulator acts rapidly, so that it is possible to maintain desired voltage with a rapidly fluctuating load without causing more than a momentary change in voltage even when heavy loads are thrown on or off.


In general, most modern synchronous generators have excitation systems that involve rectification of an ac output of the main or auxiliary stator windings, or other appropriate supply, using silicon controlled rectifiers or thyristors. These systems enable very precise control and high rates of response.



Computer-assisted (or on-line controlled) load and frequency control and economic dispatch systems of generation supervision are being widely adopted, particularly for the larger new plants. Strong system interconnections greatly improve bulk power supply reliability but require special automatic controls to ensure adequate generation and transmission stability. Among the refinements found necessary in large, long-distance interconnections are special feedback controls applied to generator high-speed excitation and voltage regulator systems.