E-05 Ac Generators

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Basic Training Module 5: Generators Electric al

MODULE E-5

AC GENERATORS


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CHAPTER 1 ........................................................................................................................... 4

INTRODUCTION TO GAS TURBINES ............................................................................. 41.1 GENERAL....................................................................................................4

1.2 THE STEAM TURBINE................................................................................41.3 THE GAS TURBINE ....................................................................................51.4 SINGLE SHAFT AND TWO SHAFT TURBINES .........................................81.5 FUEL............................................................................................................91.6 SPEEDCONTROL .....................................................................................101.7 STARTING.................................................................................................111.8 STOPPING ................................................................................................141.9 PROTECTION ...........................................................................................151.10 CONTROL .................................................................................................161.11 SPEED CONTROL ....................................................................................161.12 WASHING..................................................................................................161.13 ANTI-ICING ...............................................................................................16

CHAPTER 2 ......................................................................................................................... 18 A.C. GENERATORS .......................................................................................................... 18

2.1 GENERAL..................................................................................................182.2 ROTOR CONSTRUCTION ........................................................................192.3 INSULATION .............................................................................................212.4 COOLING ..................................................................................................222.5 EXCITATION AND VOLTAGE CONTROL ................................................232.6 NEUTRAL EARTHING RESISTOR ...........................................................23

CHAPTER 3 ......................................................................................................................... 25GENERATOR EXCITATION AND VOLTAGE CONTROL .......................................... 25

3.1 GENERAL..................................................................................................253.2 CONVENTIONAL EXCITATION................................................................253.3 STATIC EXCITATION................................................................................263.4 BRUSHLESS EXCITATION (GENERAL CASE) .......................................263.5 BEHAVIOUR UNDER SHORT CIRCUIT...................................................273.6 BRUSH LESS EXCITATION (WITHOUT PILOT EXCITER)......................293.7 THE DIODE BRIDGE.................................................................................303.8 REGULATION RESPONSE TIME .............................................................313.9 AUTOMATIC VOLTAGE REGULATORS (AVR) .......................................323.10 AVR SET-POINT .......................................................................................32

3.11 A.C.GENERATOR VOLTAGE REGULATION...........................................32CHAPTER 4 ......................................................................................................................... 34DIESEL GENERATOR SETS ........................................................................................... 34

4.1 GENERAL..................................................................................................344.2 BASIC SERVICES.....................................................................................354.3 AVAILABILITY OF BASIC SERVICES GENERATOR...............................364.4 BASIC SERVICES GENERATOR UTILITIES............................................36

CHAPTER 5 ......................................................................................................................... 38SYNCHRONISING OF GENERATORS .......................................................................... 38

5.1 GENERAL..................................................................................................385.2 D.C. GENERATORS..................................................................................38

5.3 A.C.GENERATORS...................................................................................395.4 SYNCHRONSING A.C.GENERATORS..................................................... 41


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5.5 LAMP SYNCHRONISING..........................................................................415.5.1 The 2-Lamp Method ....................................................................................... 415.5.2 The 3-Lamp Method ....................................................................................... 43

5.6 SYNCHROSCOPE ....................................................................................445.7 SYNCHRONISING AT A SWITCHBOARD................................................45

5.8 AUTOMATIC SYNCHRONISING ..............................................................465.9 CHECK SYNCHRONISING .......................................................................465.10 CLOSING ONTO DEAD BUSBAR.............................................................47

CHAPTER 6 ......................................................................................................................... 49LOAD SHARING ................................................................................................................. 49

6.1 GENERAL..................................................................................................496.2 THE D.C. CASE.........................................................................................496.3 A.C. GENERATORS..................................................................................516.4 CONTROL OG GENERATOR LOADING..................................................52

6.4.1 Voltage Adjustment ......................................................................................... 526.4.2 Speed Adjustment ........................................................................................... 536.4.3 Summary .......................................................................................................... 54

6.5 PRINCIPLES OF PARALLEL OPERATION ..............................................566.5.1 Droop ................................................................................................................. 566.5.2 Trimming and Governor Setting .................................................................... 596.5.3 Trimming and Governor Droop ..................................................................... 606.5.4 Stability .............................................................................................................. 60

CHAPTER 7 ......................................................................................................................... 63LOAD SHEDDING .............................................................................................................. 637.1 GENERAL .................................................................................................................. 637.2 ACTION ...................................................................................................................... 637.3 SHEDDING CONTROL ............................................................................................ 647.4 PLANNING ................................................................................................................. 657.5 AUTOMATIC LOAD SHEDDING ............................................................................ 65

7.5.1 Direct Shedding ............................................................................................... 657.5.2 Testing .............................................................................................................. 69

CHAPTER 8 ......................................................................................................................... 70GENERATOR PROTECTION ........................................................................................... 70

INTRODUCTION..................................................................................................70PROTECTING THE GENERATOR FROM EXTERNAL CONDITIONS ...............70SENSING INTERNAL FAULTS............................................................................72MISCELLANEOUS RELAYS AND MECHANICAL PROTECTION ......................72

TYPICAL GENERATOR PROTECTION ..............................................................72Generators and electrical safety:..........................................................................75Important Safety Points ........................................................................................75


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CHAPTER 1

INTRODUCTION TO GAS TURBINES

1.1 GENERAL

All generators send out energy in the form of electrical power, and they haveto be given the equivalent mechanical energy. This means that they have tobe driven by an engine of some sort, which derives its energy from fuel, orsome other natural source such as wind or water. The engine which drivesan electric generator is called a 'prime mover and may take many forms.

In the early days steam-, gas- or oil-driven reciprocating engines were used.Later, steam turbines became more general, especially in large powerstations. More recently gas turbines have come into use, especially on oilplatforms where gas is produced as part of the production process usually insufficient quantities to provide a source of fuel.

The modern form of oil engine, the diesel, is also much used, principally forstandby or emergency plant when gas supplies to the gas turbines fail or areshut down. On a platform, of course, it is necessary to bunker diesel fuel forthese engines.

As the various forms of gas turbine may not be familiar to some, a briefdescription of this type of engine and how it evolved is given overleaf. Firstly,however, it may be advantageous to recap the principles of operation of its

predecessor, the steam turbine.

1.2 THE STEAM TURBINE

Fuel was burned under a boiler, whose water (shown blue) was turned intosteam. This steam, at high pressure and speed, hit the inclined blades of aturbine wheel (yellow) and drove it round. In doing so it lost some of itspressure, but enough was left to drive a second wheel and a third or fourth

on the same shaft. Finally the steam was exhausted into a condenser,turned back into water and returned to the boiler to be reheated and usedagain. This was the whole steam 'cycle,' which is shown in Figure 1.1.


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It should be noted that the gas is hottest at the combustion chamber or inletend. As it expands in the turbine, it cools, and it should leave the exhaustend at a lower temperature, of about 450C. Many turbines have instrumentsto measure exhaust temperature. If it is too high, it indicates some fault in thecombustion, and the set is usually shut down to save the blades from

damage.

Figure 1.3 Compressor


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In order for the fuel to burn, oxygen, in the form of air, is needed, and it mustbe at high pressure in order to enter the combustion chamber; therefore- anair compressor is fitted integrally with the turbine. It is just like a turbine inreverse. Air is drawn in at the larger diameter end by the inclined blades

acting as a suction fan. Once in, it is compressed by the blades of thecompressor rotor (shown yellow) into a smaller volume, to be sucked in againby the next row of blades and compressed still further. Each stage ofcompression causes the air to become hotter. Eventually it emerges at thesmall diameter end as hot compressed air. The principle of the gas-turbinecompressor is shown in Figure 1.3.

To provide the power to compress the air, the compressor must be drivenmechanically. The turbine itself drives it. The gas-turbine shaft is coupled tothe compressor shaft and constitutes the complete gas-turbine assembly.This looks like perpetual motion which it is, provided that the fuelcontinues to be supplied. The combined turbine and compressor unit isshown in Figure 1.4.

FIGURE 1.4SINGLE SHAFT GAS TURBINE SET.

In practice something like 80% of the power developed in the turbine fromthe combustion is needed to drive the compressor, leaving only about 20%'payload' to drive the load. It can be seen that, if there is a drop of only about5% in the combustion efficiency, so needing 85% of the output to drive thecompressor, the effect is to reduce the payload from 20% to 15% aneffective reduction of load drive of 25%. Gas turbines are therefore verysensitive to combustion control.


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In the gas turbine on Oil platform the power developed by the gas turbine,less that part of it needed to drive the compressor, is used to drive the load,which may be a machine such as a generator, air compressor or pump.

One of the great advantages of the gas turbine over other forms of prime

mover is its high power/weight ratio. An important point to note is that, unlike other types of engine, the gasturbine needs to take in up to 70 times the amount of air actually needed forcombustion. The excess is for cooling. This means that gas turbines havevery large intake ducting, usually provided with screens and filters to preventthe entry of sea birds and other sizeable particles. In freezing weather thescreens can become iced up and restrict the flow of air(not in UAE).Therefore, anti-icing equipment and blow-in doors are often provided (seepara. 1.13).

1.4 SINGLE SHAFT AND TWO SHAFT TURBINESThe type of gas turbine shown in Figure 1.4 is known as a 'single-shaft' type

that is, the power turbine, compressor and driven load are on a single,common shaft. The power delivered by the power turbine is divided betweenthe compressor (about 70% to 80%) and the driven load (about 20% to 30%).

In some larger gas turbines the arrangement is different. A standard aircraft-type jet engine may be used, as shown in Figure 1.5, where the compressorturbine is only large enough to drive the compressor itself, with no driven

load. But the exhaust gas which, in an aircraft, would go straight to jet isducted to the input of a further power turbine on a separate shaft;

its rotor is shown blue. This drives the load, and usually at a speed differentfrom that of the compressor turbine.

This is known as a 'two-shaft' gas turbine and has the advantage that it canbe used with an existing proved aero gas-turbine design with only minormodifications to the jet end. The power turbine is a completely separatedesign, which need not even be in line with the compressor (though it usuallyis). The complete aero compressor/compressor-turbine unit is known as the

'gas generator', and the separate load-drive unit as the 'power turbine'.

In a single-shaft gas turbine the power turbine is usually coupled to thedriven load (a generator- or compressor) through a gearbox. The compressorand power turbine therefore run at the same fixed speed, which is thegenerator speed multiplied by the gear ratio. In a two-shaft turbine thecompressor and the power turbine can, and do, run at different speeds. Thepower turbine is coupled to the generator and runs at governed speed, butthe compressor speed varies with the loading. At light load it will be idling,but as loading increases it increases its own speed up to full load, when it willgenerally be running much faster man the power turbine.


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Figure 1.5Two Shaft Gas Turbine Set

1.5 FUEL

Like any other internal combustion engine, a gas turbine can burn gas orliquid fuel (usually diesel oil). Some turbines are designed for single-fuelburning that is, for gas only or liquid only whereas others may havebeen adapted for 'dual fuel'; they may be set to run on either fuel or, in somegas turbines, on a mixture of both.

On oil platforms where gas is available the turbines will be for gas only ordual fuel. If dual, they will normally run on gas, with liquid fuel as a fall-back ifgas pressure should fail. If this happens, the changeover from gas to liquid isautomatic; the turbine does not stop, but an alarm is given. When gaspressure is restored the change back must sometimes be done by hand inslow time, but on some sets the change back is automatic provided that 'GasFuel' had been selected originally and the fuel selector switch had not beenmoved.

There are exceptions to the arrangements described above. In someinstallations there is no automatic changeover, even from gas to liquid fuel.

Gas turbines can operate on a variety of fuels which range from crude oil(with some de-rating of the turbine) through to fuel gas.


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1.6 SPEEDCONTROL

A governor always controls the turbine speed. In single-shaft sets the

governor controls the shaft speed, but in two-shaft sets it controls only thepower turbine speed and so the speed of the driven load. The gas-generator shaft is free to take up its own speed, depending on the load, asexplained below (see also Figures 1.6 and 1.7).

In single-shaft turbines the governor controls speed by regulating the gascontrol valve or the liquid fuel valve. In two-shaft sets the governor itself isdriven from the power turbine shaft but regulates the fuel input to the gasgenerator. This runs at such a speed as to provide just enough gas to thepower turbine to keep it at its correct speed. Thus, as load increases, the gasgenerator speeds up, but the power turbine stays at constant speed. The

skilled operator can detect load changes by the note of a two-shaft machine,but not with a single-shaft.

Speed control is discussed in detail in Chapter 5.

Figure 1.6Single shaft Gas Turbine Control


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FIGURE 1.7TWO SHAFT GAS TURBINE CONTROL.

1.7 STARTING

An external starter must start gas turbines. This may be electric, air-motor oreven a diesel engine. For some sets a separate small gas turbine is used,which itself is electrically started. Electric Starting requires a separate batteryand charger.

Because in single-shaft machines not only the compressor and turbine butalso the driven load (usually a generator) and gearbox must all be startedtogether, the starting unit must be relatively heavy, and this precludes electricstart, with battery, on any but the smallest machines. On the other hand in atwo-shaft set the starter has only to spin up the compressor/turbine unit ofthe gas generator, so that the starter need only be quite small and is suitedto an electric motor.

On some of the largest sets the d.c. power for the starting motor is taken notfrom a battery but is rectified from the set's general a.c. supplies. Thisrequires that auxiliary a.c. supplies for the turbine set shall be availablebefore the set can be started.

When the start button is pressed, but before the gas-turbine shaft actuallybegins to move, automatic circuits put into action a sequence programmewhich normally includes starting a lubrication pump to pre-lubricate theturbine, gearbox and generator bearings. In some machines the turbine hoodis also purged with air to remove any gas present. When the lubricating oilpressure has reached a certain level, the start motor is actuated and beginsto rotate and accelerate the turbine shaft (in the case of a two-shaft turbine,the gas-generator shaft only). When it reaches a certain speed, usually about20%, fuel is admitted to the combustion chamber, which is by now receivingsome air from the compressor. Automatic ignition by spark-plug and torch


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follows, and the hot burning gas passes to the power turbine in the case of asingle-shaft set, or to the compressor turbine of a two-shaft set.

The turbine gradually takes over the job of driving the compressor, and thestarting motor steadily becomes off-loaded. When its load falls to a

predetermined level, the electric start motor is switched off, or themechanical start motor un-clutched and stopped. The ignition is alsoswitched off. As the set runs up, a mechanically driven lubricating oil pumpbegins to deliver lubrication to the bearings. When its pressure reaches acertain level, the electrically driven pump stops automatically.

The turbine is now self-sustaining, and the speed continues to build up until itcomes under governor control and settles at its correct level. In a two-shaftset the gas-generator speed continues to build Lip, and the hot burning gasfrom it passes on to the main power turbine, which then starts to move byitself without any mechanical starting. Its speed too builds up until it comesunder governor control, when the fuel to the gas generator is cut back and itsettles down to its no-load or 'idling' speed. The power turbine, however, isnow running at its controlled speed.


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1.8 STOPPING


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To stop the set, fuel is simply cut off, and the set runs down steadily. Part ofthe stopping sequence includes the starting of the electrically drivenlubricating oil pump so that bearing lubrication continues as the set runsdown and the mechanical lubricating oil pump becomes less effective. Theelectric pump continues to run for some time after the set has actually

stopped.Some larger sets have a hydraulic ratchet arrangement which slowly turnsthe turbine rotor, after stopping, for up to 24 hours to prevent 'bowing' of therotor due to uneven cooling.

All sets are arranged so that, when the 'Stop' button is pressed or when ashutdown signal is given for any other reason, the associated generatorsupply breaker is tripped, if not already open, to off-load the turbine.Sometimes automatic provision is made to off-load the set gradually (exceptwith an emergency stop) before the supply breaker actually trips.

1.9 PROTECTION

The turbine (as distinct from the electric generator) has a number ofprotective devices to guard against malfunction both during starting and whilerunning. During starting each stage of the sequence is monitored to ensurethat it is completed within a certain time; if it is not so completed, the start is'aborted', and the set, if already moving, is stopped and the appropriatealarm given.

During running other protection operates, including such obvious things asover speed, excessive vibration, loss of lubricating oil pressure, highlubricating oil temperature and excess exhaust temperature (which indicatesa combustion fault). All these and other malfunctions shut the set down(having also tripped the generator breaker) and give the appropriate alarm.

Alarms are visual and audible. The visual alarms are grouped into'annunciator' lamp boxes on the control board, each lamp window beingannotated with the fault it announces. At the onset of a fault the lamp flashesand a buzzer sounds. When the 'Accept' button is pressed the buzzer stopsand the lamp burns steadily. It does not go out however until the fault hasbeen cleared; even then a 'Reset' button must be pressed before the set canbe started again. If a turbine stops during starting or shuts down during

running, it should be possible for the operator to diagnose from the lampindications the cause of the trouble.

On some sets the starting sequence lamps, which merely monitor the startingstages but do not indicate a malfunction, are segregated from the faultlamps. On most platforms the turbine malfunction alarms are repeated in theElectrical Control Room, though they are usually grouped to reduce thenumber of lamps there.


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One of the most damaging things that can happen to a turbine is failure oflubrication. This can cause bearings to fail at high speed with probablecatastrophic damage and danger to personnel. To guard against this all setshave not only the mechanically driven oil pumps and the a.c. electricallydriven pumps for starting pre-lubrication, but also a d.c.-driven emergency

pump fed from a battery, which operates automatically on failure oflubricating oil pressure. This is a vital piece of equipment, and it must beregularly tested to prove its proper functioning.

1.10 CONTROL

With every gas turbine there is a Local Control Panel, usually adjoining thecontrol panel for the generator. The turbine control panel has instruments toindicate speed, temperatures and pressures at various points and fuelpressure, as well as controls for starting and stopping, for fuel selection andfor setting the speed. There are a number of lamps in an annunciator panel

to indicate malfunction, and others to indicate successful completion of eachstarting sequence step. On some larger sets the governor control occupies acomplete panel on its own.

1.11 SPEED CONTROL

Control of speed by automatic governor is dealt with in Chapter 5.

1.12 WASHING

Air pollution, especially salt, can cause encrustation of the compressorblades, distorting their aerodynamic form and reducing the efficiency ofcompression. This shows up as higher exhaust temperature.

When this situation occurs (and high exhaust temperature indication is apointer; the compressor must be 'washed'. Turbine manufacturers makearrangements for this to be done at reduced turbine speed using water, withor without detergent or such as ground walnut husks or bran. The twomethods are sometimes known, as 'Crank Soak' (liquids) and 'AbrasiveCleaning' (solids).

Details of the recommended methods are given in the manufacturers'operations Maintenance Manuals.

1.13 ANTI-ICING (NOT IN UAE)

In freezing weather the air intake filter screens can become iced up; in thisstate they can severely restrict the intake of air and cause seriouscombustion problems.

One way of dealing with this is to duct warm air from the engine to the areaof the screens. On some makes of turbine this is taken from the exhaustducting on others it is bled off the later stages of the turbine compressor.


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Blocked screens, whether due to icing or other causes, can bring aboutproblems, and immediate steps should be taken to clear them. One featurewhich assists the air flow in the short term is the 'blow-in* door. This is adoor, usually on the side of the intake and downstream of the screens, whichis loosely hinged and is just kept closed by gravity. If the screens become

blocked, the differential pressure across them increases, and the lowerpressure inside the ducting sucks the door open, so allowing air to bypassthe screens until action to clear them can be taken. The opening of a blow-indoor gives an alarm co the operator. The door usually has a de-icing heaterto prevent its becoming iced up.


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CHAPTER 2

A.C. GENERATORS

2.1 GENERAL

The principle of a.c. generation is fully covered in the manual 'Fundamentalsof Electricity 2', where it is developed from Faraday's Law of ElectromagneticInduction to the idea of a modern generator with a rotating field and astationary armature. This chapter assumes familiarity with that concept anddeals with the actual hardware. Chapter 3 discusses the various methods ofexcitation.

Figure 2.1 shows, in cutaway form, a typical a.c. generator in the 15-megawatt (20 000 hp) size range. The generator proper is enclosed in a boxor 'hood'; this is both to exclude noise and to contain the closed ventilationsystem. It also assists purging before starting if gas has been present. Therotating parts are colored yellow and the stator blue.

Figure 2.1Typical A.C. Generator


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The armature (normally the stator) windings carry the load current, whichvaries with the loading. These windings have resistance and generate heat ata rate proportional to the square of the current (W = I2 R). The field's excitingwinding (normally on the rotor) also carries current. It too has resistance andgenerates I2 R heat. These two sources of heat, together with iron loss

heating, combine to raise the temperature of the machine. All the heat mustbe taken away by the cooling system if the temperature rise is to be heldbelow the designed limit.

Since the stator heating varies with the square of the load current, doublingthe load current gives rise to a four-fold increase in the stator heat generated.It is important therefore that the machine never becomes excessivelyoverloaded. If it does, the cooling system may be unable to handle the heat,and dangerously high temperatures may result.

The generator is cooled by a shaft-driven fan, which circulates air in a closedair circuit through all the windings. The air, in circulating, passes through airwater hear exchanger. Here the heated water is discharged and the cooledair re circulated, as shown by the arrows in the figure. Temperature detectorsat various points give warning of overheating; if it is seriously high andcontinues unchecked, the whole set is usually shut down.

If the cooling system should break down for any reason, panels in the hoodcan be removed and the machine cooled by natural ventilation through thefan. Under these circumstances however the loading on the generator mayhave to be curtailed to a value well below its normal rating.

The stator (armature) carries a 3-phase winding consisting of insulatedconductors in slots round the inside face. These conductors must beinsulated up to the full working voltage of the system. Serious or sustainedexcess temperature of the winding will cause this insulation to deteriorate oreven to break down completely, resulting in an internal flashover andpossibly complete write-off of the generator.

The rotor windings, which provide the field, operate at a much lower voltage may be below 100V-d.c. So insulation is less of a problem.Nevertheless, if the automatic voltage regulator calls for too much voltageand therefore too much fields current it is still possible to overheat anddamage the rotor.

2.2 ROTOR CONSTRUCTION

A.C. generators with rotating fields have rotors which fall into two types:salient pole and cylindrical. They are both shown in Figure 2.2.

The salient-pole type is illustrated in Figure 2.2(a). It is common with offshoregenerators and also with the smaller sizes onshore. It consists of a solid ironmotor body (square in the case of a 4-pole rotor) onto which pole pieces arebolted. Each pole piece carries one of the field windings as shown in thefigure. The poles terminate in pole shoes which spread out the magnetic fieldin the air gap, but it should be noted that with the salient-pole arrangement


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the air gap, and so the air gap flux, is far from uniform. Some rotors havedamper windings embedded in the pole shoes, but these are not shown inFigure 2.2(a).

FIGURE 2.2 A.C. GENERATOR ROTORS.

The salient-pole rotor is commonly used with low speed while the cylindricalrotor normally used with higher speed.

The cylindrical rotor (sometimes also called 'turbo type) is, as the nameimplies, completely cylindrical and has no projections. It is illustrated inFigure 2.2(b). The field windings are embedded and wedged into slots in therotor surface in a similar way to the stator slots. (The overhang of the endwindings has been exaggerated in the figure to make the constructionclearer.) The rotor slots cover only part of the surface and are disposedeither side of the poles, the whole field winding forming a spiral around eachpole centre.

The air gap is uniform, and consequently the air gap flux due to the fieldwinding is almost purely sinusoidal around the gap, being maximum oppositeeach pole centre. The smooth surface also results in low windage resistance.

Cylindrical rotors are very sound mechanically and are favoured for large,high-speed generators (3 000 or 3 600 rev/min), where centrifugal forces ona salient-pole rotor would present severe problems. Consequently cylindricalrotors are common with 2-pole generators and are sometimes used with 4-


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pole types. They are never used with six poles or more, where the rotorconstruction would become far too difficult.

2.3 INSULATIONGenerator windings are insulated against the highest voltages to which they

may be subjected, and the insulation must withstand a certain specifiedmaximum temperature without deteriorating. There are many insulatingmaterials with different and often conflicting properties. They aregrouped into a number of classes, depending on the maximum temperatureto which they will be exposed and on the insulating material used. Theclassification is as follows (in accordance with BS 2757 ).

Class Typical Insulating Material UltimateTemperature

Y Cotton, silk, paper, etc., un impregnated 90C

A Impregnated cotton, silk, etc.; paper; enamel 105C

E Paper laminates; epoxies 120C

B Glass fibre, asbestos (un impregnated ); mica 130C

F Glass fibre, asbestos, epoxy impregnated 155C

H Glass fibre, asbestos, silicone impregnated 180C

C. Mica, ceramics, glass, with inorganic bonding > I80C

Note: Asbestos no longer uses because of environmental condit ions

It should be noted that the classification letters do not follow an alphabeticalsequence. This is because there were originally only three classes 'A', 'B'and 'C'. Later intermediate classes were added, and it was decided not todisturb the original well-understood three. Most platform and shore-installedgenerators are Class 'B' or 'F'.

Certain of the higher-temperature insulation materials may be hygroscopicand therefore not always suitable in any particular environment, particularlywhere dampness is severe.

It should be particularly noted that the classification depends on the ultimatetemperature to which the insulating material may be subjected, for it is thiswhich determines whether or not it will suffer damage when heated. It doesnot depend on temperature rise alone, for instance, the ambient temperatureis 40C, a Class 'B' material may be used if the designed temperature risewill not exceed 90C, so making the ultimate maximum temperature 130"C.Designed temperature rises therefore must take into account the greatestexpected ambient temperature in which the machine will operate.


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2.4 COOLING

All generators used on platforms and in shore installations are air cooled.The air is circulated past the stator and rotor windings by a fan on thegenerator shaft. The warmed air itself may be discharged to atmosphere andnot used again ('Circulating Air' or 'CA'); or it may be water cooled in aseparate cooler with a forced water circulation ('Circulating Air, Forced Wateror 'CAFW'); or in a radiator-type cooler ('Circulating Air, Natural Water' orCANW). There are usually alarms if the air or water temperatures exceedcertain limits. All the largest gas-turbine generators are CAFW cooled.

The above letter coding was formerly in general use and is well understood.Recently however a new international coding system for cooling methods hasbeen introduced for all rotating machines (BS 4999, Part 21) and is likely tobe met with on modern drawings. It consists of the letters 'IC' followed by twodigits The meanings of these digits are given below for typical platform orshore-installed generators ;

First Digit Second Digit

0 free circulation1 Inlet duct ventilated2 Outlet duct ventilated3 Inlet and outlet duct ventilated4 Frame surface cooled5 Integral heat exchanger (using

surrounding medium)6 Machine-mounted heat exchanger

(using surrounding medium)7 Integral heat exchanger (not using

surrounding medium)8 Machine-mounted heat exchanger

(not using surrounding medium)9 Separately mounted heat exchanger

0 free convection1 Self-circulation2 Integral component mounted on

separate shaft3 Dependent component mounted

on the machine5 Integral independent component6 Independent component mounted

on the machine7 Independent and separate device

or coolant system pressure8 Relative displacement

Where it is desired to specify the nature of a coolant, the following letter codeis used in conjunction with the cooling code:


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When nothing but air is used, the letter 'A' may be omitted.

Thus a generator cooled by air with an internal fan and with an air/water heat

exchanger using pressurised water from the platform system would beclassified IC87, or IC8A/7W, instead of the former CAFW.

The larger generators also have thermocouple-type temperature detectorsembedded at various points in the windings. If anyone of them exceeds acertain temperature, an alarm is given on the control panel. The panel also hasfacilities for the operator to scan all the detectors in turn and to read off theactual temperatures.

2.5 EXCITATION AND VOLTAGE CONTROL

The different forms of excitation and automatic voltage control are dealt with inChapter 3.

2.6 NEUTRAL EARTHING RESISTOR

The star points of all high voltage generators on platforms are earthed througha current- limiting 'neutral earthing resistor' (NER). Its purpose is to limit thefault current flowing through the generator if an earth fault develops anywhereon the system.

The N E R is separately mounted near the generator and usually consists of aframe containing a heavy grid-type resistance element capable of carrying alarge current for a short time. This short-time rating is possible because anyheavy fault current will be quickly cleared by the earth-fault protection.

NER are therefore given a maximum current rating for a maximum time

For example 200A for 30 sec, they may also have a continuous current rating

For example 25A cont.- to cover small earth leakage and harmonic currentswhich are not large enough to operate the protection. Their ohmic value goesdown to about 10 ohms for the largest off shore generators.

The NER unit sometimes contains also a current transformer to measure thepresence of any earth fault current in order to initiate the protection


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INSULATED BEARINGS

Bearings of a large machine arc often insulated to prevent stray currents fromcirculating through them. Such currents can arise from emfs being generatedin the rotor shaft due to stray magnetic fields. Under fault conditions these

stray fields can bc very large. Figure 2.5(a) shows how such currents may flowthrough the bearings.

FIGURE 2.5

INSULATION OF BEARINGS.

These currents, if allowed to flow, would arc across the bearing surface andcause small craters, which would eventually destroy the bearings. Figure 2.5shows pedestal sleeve bearings, hut the same principles apply to ball androller bearings.

The current path of Figure 2.5(a) can be broken by insulating one or bothbearings the insulation may be at the bearing housing or, more commonly,beneath the pedestal where it seats on the bed plate stool as shown in Figure2.5(b). The insulation of only one bearing is more usual, but insulating bothallows the insulation to be checked.

For reasons of safety the shaft must be at earth potential. Consequently onmost machines one bearing (the un insulated end if only one is insulated) isfitted with an earth strap, one end of which terminates in a brush running onthe dry shaft. If the generator is of the 'over- hung' type with only one outboardbearing, such as with certain diesel-generator sets, this bearing is insulatedand the earthing of the rotor shaft is made through the engine and coupling.

The insulation of the pedestal is carried out by a shim of insulating materialbetween base of the pedestal and its stool. The holding-down bolts are bushedwith insulating material. Sometimes two insulating shims are used with a thinmetal sheet between them. This enables the insulation resistance of each partto be measured separately, since the shaft and bed plate are normally both atearth potential.

It is important that, where a bearing pedestal is insulated, no waste material ortools should be allowed to lean against it, as they would short-circuit theinsulation.


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CHAPTER 3

GENERATOR EXCITATION AND VOLTAGE CONTROL

3.1 GENERAL

The excitation of a generators field system has already been mentioned inChapter 2, as it is not possible to describe a.c. generators without referring totheir field system and excitation. This chapter discusses the three practicalmethods of field excitation, which may be encountered.

Fig.3.1 A.C. GENERATOR EXCITATION

3.2 CONVENTIONAL EXCITATION


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Fig. 3.1(a) shows the conventional method described in the manualFundamentals of Electricity 2, where a driven d.c. exciter (in this case belt-driven) feeds its d.c. output through slip rings to the main generator field.

The output voltage is sensed by an automatic voltage regulator (AVR), whichregulates the exciters field so that the exciter output holds the main field atwhatever level is necessary to maintain the generator output voltage constant.

AVRs are discussed later in this chapter. It will be seen that the control ofvoltage is a closed loop, and, like any other closed loop servo- mechanism it issubject to certain errors.

3.3 STATIC EXCITATION

Fig. 3.1 (b) shows a development where the rotating d.c. exciter is replaced bya static electronic exciter, which usually incorporates the AVR. Voltagesensing and excitation power are derived from the main generator output;

excitation current is controlled by the AVR, rectified and fed in to the main fieldthrough slip rings, just as in the conventional case. This is called the staticexciter method, and it should be noted that it still requires brushes and sliprings. It is not found on platforms but is widely used onshore, although not toany great extent in oil installations.

3.4 BRUSHLESS EXCITATION (GENERAL CASE)

A further significant development is shown in Fig.3.1 . Here the shaft-drivenrotating exciter has been restored, but it now takes the form of an a.c.generator of the fixed-field type mounted on the main shaft itself. Its a.c.output is taken through connections inside the shaft, through a diode bridgewhich rotates with the shaft, to the main rotating field of the generator. Thefield is thus excited by d.c. without the need for brushes and slip rings. It willbe seen that this exciter cannot be belt-driven; it must be integral with the mainshaft.

As with static excitation, voltage sensing and excitation power is derived fromthe main generator output. Excitation current is controlled by the AVR, rectifiedand fed in to the fixed field of the a.c. exciter. The a.c. output of the exciterfollows the AVR signal, and its output current is rectified by the diodes whichrotate with the shaft; the d.c. output from them is in turn passed to thegenerators main field. The field current thus follows the AVR signal almost

exactly.It will be seen that the only link between the fixed and moving parts is themagnetic one between the exciter field and its rotating armature: no slip ringsand brushes are needed. The method is for this reason called brushlessexcitation, and it will be found, in one form or another, an all platform andonshore main and auxiliary generators.

The principal advantage of brushless excitation over the other two types is thatthe absence of brush gear and slip rings greatly eases the maintenanceproblem. And to avoid sparks that can cause fire in the flammable atmosphere


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3.5 BEHAVIOUR UNDER SHORT CIRCUIT

In the conventional case (Fig. 3.1 (a) excitation power is derived from aseparate d.c. generator which is not affected by the voltage on the main

generators output lines. However, with both static excitation and the brushlessexcitation described above (Fig. 3.1(b) and (c) excitation power (as well assensing) is derived from the output of the generator itself true shuntexcitation.

Under normal conditions this is quite satisfactory, but under short-circuitconditions the generators output voltage will drop heavily it might evenvanish. Under this low-voltage output situation the AVR will try to force up theexcitation, but just at the moment it wishes to do so, it has no power available.Under these conditions a collapse of system voltage is possible.

To overcome this a method is employed which makes use of the short-circuitcurrents themselves to provide the missing excitation.


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Fig.3.2 A.C. GENERATOR EXCITATION (2)


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3.6 BRUSH LESS EXCITATION (WITHOUT PILOT EXCITER)

Three heavy current transformers are arranged in the generator output lines asshown in Figure 3.2(a). Their secondary outputs are rectified and passed to

the main exciter's field either in parallel with the normal excitation (as shown)or sometimes to a separate field winding in the exciter. Although they take theform of current transformers, these units, when used in this application, arereferred to as 'short-circuit CTs'.

Under short-circuit conditions when the generator output voltage is very low,the short-circuit CTs pick up the heavy short-circuit currents and, after theyhave been rectified, use them to boost the main exciter field, and so the mainfield. This serves to maintain the generator output voltage under short-circuitconditions - a necessary requirement in network operation so that protectionmay operate reliably.

Short-circuit CTs are used generally with medium-sized generators with eitherstatic or brushless excitation where no 'pilot exciter' is fitted (see below) andwhere excitation power is drawn from the generator's output. This applies tomost basic services generators on platforms and to some main sets.

3.7 Brushless Excitation ( with pilo t exciter)

With large brushless generators a different method is used. Instead of drawingexcitation power from the generator output, the AVR has only a voltage-sensing connection. The arrangement is shown in Figure 3.2 (b)

The exciters field is powered independently from a separate high-frequencyinductor-type generator called a 'sub-exciter' or 'pilot exciter'. It has permanentmagnets as rotating field and is driven by the main shaft. It also providesoperating power to the AVR itself. Only the voltage sensing leads to the AVRare taken from the main generator output. The AVR regulates and rectifies thepower from the pilot exciter to the main exciter field. This in turn regulates thea.c. exciter output, and thence the d.c. rectified input to the main field throughthe diodes, to hold the generator output voltage constant.

The pilot exciter is mounted on the main shaft, usually immediately next to the

main exciter not exactly as in Figure 3.2(b), which is schematic only). It isusually arranged in a single enclosure with the main exciter and the diodeplates. Figure 3.3 shows this arrangement.

As in the conventional case, the excitation of the generator is now independentof the generators output voltage and so is maintained even under short-circuitconditions and without the use of short-circuit CTs. This is the arrangement onalmost all platform main generators.


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3.8 THE DIODE BRIDGE

In Figures 3.1(c) and 3.2(a) and (b) the diodes are shown for clarity as insidethe shaft between the exciter and the main generator. The exciter output is 3-

phase, and the diodes are in fact a 3-phase full-wave bridge, requiring sixdiode elements. Clearly they cannot be buried in the middle of the shaft, and inpractice they are mounted on a rotating plate on the extreme end of the shaftat the exciter end, as shown in Figure 3.3 in green. This makes them easilyaccessible for inspection, testing or replacement.

Fig.3.3 GENERATOR AND DIODE PLATE

A point on the use of diodes should be noted. If one of the six should fail,either by open or short-circuiting, harmonic currents flow in the main fieldcircuit. These harmonics are reflected into the field circuit of the main exciterand are detected by a 'diode failure' relay tuned to respond to the principalharmonic frequency; the alarm (or trip) signal from this relay is time-delayed by10 or 15 seconds to prevent false operation.

A diode failure would have no discernible effect, from the consumer's point ofview, on the generator's output voltage. The reduced d.c. output from thediode bridge with one diode faulty would lower the main field's d.c. currentslightly, and with it the main generator's output voltage. This would beimmediately detected by the AVR, which would increase the excitation until thevoltage was restored, and the consumer would not be aware of it. However,the remaining healthy diodes might then be somewhat overloaded, and thesituation should be corrected.


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significant after a short circuit has been cleared. During the period of the faultthe voltage will have dropped and the AVR will have forced to the excitation,probably to its limit. When the fault is cleared this over excitation shows as alarge overvoltage on the whole system, which is comparatively slow torecover. This could involve a risk to burnout of lamps or delicate apparatus.

3.10 AUTOMATIC VOLTAGE REGULATORS (AVR)

AVRs are of many different makes, and various types are found on platformsand onshore installations.

All, however, have certain features in common when used with brushlessgenerators. They are nowadays entirely electronic; they take their operatingpower from either the main output or the shaft-driven high-frequency sub-exciter (typically at 400 Hz), but they sense the voltage to be controlled fromthe output side of the generator before the circuit-breaker terminals. In high-voltage generators this sensing circuit is taken through a measuring voltagetransformer of at least Class 0.5 accuracy

The detailed electronic circuits are not discussed here, but power from themain output or the high frequency pilot-exciter is rectified through thyristors,which are controlled by the voltage-sensing circuits to provide the correct d.c.current to the field of the main a.c. exciter.

3.10 AVR SET-POINT

Like any closed-loop servo, an automatic voltage regulating system holds thevoltage constant, within stated errors, at whatever level it has been set. Thislevel is referred to as the set-point

In an electronic AVR the set point is adjusted by a variable resistance (rheostat), in the appropriate part of the circuit. On some generators thisrheostat is outside the AVR proper and is mounted on the adjacent generatorcontrol panel for manual control; it is usually marked Raise Volts/Lower Volts.On other makes of generator it is arranged for remote control from somedistant panel. In such a case the rheostat is motor-driven, the motor beingcontrolled forward or backward by a 2-way-and-off spring-loaded switchmarked as above.

When used with a single generator, the AVR set-point control does indeedregulate the machines voltage output, but when used on a generator runningin parallel with others, the prime function of the AVR control is not so much toregulate voltage but to adjust the sharing of reactive load between thegenerators, despite the marking of the control knob or switch. It does,however, have some effect on voltage level, but this is only secondary.

3.11 A.C.GENERATOR VOLTAGE REGULATION

When a load is applied to the terminals of a generator previously running at noload and without AVR control, the terminal voltage will drop by an amount

x 100%Vo - V


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which depends on the nature of the load. It is usually quoted at full rated load that is, at the full-load rated current and rated power factor and is expressedas a percentage of the no-load or system voltage. Thus, if Vo is the no-loadvoltage and V the generator terminal voltage at full rated load and power factorand with the excitation unaltered, then

Is the percentage full-load regulation.

In practice of course the reduced voltage V would be immediately detected by

the AVR, which would increase the excitation until the terminal voltage wasrestored to the system value Vo.

Vo


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CHAPTER 4

DIESEL GENERATOR SETS

4.1 GENERAL

In large onshore installations power is derived from the National Grid. Onplatforms the main generating sets are always driven by gas turbine, using theplatforms own gas as fuel when available, with liquid fuel as an alternative insome cases.

Onshore the grid supply can sometimes fails, and on platforms maingenerators may also fail, or under certain conditions they may be deliberatelyshut down. In either there is loss of main power supply, and it is important thatthere should be immediately available a quick-starting alternative supply andthis means diesel generation.

All platforms, and most large onshore installations, have one or more diesel-generator sets. In many cases they are arranged to start automatically on lossof main voltage and to switch themselves onto an emergency switchboard. It isnever the intention that such generators should replace the lost main ones, but

they should provide limited power for only really essential services such assome degree of lighting, safety, instrumentation, communications, fire and gasdetection and so on.

Diesel-driven generators are also required for black-start conditions when nomain generators are running out whose auxiliaries must be run in order to startthem. Such diesel sets must of course be entirely self-contained, requiring noexternal assistance to start them.

The construction of a diesel engine is well known and will not be describedhere. It is usually multi-cylinder, turbo-charged and jacket-cooled through awater/air radiator, sometimes assisted by a cooling fan. It is usually battery-started, and some sets have an alternative hydraulic starter, hand pumped, foruse if the battery becomes discharged, for example after a prolongedshutdown. It is vitally important for diesels which drive emergency generatorswhich are automatically started that the batteries are maintained fully chargedready for an instant start; also that practice starts should be exercisedregularly.


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4.2 BASIC SERVICES

In all installations the really essential services, which it is vital to keep runningeven when the normal main power has been lost, are offshore termed BasicServices (it called also essential services or emergency). The diesel-drivengenerator is called the Basic Services Generator and its switchboard theBasic Services Switchboard, both shown in red in Figure 6.1. The system isusually at low voltage (415V), and positive steps are taken to see that thebasic services generator does not feed back into any non-basic low-voltageservices or into the high-voltage system. (There are however some exceptionsto this practice.)

Under normal conditions on an offshore platform the basic servicesswitchboard is part of the complete 415V distribution system. It is incontinuous use and is normally fed through an interconnector from a main415V board, as shown in Fig. 4.1. If power on the main board fails the basicservices board is isolated from it and can be fed direct by its basic services

generator, which normally has sufficient capacity for that board and no more.The generator may start automatically on failure of the main 415V power, butquite commonly it must be manually started. The incomer circuit-breaker fromthe generator, is interlocked with the incomer from the main 415V board sothat both cannot be closed at the same time; therefore the generator can never

Fig. 4.1--Typical Basic Services and Black Start Generator Arrangement

feed back into the remainder of the 415V system or, through the transformer,into the HV system ( other than with the exceptions mentioned above ).


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Auto starting is achieved by providing the basic service busbar with anundervoltage relay, which causes the interconnector to open on loss of mainsupply and the basic service generator to start. When the generator has

started and run up, it closes its incomer breaker automatically and in so doinglocks out the interconnector. Even when main power is restored, theinterconnector breaker cannot be reclosed onto the basic services board untilthe operator has first opened the generator incomer breaker, so lifting theinterlock. The normal interconnector incomer breaker can then be closed, andthe system reverts to normal. The basic services generator is afterwardsstopped manually and left in a condition to restart whenever needed.

Where the start is manual no undervoltage trip is provided, but instead the actof manually closing the generator incomer breaker also trips and locks out theinterconnector incomer breaker. When power is restored, the process isreversed manually.

With regard to the exceptions referred to above, on some of the newerplatforms larger diesel-generator sets are fitted which have a capacityappreciably greater than that needed only for the basic services switchboardand its essential loads. In those cases some limited feedback into the systemis allowed to power other less essential but still important loads, such asutilities. In that case the interlock between the generator and interconnectorbreaker is not fitted.

4.3 AVAILABILITY OF BASIC SERVICES GENERATOR

A basic services generator is nearly always needed in a hurry, whetherautomatically or manually started. It is therefore always left in a ready-to-runstate. If automatic, the selector switch is left on Auto, even if it had beenturned to Local for the previous manual stopping. Ready-use fuel tanks arekept full, oil and water levels correct, battery fully charged and heaters on.These things are checked daily and always after the machine has been run.

Where basic services generating sets are automatically started on loss of mainsupply, this feature is regularly tested to ensure that it functions correctly.Manual starts on all auxiliary sets are also regularly exercised.

4.4 BASIC SERVICES GENERATOR UTILITIES


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Most diesel engines are electrically started from a local battery, usually 24V.When the engine is not in use this battery is kept fully charged by a chargerfed from the main a.c. system. An engine-driven (d.c. Generator) charges thebattery when the engine is running.

Basic services diesel generator unit is provided with ready-use fuel tanks witha capacity sufficient for at least 24 hours full-load running. As main suppliers

are assumed to have been lost, fuel pumping facilities may not be available,and it may be necessary to refill the tank by hand-pumping from barrels.

Each diesel generator unit is provided with a local control panel on or near theengine mounting, from which the output can be controlled and monitored forspeed and voltage. No remote control is exercised from the Electrical ControlRoom on the generator and interconnector circuit breakers. All control is local,but there is usually some remote instrumentation in the Electrical ControlRoom.( some remote actions may be available as exception of the above)

Typical Emergency Generator Circuit


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CHAPTER 5

SYNCHRONISING OF GENERATORS

5.1 GENERAL

The idea of synchronising is not new. Every time you change gear in a car yousynchronise the engine to the road speed so that, when the clutch is let in,both shafts are running at the same speed and there is no jerk. Conversely, ifyou synchronise badly there is a jerk and possibly a lot of noise. The sameapplies with electrical machines when they are put in parallel.

Only offshore installations have main and subsidiary generators. Onshore

there are only emergency generators, usually only one per installation.Consequently this chapter applies only to offshore installations.

5.2 D.C. GENERATORS

The simplest case of synchronisation occurs with d.c. generators,

Figure 3.1 represents two d.c. generators, both on open circuit but about to beparalleled by a switch. Each is separately excited such that machine 'A' has anopen- circuit voltage V A and machine 'B' VB. Machine "A' is assumed to be the'running' generator, and machine "B' is the 'incoming' generator which is to beparalleled to A'.

Before closing the switch which puts the two generators in parallel it isnecessary only to ensure that their voltages are the same - that is, VB = V A;then the switch may be closed, and no sudden current will flow - there will beno electrical jerks'.

If the voltages were different, suppose that V A is greater than VB. On closingthe switch there will be a closed loop with the emfs V A and VB opposing oneanother. Since V A is greater than VB, there is a net clockwise emf in the loop,

which will cause a clockwise current Ic to flow round it (shown in red), limitedonly by the resistances of the two armatures. This current appears suddenlyas the switch is closed, putting a sudden load onto generator 'A', so causing itto slow with a jerk, and causing generator B' to motor, making it acceleratewith a jerk. This circulating current, which occurs on closing the switchwhenever V A and VB are not equal, is also called the 'synchronising current'.To avoid it and its consequent jerking effect on the system, the incomingmachine voltage must first be matched to the voltage of the running machine -normally done by trimming the field of the incoming generator.


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5.3 A.C.GENERATORS

With a.c. generators the problem is more complicated. It can be seen in thed.c. case how a circulating current is caused by differing opposing voltages. Ind.c. this is straightforward, but in a.c. a voltage difference can be caused eitherby differing voltage amplitudes or, for the same voltage amplitudes, bydiffering phase.

In Figure 5.2(a) the two voltages VA and VB are in phase with one another,but their amplitudes are different. At any instant such as time T, theinstantaneous voltage of machine A' is TA and that of machine 'B' is TB.Therefore there is, at that instant, a voltage difference AB which will cause acirculating current to flow between the generators when the paralleling switchis closed. This is true at any instant other than a common voltage zero.

In Figure 5.2(b) the two voltages have equal amplitudes but are displaced in

phase, VB lagging on VA. At any instant such as time T the instantaneousvoltage of machine 'A' is TA and that of machine 'B' is TB. Although the twovoltages are equal in amplitude, there is still an instantaneous difference ofvoltage AB which will cause a circulating current to flow between thegenerators when the paralleling switch is closed. Therefore, even though thevoltage levels, (as read by voltmeters) are the same, a difference of phasewill still cause a circulating, or 'synchronising', current to flow between themachines, causing one to accelerate and the other to decelerate and to jerkthem into phase with each other as the switch is closed.


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Fig. 5.2

Voltage and Phase Difference

Therefore, to prevent sudden circulating currents occurring and to achievesmooth paralleling, the voltages of both machines must first be equalised andthe machines then brought into phase. This is described in para.5.4.

There is one further requirement. As when changing gear in a car, the twogenerator speeds must also be equalised before paralleling. If this is not done,the faster machine will be jerked back and the slower jerked forward, whichcould cause serious mechanical problems in large machines, as well as to thecouplings, gear trains and prime movers.

If the two machines are running at different speeds before paralleling, this willshow as different (frequencies on the frequency meters. Therefore apreliminary to synchronising is to equalise as nearly as possible not only themachine voltages but also their frequencies, using the switchboard voltmetersand frequency meters.


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5.4 SYNCHRONSING A.C.GENERATORS

It is assumed that one machine 'A' (the 'running' generator) is already inservice on the busbars and is on load, and that a second machine 'B' (the

'incoming' generator) has been started and run up and is ready to be put inparallel with 'A' in order to share its load. Before this can be done the incominggenerator 'B' must be synchronised with the running machine 'A'.

As already described, the first step is to match the incoming to the runningvoltage by reference to the voltmeters on the two generator control boards,and by using the incoming voltage regulator to trim it. Similarly the incomingfrequency is matched to the running frequency by reference to the twofrequency meters and by trimming the incoming speed regulator. Note that therunning machine controls should not be touched - the incoming machine isalways matched to the running, not vice versa.

It now remains to bring the generators into phase. Even after matching thefrequencies by meter, the speeds will still not be exactly equal, and onemachine will be slowly overtaking the other. As this occurs, their phaserelationship will be steadily, but slowly, changing. The idea is to make this takeplace as slowly as possible and, as they momentarily pass through the in-phase' state, to catch them at that point, to close the paralleling switch and tolock them there.

There are two ways in which the correct phase may be detected - the first is bylamps, and the other is by an instrument called a synchroscope.

5.5 LAMP SYNCHRONISING

5.5.1 The 2-Lamp Method

Synchronising by lamps makes use of the circuit shown in Figure 5.3;two lamps in series are connected across the same phase of eachgenerator. Only when the two systems are in phase is the voltageacross the lamps continuously zero, and both lamps are out. At all othertimes there is a voltage difference, and the lamps glow. This is known

as the 'lamps dark' method of synchronising.The voltage phase vectors of both generators are shown. Machine No 1is the 'running' and its vectors are in full line. Machine No 2 is the'incoming' and its vectors are dotted. It is approaching synchronism withNo 1.


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Fig. 5.3 Lamp Synchron ising (2-Lamp Method)

When the machine frequencies are nearly equal, the lamps areswitched on and alternately glow and go out, giving a slow flashingappearance. The nearer the frequencies are to being equal, the slowerthe lamp flashing period. Therefore to achieve phase matching, theincoming machine's speed is slowly trimmed, until the lamps areflashing very slowly; then, as they are changing from bright to dark, theoperator places his hand over the breaker control button or handle and,at the moment when the lamps go completely out, operates it to closethe breaker. The lamps then stay out, but they should be switched offafter completing the synchronising.

NOTE The lamps could be connected to burn at their brightest, insteadof being dark, when the systems are in phase, but this 'lampsbright' method is seldom used today. It is easier to detect theexact point of 'no light' in a lamp than to estimate when it is at itsbrightest. The 'lamps dark' method is almost universally found.

It is necessary to use two lamps in series because, when the systems

are fully out of phase (lamps at brightest), the voltage difference is thendouble the system phase voltage.


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5.5.2 The 3-Lamp Method

An alternative method, know as 3-lamp synchronising is found on manyplatforms. It is shown in Figure 5.4.

The three lamps are connected as shown: No.1 (yellow-to-yellow), No.2(blue-to-red) and No.3 (red-to-blue). In the centre diagram the full linesrefer to generator A (R1, Y1 and B1), and the dotted lines to generatorB (R2, Y2 and B2). Machine B is shown approaching synchronismwith machine A.

With the lamps so connected, the voltage across No.1 lamp (Y1-Y2) issmall, and the lamp glows dimly. The voltages across No.2 and No.3lamps (B1-R2-B2) are large, and both lamps are bright. As synchronism

is reached (left-hand of the three lowest diagrams), No.1 lamp goes outand the other two have equal brightness.

When the two generators are 120 o out of synchronism (centre of thethree diagrams) it can be seen that it is No.2 lamp (B1-R2) which hasno voltage and goes out. 120o later (right-hand diagram) No. 3 lamp(R1-B2) goes out.


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Fig. 3.4

Lamp Synchronising (3-Lamp Method)

Thus, as generator 'B' catches up with generator 'A', each lamp goesout in turn, and at a decreasing rate, as synchronism is approached.Finally, at synchronism, No.1 lamp remains extinguished long enoughfor the generator breaker to be closed.

The lamps are arranged either in a triangle with No.1 at the top, or in aline with No.1 in the centre. They may be lettered 'A', 'B' and 'C' insteadof being numbered. Depending on whether the order of going out isclockwise or anti-clockwise with the triangular arrangement, or left-to-right or right-to-left with the in-line arrangement, the operator candetermine whether the incoming generator is fast or slow - which cannotbe done with the 2-lamp method.

5.6 SYNCHROSCOPE


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The synchroscope method is normally used on those offshore installationswhere they are provided, but the synchronising lamps are often retained as afall-back in case the synchroscope should fail. Therefore synchronising bylamp should be regularly exercised where this facility is available.

Fig.5.5 SYNCHROSCOPE

A typical synchroscope is shown in Fig. 5.5. It is an instrument with amovement similar to that of a power-factor meter, but with the two windings fedfrom the running and incoming voltages. Whereas in a power-factor meter thecurrent/voltage phase relationship is fixed and the pointer is stationary, in asynchroscope the phase relationship between the two voltages is constantlychanging and the pointer rotates continuously, the direction of movementdepending on whether the incoming machine is rotating faster or slower thanthe running. The face is marked with arrows denoting FAST or SLOW; theseterms always refer to the incoming generator. When the pointer passesthrough the 12 o Clock position, the machines are momentarily in phase.(Some synchroscope are marked + and -. The plus sign corresponds toFAST and the minus to SLOW).

5.7 SYNCHRONISING AT A SWITCHBOARD

Most switchboards control two or more generators, and some have sectionbreakers or interconnectors to other switchboards, any one of which may haveto be synchronised with running machines. It would not be economic to haveseparate synchroscope for each one, as it is used only infrequently.

The practice is therefore to have one synchroscope (usually with back-uplamps) in a central or conspicuous position on or near the switchboardtogether with selector switches whereby any chosen machine may be madethe incomer. Selection may be by manual switch, key or plug. The running sideis usually taken from the busbar. Where the switchboard handles high voltagethe incoming and running voltage signals are taken through voltagetransformers. The synchroscope is provided with fuses and an isolating switch,as it is not good practice to leave it in circuit when it is not in use.


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To use the synchroscope, having selected which is to be the incominggenerator, the voltages and frequencies are first matched as already describedin para. 5.4. The synchroscope is then switched on; its pointer will be rotating.The incoming speed regulator is trimmed until the pointer is moving veryslowly in the FAST direction. As it next approaches the 12 o'clock position, the

hand is placed over the breaker control button or handle and, just before thepointer reaches 12 o'clock, it is operated to close the breaker. Thesynchroscope will then stop and remain locked in the 12 o'clock position as thegenerators remain in synchronism. Finally, the synchroscope must beswitched off.

The reason why the incoming generator should be running the faster is that,when the breaker is closed, it will immediately take up a small part of the load.If it were running slower, that load would be negative - that is, the machinewould 'motor' - and a reverse power situation would exist. The generator'sreverse power protection might then cause the breaker to trip.

5.8 AUTOMATIC SYNCHRONISING

Most offshore switchboards are provided with an automatic synchronisingfeature. This consists of a number of relays (usually in a single case) whichcompare the incoming and running voltages and frequencies as well as theirphase relation. Should any of these be outside limits, the incoming voltageregulator or speed regulator is automatically trimmed. Only when all three arewithin predetermined limits is a signal given automatically to the circuit breakerto close.

Here again there is usually only one auto-synchronising unit to eachswitchboard; it is connected automatically to whichever machine is beingstarted so long as the synchronising selector switch is set to AUTO.

Auto synchronising is usually reserved for generators only. All othersynchronising for example across section breakers or interconnectors or onLV switchboards - is normally by hand.

5.9 CHECK SYNCHRONISING

In many instances, particularly with smaller generators and in the cases justmentioned, automatic synchronising is not used, and the exercise must becarried out manually by lamp or synchroscope. In such cases there is a dangerthat, if the manual synchronising is carried out unskilfully or by an operatorunder instruction, the switch could be closed at the wrong instant and severedamage could result to expensive machinery.

This can be prevented by 'check synchronising'. The equipment is similar to

that used for auto-synchronising, but it does not automatically trim theincoming voltage, frequency and phase - it only monitors them. Nor does itcarry out the final act of closing the circuit breaker automatically; these all have


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to be done manually by the operator. However it does inhibit the breaker'smanual closing circuit so that, unless all three synchronising conditions aresatisfied together, the operator cannot close the breaker even though hepresses the CLOSE button. If the breaker then fails to close, the wholesynchronising process must be repeated.

Some check synchronising units sense only phase angle difference and do notmonitor voltage or frequency differences. They rely on manual adjustment ofvoltage and frequency and only inhibit the closing of the breaker when thephase angle difference is excessive. It should be noted that voltage differencewill cause circulating reactive current only. Although this is not desirable, itdoes not cause any mechanical shock and consequent damage to thetransmission or the turbine since no active power is involved.

When, and only when, the check synchroniser is satisfied that the voltages,frequencies and phase difference are within acceptable limits, (or, in the caseof the 'phase only' type, that the phase difference is within limits), it closes acontact which 'arms' the circuit-breaker closing circuit, so permitting closurewhen the operator presses the CLOSE switch. The same contact on the checksynchroniser also momentarily lights an IN SYNCHRONISM or READY TOSYNCHRONISE lamp, indicating to the operator that the breaker is ready forclosing. Once this lamp has gone out again, he cannot close the breaker untilit illuminates a second time.

Where check synchronising is fitted, it is brought automatically into circuitwhenever a second or subsequent generator has been started and selectedfor switching on-line; it so serves as a protection against incorrect operation.

Check synchronisers may also be fitted across section breakers,interconnectors and LV incomers from transformers - in fact at any point in thenetwork where it might be possible to close across two unsynchronisedsystems accidentally. They are also fitted across main generator incomer

breakers even when auto synchronising is provided. They come into actionautomatically if manual synchronising is selected.

Sometimes operators form the bad habit of holding the breaker control switchclosed before synchronism is reached, and relying on the arming contact ofthe check synchroniser to complete the closing circuit. This is bad practice andmust be avoided.

5.10 CLOSING ONTO DEAD BUSBAR


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If it is required to connect an incoming generator, or LV transformer incomer,onto a dead busbar, the check synchroniser will not allow it to happenbecause, one side being dead, the two sides can never be in synchronism. Inthat case the check synchroniser must be temporarily 'cheated' while theconnection is made. On most switchboards a special switch is provided for this

purpose. It is spring-loaded to return to the OFF position so that the checksynchroniser cannot be left permanently out of operation. This cheating switchmay be tagged CLOSE ONTO DEAD BUSBAR or CHECK SYNC. OVERRIDEor other similar wording.


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CHAPTER 6

LOAD SHARING

6.1 GENERAL

When two or more a.c. generators have been synchronised as described inChapter 3, and they are feeding a common load, ideally they share that loadbetween them in proportion to their sizes. However, to allow for flexibility inoperating the system, each generating set is provided with field excitation andprime-mover governor controls whose settings affect not so much the voltageand speed of the sets but rather the share of the load taken by each whenoperating in parallel.

6.2 THE D.C. CASE

To see what happens when a power source is connected to an external circuit,consider first the simple d.c. system of Figure 6.1 (a), where a battery is shownas the source of d.c. power.

The battery develops an emf of E volts and has an internal resistance of rohms. Only the terminal voltage V is available to be measured, using avoltmeter. This voltage is used to drive the current IL through the loadresistance of R ohms. When the battery is not supplying current, there is no

internal voltage drop and V=E. when current IL is flowing, the direction of flowinside the battery is from the negative terminal to the positive and, is passingthrough the internal resistance r, it causes a voltage drop IL.r of oppositepolarity to the battery emf. Thus the terminal voltage V is equal to E- IL.r


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Fig.6.1 D.C. SOURCES OF POWER

Figure 6.1 (b) shows two different batteries connected in parallel without anexternal load. The emf EA of battery' A' is assumed to be higher than the emfEB of battery 'B'. The difference between these emfs causes a circulatingcurrent 'c to flow out of battery' A' and into battery 'B', thereby discharging 'A'and charging 'B'; it is limited in value by the internal resistances of the twobatteries in series. (In the example shown these resistances r are assumed tobe the same.) Because the batteries are connected in parallel, they have acommon terminal voltage V. In this particular example, V has a value mid-waybetween EA and EB; this is because the common circulating current causesequal voltage drops in each battery. The drop in battery' A' is subtracted fromthe emf EA and the drop in battery 'B' is effectively added to the emf EB' sinceit is being charged by battery' A '.


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6.3 A.C. GENERATORS

With a.c. sources of power, the principal ones involved in load-sharing aregas-turbine-driven generators. The electrical indicating instruments of onesuch generator are shown in Figure 6.2(a), and it is the interpretation of thereadings of these instruments which enables the operator to adjust the outputof each set to achieve correct parallel operation.

FIG. 6.2 A.C. SOURCES OF POWER

The governor and excitation controls of a generating set are represented inFigure 6.2(b). The emf E of the generator is controlled by setting the excitationcurrent in the field coils; an excitation setting or 'volts adjust' control is providedto vary the emf produced by the generator. The governor or 'speed' settingvaries the mechanical power supplied to the generator by the gas-turbine. Likethe battery, the generator has an internal impedance, z ohms. This consists ofboth resistance (r) and inductive reactance (x), but the resistive component isso small that it may be disregarded and only the reactance x is considered.


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6.4 CONTROL OF GENERATOR LOADING

Consider now the effect of operating the volts adjust and the speed adjustcontrols of a generator which is running in parallel with another. Eachoperation will be considered separately and independently.

Fig.6.3 LOAD SHARING CONTROL

6.4.1 Voltage Adjustment

In Fig. 6.3 EA and EB represent the generated emfs of two generators A andB running in parallel, and for simplicity assume that EA and EB have thesame length and direction initially. If the excitation of generator B is nowincreased by operating the voltage adjust control in the RAISE VOLTSdirection, The voltage vector EB becomes longer than EA but is still in the

same direction. The difference between EB and EA is then a net emf e whichcauses a circulating current IC to flow from machine B to machine A. Sincethese machines have reactance but negligible resistance, the circulatingcurrent is limited by the combined reactance of the two machines and lags 90 o on (e).

This is shown in Fig. 6.3 (a). Since Ic is at right angles to EA and EB,generator B is producing, and generator A is receiving, lagging wattless orreactive power, as would be shown by their respective varmeters. (Thevarmeter of generator A would read in the negative sector unless preventedby a stop).


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Thus adjustment of the voltage control causes lagging reactive power, or vars,to circulate between the generators from the one with the higher to the onewith the lower excitation.

If both machines are already on load and producing vars for the system,

raising the voltage setting on one generator increases its var-loading anddecreases the var-loading on the other, so varying the sharing of reactivepower between the sets, as indicated by different readings on their varmeters.Lowering the voltage setting has the opposite effect.

Note that, although the control may be marked VOLTS ADJUST, with parallelgenerators it has little effect on the system voltage but becomes principally areactive (or var) load sharing control.

6.4.2 Speed Adjustment

Consider next the effect of operating the 'speed adjust' control of one of thegenerators. Suppose the speed control of generator 'B' is moved in the RAISESPEED direction.

This causes the fuel valve of generator B's prime mover to open more, soadmitting more fuel and increasing the engine torque. It will drive the generatorrotor more strongly in the forward direction of rotation. Since the rotor carriesthe field system, the emf of that generator advances relative to that of theother. In Figure 6.3(b), if EA is the emf of generator' A' and EB that ofgenerator 'B " then EB is advanced in phase because of the rotor position, butits length is not altered since there has been no change in the excitation. Theangle between the new rotor position and the old i

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E-05 Ac Generators