2- hydro power and synchronous machines(1)

8/9/2019 2- Hydro Power and Synchronous Machines(1)

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The University of Texas at Dallas www.utdallas.edu

Hydroelectric Power Plantsand

Synchronous Machines


HydroelectricPowerPlants


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Power Plants for Primary Resources The vast majority of electricity generated worldwide

(about 99%) is generated from power plants usingprimary energy resources such as hydropower, fossilfuel, and nuclear fuel.

The geological and hydrological characteristics of thearea where the power plant is to be erected determine, toa large extent, the type of the power plant.

Fossil fuel power plants in the United States areconcentrated mainly in the east and Midwest regionswhere coal is abundant.

Hydroelectric power plants are concentrated in thenorthwest region where water and water storage

facilities are available.

3


Power Plants for Primary Resources Nuclear power plants, however, are distributed in all

regions since their demand for natural resources is

limited to the availability of cooling water.

Fossil fuels are the main source of electric energy (over

80%), and about 63% of the electric energy is produced

by coal and oil-fired power plants.

In the United States, coal counts for about 50% of the

fuel used to generate electricity.

4


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ElectricityGenerationinUSA5


Renewable

Energy

Consumption

by

major

source6


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Hydroelectric Power Plants Hydro is a Greek word meaning water, hydropower means

the power in the moving water and hydroelectric is the processby which hydropower is converted into electricity.

The hydroelectric power plant harnesses the energy of the

hydrologic cycle. The motion of water toward oceans is due to its kinetic energy,

which can be harnessed by the hydroelectric power plant thatconverts it into electrical energy.

If water is stored at high elevations, it possesses potential energyproportional to that elevation.

When this water is allowed to flow from a higher elevation to alower one, the potential energy is transformed into kineticenergy, which is converted into electrical energy by

hydroelectric power plants.

7


Hydroelectric Power Plants The worlds first hydroelectric power plant was constructed

across the Fox River in Appleton, Wisconsin, and began its

operation on September 30, 1882.

The plant generated only 12.5 kW, which was enough to power

two paper mills and the private home of the mills owner.

The latest and largest hydroelectric power plant, so far, is the one

being built in Chinas Three Gorges, which has a capacity of 22.5

GW.

8


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Worlds Largest Hydroelectric Power Plants


Types Of Hydroelectric Power Plants

Thecommontypesofhydroelectricpowerplantsare

impoundmenthydroelectric,

diversionhydroelectric,and

pumpedstoragehydroelectricpowerplants.


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ImpoundmentHydroelectricPowerPlants

Impoundmenthydroelectricisthemostcommontypeof

hydroelectricpowerplantandissuitableforwaterbodies

with

high

heads.

Thedaminthesepowerplantscreatesareservoiratahigh

elevationbehindthedam.AgoodexampleistheGrand

CouleeDamshownintheFigure

Atypicalimpoundmenthydroelectricpowersystemhassix

keycomponents:dam,reservoir,penstock,turbine,generator,

andgovernor.Aschematicofahydroelectricpowerplantis

shownintheFigure.


Energy

changes

Energychangesinhydroelectricpowerstations:

Potentialenergy>Kineticenergy>Electricity

12


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Impoundment Hydroelectric Power Plants

The Grand Coulee Dam and Franklin D. Roosevelt Lake (Washington State)

13


Key Components of Power Plants14


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Diversionhydroelectricplant

Strongcurrentsofriversareutilizedbylowheadturbinestogenerateelectricity.This

hydroelectricplantdoesnotrequireawaterreservoirathighelevation,soitsgenerating

capacityislessthanthatfortheimpoundmenthydroelectricpowerplant.

15


PumpedStorageHydroelectricPlantA

pumped

storage

plant

uses

two

reservoirs,

one

located

at

amuch

higher

elevation

than

theother.Duringperiodsoflowdemandforelectricity,suchasnightsandweekends,

energyisstoredbyreversingtheturbinesandpumpingwaterfromthelowertothe

upperreservoir.Thisincreasesthepotentialenergybehindthedamforlateruse.The

storedwatercanlaterbereleasedtoturntheturbinesandgenerateelectricityasitflows

backintothelowerreservoir.OneofthelargestpumpedstoragehydropowerfacilitiesintheworldLudington

PumpedStoragePlantinMichigan.

16


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Dam It is a barrier that prevents water from flowing downstream,

thus creating a lake behind the dam.

The potential energy of the water behind the dam is directlyproportional to the volume and height of the lake.

The dam can be enormous in size; the Grand Coulee Dam in

Washington State is 170 m in height, 1.6 km in length, and itscrest is 9 m wide. Its base is 150 m wide.

The volume of the concrete used to build the dam is almost9.16 x 106 m3.

The Three Gorges Dam in China is the biggest dam ever builtso far, followed by the Itaipu Dam in Brazil.


Power

Output

from

a

Dam IfQisthevolumeflowrate(cubicmeters/sec),Histhe

effectiveheadinmeters,istheefficiencyoftheturbogeneratorsystem,thenthepoweroutputofthedamis;

P=.1000.H.Q.g Watts

IfH=50m,Q=20m3/sec,=1, P=1000x20x50x9.81= 9.81MW


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Comparative Data on

Two Large Hydroelectric Dams


ReservoirThe dam creates a lake behind its structure called a

reservoir and often covers a wide area of land.

The Grand Coulee Dam created the Franklin D.Roosevelt artificial lake, which is about 250 km longand has over 800 km of shore line.

Its surface area is about 320 km2, the depth of thelake ranging from 5 to 120 m.


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Reservoir Energy The water behind the dam forms a reservoir (lake).

The potential energy of the water in the reservoir PEr is a linear

function of the water mass and head.

PEr= MgH (1)

Where M is the water mass (kg)

g is the acceleration due to gravity (m/s2)

H is the water head (average elevation) behind the dam (m)

The unit of PEr is joule (Ws). The mass of the water is a function

of the water volume and water density

M = Vol (2)

Where vol is the volume of water (m3)

is the water density (kg/m3)

At temperatures up to 20C, is 1000 kg/m3

21


PenstockPenstock: It is a large pipeline that channels water from

the reservoir to the turbine.

The water flow in the penstock is controlled by avalve called governor.

Penstock of the Grand Coulee Dam.


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Penstock Energy The potential energy of the water entering the

penstock, PE, is

PE=mgH (3)

where m is the mass of water entering the penstock.

This potential energy is converted into kinetic energyas the water moves inside the penstock.

The kinetic energy, KE, of the water leaving thepenstock is

(4)

where v is the velocity of water exiting the penstock(m/s). The penstock is generally inclined; therefore, its

length is longer than the water head, which results insome energy losses.

23


Penstock Energy

Hence, the penstock efficiency, p is defined as the ratio of itsoutput energy KE to its input energy PE.

(5)

The mechanical power of the water exiting the penstock (alsoknown as hydropower) is given by

where f is the flow of water inside the penstock (kg/s) and isdefined as

24


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Penstock Energy The volume of water passing through the penstock

during an interval time, t, is

whereA is the cross-sectional area of the penstock

t is the time interval

The mechanical power of the water exiting thepenstock (also known as hydropower) is given by

25


Turbine A turbine is an advanced water wheel. The high-pressure water

coming from the penstocks pushes against the blades of theturbine causing the turbine shaft to rotate.

The electrical generator is mounted directly on the same shaft ofthe turbine, thus the generator rotates at the speed of the turbine.

Hydroelectric turbines are specially designed water wheels thatcome in three main types.

Kaplan turbines, named after Viktor Kaplan, are used mainly indiversion power plants with small heads.

Pelton turbines, invented by Lester Pelton, are used in high headimpoundment power plants.

Francis turbines, invented by James Francis, are used in eithertype of power plant


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HydroTurbines

KaplanTurbine

AssemblyofaPeltonTurbineinthe''Walchensee''

PowerPlant,Germany

27


Francis turbine


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Hydroelectric TurbineGenerator Units

Turbinesinside

Hoover

Dam

in

ArizonaFrancis

Turbine


Generator

The electrical generator is mounted directly on thesame shaft of the turbine, thus the generator rotates atthe speed of the turbine.

It is an electromechanical converter that converts themechanical energy of the turbine into electricalenergy. The generators used in all power plants are thesynchronous machine type.

The generator is equipped with various controlmechanisms such as the excitation control and variousstabilizers to maintain the voltage constant and toensure that the generators operation is stable.


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Example1

ThePenstockoutputofGrandCouleedamisabout800MW

whentheeffectivewaterheadis87m.TheturbineisFrancise

design.Computethewaterflowrateinsidethepenstock.

PE=mgH m is in kg, H is in meters

Flow rate is f = vol/t m3/s

Mass m = vol.

PE= vol. .g. H = f.t. .g. H

Power=PE/t=f..g. H

f = Power/(.g. H) = (800 x 106)/(1000 x 9.81 x 87)

= 837.35 m3/sec


Problems1. Inahydroelectricplant,twomillioncubicmetersofwaterisstoredinareservoirataheight

of60metersfromahydroturbine. CalculatetheamountofenergyinMWHifallthewater

usedforproducingelectricity?Neglecttheleakageandotherlosses. Thedensityofwateris

1000kg/m3

2. Thereservoirofahydroelectricpowerplantisataheightof80ft. Thedensityofwateris

1000kg/m3. Whatistheminimumamountofwaterrequiredtoobtain500MWh ofelectric

power?(1kWh=3.6MJandg=9.8m/sec2)

3. Inahydroelectricplant,thewaterinthereservoircoversthearea5squarekilometers. The

depthofthewateris20meters.Theheightofthereservoiris70metersfromahydro

turbine. CalculatetheamountofenergyinKWHifallthewaterisusedforproducing

electricity?Neglect

the

leakage

and

other

losses.

The

density

of

water

is

995

kg/m3

4. Estimatethepoweroutputofadamwithaheadof50mandvolumeflowrateof

20m3/sec(Neglectanylosses).Densityofwateris1000kg/m3


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2.0 Synchronous Machines

37

38

Power Generation

99+ % of all power are generated by the

synchronous generators

Synchronous machines can operate as

generators or motors


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Synchronous Machines

Synchronous generators or alternators are used to convert

mechanical power derived from steam, gas, or hydraulic-turbine

to ac electric power

Synchronous generators are the primary source of electrical

energy we consume today

Large ac power networks rely almost exclusively on synchronous

generators

Synchronous motors are built in large units compared to induction

motors (Induction motors are cheaper for smaller ratings) and

used for constant speed industrial drives

39

Construction of synchronous

machines

Synchronous machines are AC machines that have a field circuit supplied

by an external DC source.

In a synchronous generator, a DC current is applied to the rotor

winding producing a rotor magnetic field. The rotor is then turned

by external means producing a rotating magnetic field, which

induces a 3-phase voltage within the stator winding.

Field windings are the windings producing the main magnetic field

(rotor windings for synchronous machines); armature windings are

the windings where the main voltage is induced (stator windings

for synchronous machines).


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machines

The rotor of a synchronous machine is a large electromagnet. The magnetic poles

can be either salient (sticking out of rotor surface) or non-salient construction

(Cylindrical).

Non-salient-pole rotor: usually two- and four-pole rotors. Salient-pole rotor: four

and more poles.

Rotors are made laminated to reduce eddy current losses.

1. Most hydraulic turbines have to turn at low speeds

(between 50 and 300 r/min)

2. A large number of poles are required on the rotor

Hydrogenerator

Turbine

Hydro (water)

D 10 m

Non-uniform

air-gapN

S S

N

d-axis

q-axis

Salient-Pole Synchronous Generator

42


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Salient-Pole Synchronous Generator

Stator

43

L 10 m

D 1 mTurbine

Steam

Stator

Uniform air-gap

Stator winding

Rotor

Rotor winding

N

S

High speed

3600 r/min -pole

1800 r/min -pole

Direct-conductor cooling (using

hydrogen or water as coolant)

Rating up to 2000 MVA

Turbogenerator

d-axis

q-axis

Cylindrical-Rotor Synchronous Generator

44


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Cylindrical-Rotor Synchronous Generator

Stator

Cylindrical rotor45


machines

Salient pole with field

windings

Salient pole without

field windings

observe laminations

A synchronous rotor with 8 salient poles


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machines

Two common approaches are used to supply a DC current to the field circuits on

the rotating rotor:

1. Supply the DC power from an external

DC source to the rotor by means of

slip rings and brushes;

2. Supply the DC power from a special

DC power source mounted directly on

the shaft of the machine.

Slip rings are metal rings completely encircling the shaft of a machine but insulated

from it. One end of a DC rotor winding is connected to each of the two slip rings onthe machines shaft. Graphite-like carbon brushes connected to DC terminals ride on

each slip ring supplying DC voltage to field windings regardless the position or speed

of the rotor.


machines

Slip rings

Brush


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machines

Slip rings and brushes have certain disadvantages: increased friction and

wear (therefore, needed maintenance), brush voltage drop can introduce

significant power losses. Still this approach is used in most smallsynchronous machines.

On large generators and motors, brushless exciters are used.

A brushless exciter is a small AC generator whose field circuits are

mounted on the stator and armature circuits are mounted on the rotor

shaft. The exciter generators 3-phase output is rectified to DC by a 3-

phase rectifier (mounted on the shaft) and fed into the main DC field

circuit. It is possible to adjust the field current on the main machine by

controlling the small DC field current of the exciter generator (located on

the stator).

Since no mechanical contact occurs between the rotor and the stator,

exciters of this type require much less maintenance.


machines

A brushless exciter: a

low 3-phase current is

rectified and used to

supply the field circuit

of the exciter (located

on the stator). The

output of the excitersarmature circuit (on the

rotor) is rectified and

used as the field

current of the main

machine.


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machines

To make the

excitation of a

generator completely

independent of anyexternal power

source, a small pilot

exciter is often added

to the circuit. The pilot

exciter is an AC

generator with a

permanent magnet

mounted on the rotor

shaft and a 3-phase

winding on the stator

producing the powerfor the field circuit of

the exciter.


machines

A rotor of large

synchronous machine

with a brushless exciter

mounted on the same

shaft.

Many synchronous

generators having

brushless exciters also

include slip rings and

brushes to provide

emergency source of

the field DC current.


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machines

A large

synchronous

machine with

the exciter

and salient

poles.

Rotation speed of synchronous

generator

By the definition, synchronous generators produce electricity whose

frequency is synchronized with the mechanical rotational speed.

120

me

n Pf

Where fe is the electrical frequency, Hz;

nm is mechanical speed of magnetic field (rotor speed for synchronousmachine), rpm;

P is the number of poles.

Steam turbines are most efficient when rotating at high speed; therefore,

to generate 60 Hz, they are usually rotating at 3600 rpm and turn 2-pole

generators.

Hydraulic turbines are most efficient when rotating at low speeds (200-300

rpm); therefore, they usually turn generators with many poles.


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Internal generated voltage of a

synchronous generator

The magnitude of internal generated voltage induced in a given stator is

where Kc is a constant representing the construction of the machine, is flux in

it andfe = electrical frequency

.

Since flux in the

machine depends

on the field current

through it, the

internal generated

voltage is a

function of therotor field current.

Magnetization curve (open-circuit characteristic) of a

synchronous machine

Equivalent circuit of a synchronous

generator

The internally generated voltage in a single phase of a

synchronous machine EA is not usually the voltage appearing

at its terminals. It equals to the output voltage V only when

there is no armature current in the machine. The reasons

that the armature voltage EA is not equal to the output

voltage V are:

1. Distortion of the air-gap magnetic field caused by the

current flowing in the stator (armature reaction);

2. Self-inductance of the armature coils;

3. Resistance of the armature coils;

4. Effect of salient-pole rotor shapes.


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59

Xs RA

EA V

sA XR

cesisArmatureR

ceacsSynchronouX

A

s

tanRe

tanRe

Equivalent Circuit

60

Generator Equivalent Circuit

IA

Xs

EA V


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Phasor diagram of a synchronous

generator

Since the voltages in a synchronous generator are AC voltages, they are usually

expressed as phasors. A vector plot of voltages and currents within one phase is

called a phasor diagram.

A phasor diagram of a synchronous generator

with a unity power factor (resistive load)

Lagging power factor (inductive load):

Leading power factor (capacitive load).

For a given field current and magnitude of

load current, the terminal voltage is lower for

lagging loads and higher for leading loads.

The Synchronous generator

operating alone

The behavior of a synchronous generator varies greatly under

load depending on the power factor of the load and on

whether the generator is working alone or in parallel with other

synchronous generators.

Most of the synchronous generators in the world operate asparts of large power systems.

Unless otherwise stated, the speed of the generator is

assumed constant.


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operating alone

1. For lagging (inductive) loads, the phase (and terminal) voltagedecreases significantly.

2. For unity power factor (purely resistive) loads, the phase (and

terminal) voltage decreases slightly.

3. For leading (capacitive) loads, the phase (and terminal) voltage rises.

Generally, when a load on a synchronous generator is added, the following

changes can be observed:

Effects of adding loads can be described by the voltage regulation:

100%nl fl

fl

V VVR

V

Where Vnl is the no-load voltage of the generator and Vfl is its full-load voltage.


operating alone

A synchronous generator operating at a lagging power factor has a fairly large

positive voltage regulation. A synchronous generator operating at a unity power

factor has a small positive voltage regulation. A synchronous generator operating

at a leading power factor often has a negative voltage regulation.

Normally, a constant terminal voltage supplied by a generator is desired. Since the

armature reactance cannot be controlled, an obvious approach to adjust the

terminal voltage is by controlling the internal generated voltage EA= K. This

may be done by changing flux in the machine while varying the value of the fieldresistance RF, which is summarized:

1. Decreasing the field resistance increases the field current in the generator.

2. An increase in the field current increases the flux in the machine.

3. An increased flux leads to the increase in the internal generated voltage.

4. An increase in the internal generated voltage increases the terminal voltage of

the generator.

Therefore, the terminal voltage of the generator can be changed by adjusting the

field resistance.


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operating alone: Example

A 480 V, 60 Hz, Y-connected six-pole synchronous generator has a per-phase

synchronous reactance of 1.0 . Its full-load armature current is 60 A at 0.8 PFlagging. Its friction and windage losses are 1.5 kW and core losses are 1.0 kW at 60

Hz at full load. Assume that the armature resistance (and, therefore, the I2R losses)can be ignored. The field current has been adjusted such that the no-load terminal

voltage is 480 V.

a. What is the speed of rotation of this generator?

b. What is the terminal voltage of the generator if

1. It is loaded with the rated current at 0.8 PF lagging;

2. It is loaded with the rated current at 1.0 PF;

3. It is loaded with the rated current at 0.8 PF leading.

c. What is the efficiency of this generator (ignoring the unknown electrical losses)

when it is operating at the rated current and 0.8 PF lagging?

d. How much shaft torque must be applied by the prime mover at the full load?how large is the induced countertorque?

e. What is the voltage regulation of this generator at 0.8 PF lagging? at 1.0 PF? at

0.8 PF leading?



Since the generator is Y-connected, its phase voltage is

3 277TV V V At no load, the armature current IA = 0 and the internal generated voltage is EA =

277 V and it is constant since the field current was initially adjusted that way.

a. The speed of rotation of a synchronous generator is

120 12060 1200

6m en f rpm

P

which is1200

2 125.760

m rad s

b.1. For the generator at the rated current and the 0.8

PF lagging, the phasor diagram is shown. The phase

voltage is at 00, the magnitude of EA is 277 V,


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The Synchronous generator operating

alone: Example

1 60 36.87 60 53.13S AjX I j and that

Two unknown quantities are the magnitude of V and the angle of EA. From thephasor diagram:

2 22 sin cos

A S A S AE V X I X I

Then:

Since the generator is Y-connected,

3 410TV V V


alone: Example

b.2. For the generator at the rated current and

the 1.0 PF, the phasor diagram is shown.

Then:

3 468.4TV V V and

b.3. For the generator at the rated current and the

0.8 PF leading, the phasor diagram is shown.

Then:

3 535TV V V and

VIXIXEV ASASA 8.308)sin())cos((22


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alone: Example

c. The output power of the generator at 60 A and 0.8 PF lagging is

3 cos 3 236.8 60 0.8 34.1out A

V I kW

The mechanical input power is given by

34.1 0 1.0 1.5 36.6in out elec loss core loss mech lossP P P P P kW

The efficiency is

34.1100 % 100% 93.2%

36.6

out

in

P

P

d. The input torque of the generator is

36.6 291.2125.7

inapp

m

P N m -



The induced countertorque of the generator is

e. The voltage regulation of the generator is

34.1271.3

125.7

convapp

m

PN m

-

Lagging PF:

480 410

100% 17.1%410VR

Unity PF:

Lagging PF:

480 468100% 2.6%

468VR

480 535100% 10.3%

535VR


34/49

Terminal characteristics of


All generators are driven by a prime mover, such as a steam, gas, water, wind

turbines, diesel engines, etc. Regardless of the power source, most of prime

movers tend to slow down with increasing the load. This decrease in speed is

usually nonlinear but governor mechanisms of some type may be included to

linearize this dependence.

The speed drop (SD) of a prime mover is defined as:

100%nl fl

fl

n nSD

n

Most prime movers have a speed drop from 2% to 4%. Most governors have a

mechanism to adjust the turbines no-load speed (set-point adjustment).



A typical speed

vs. power plot

Since the shaft speed is linked to the electrical frequency as

120

me

n Pf

the power output from the generator is related to its frequency:

A typical

frequency vs.

power plot

p nl syss f f Operating frequency of the systemSlope of curve, W/Hz


35/49



A similar relationship can be derived for the reactive power Q and terminal voltage

VT. When adding a lagging load to a synchronous generator, its terminal voltage

decreases. When adding a leading load to a synchronous generator, its terminal

voltage increases.

The plot of terminal voltage vs.

reactive power is not necessarily

linear.

Both the frequency-power and

terminal voltage vs. reactive power

characteristics are important for

parallel operations of generators.

When a generator is operating alone supplying the load:

1. The real and reactive powers are the amounts demanded by the load.2. The governor of the prime mover controls the operating frequency of the system.

3. The field current controls the terminal voltage of the power system.


synchronous generators: Example

Example: A generator with no-load frequency of

61.0 Hz and a slope sp of 1 MW/Hz is connected

to Load 1 consuming 1 MW of real power at 0.8

PF lagging. Load 2 (that is to be connected to the

generator) consumes a real power of 0.8 MW at

0.707 PF lagging.

a. Find the operating frequency of the system before the switch is closed.

b. Find the operating frequency of the system after the switch is closed.

c. What action could an operator take to restore the system frequency to 60 Hz

after both loads are connected to the generator?

The power produced by the generator is

p nl syss f f

Therefore:sys nl

p

Pf f


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synchronous generators: Example

a. The frequency of the system with one load is

1

61 601sys nl p

P

f Hzs

b. The frequency of the system with two loads is

1.861 59.2

1sys nl

p

Pf Hz

s

c. To restore the system to the proper operating frequency, the operator should

increase the governor no-load set point by 0.8 Hz, to 61.8 Hz. This will restore

the system frequency of 60 Hz.

Parallel operation of synchronous

generators

Most of synchronous generators are operating in parallel with other

synchronous generators to supply power to the same power system.

Obvious advantages of this arrangement are:

1. Several generators can supply a bigger load;

2. A failure of a single generator does not result in a total power loss to the load

increasing reliability of the power system;3. Individual generators may be removed from the power system for maintenance

without shutting down the load;

4. A single generator not operating at near full load might be quite inefficient.

While having several generators in parallel, it is possible to turn off some of

them when operating the rest at near full-load condition.


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Conditions required for paralleling

A diagram shows that Generator 2

(oncoming generator) will be connected

in parallel when the switch S1 is closed.However, closing the switch at an

arbitrary moment can severely

damage both generators!

If voltages are not exactly the same in both lines (i.e. in a and a, b and b etc.), a

very large current will flow when the switch is closed. Therefore, to avoid this,

voltages coming from both generators must be exactly the same. Therefore, the

following conditions must be met:

1. The rms line voltages of the two generators must be equal.

2. The two generators must have the same phase sequence.

3. The phase angles of two a phases must be equal.

4. The frequency of the oncoming generator must be slightly higher than the

frequency of the running system.

Conditions required for paralleling

If the phase sequences are different,

then even if one pair of voltages

(phases a) are in phase, the other two

pairs will be 1200 out of phase creating

huge currents in these phases.

If the frequencies of the generators are different, a large power transient may occur

until the generators stabilize at a common frequency. The frequencies of two

machines must be very close to each other but not exactly equal. If frequencies

differ by a small amount, the phase angles of the oncoming generator will change

slowly with respect to the phase angles of the running system.

If the angles between the voltages can be observed, it is possible to close the

switch S1 when the machines are in phase.


38/49

Operation of generators in parallel

with large power systems

Often, when a synchronous generator is added to a power system, that system is

so large that one additional generator does not cause observable changes to the

system. A concept of an infinite bus is used to characterize such power systems.

An infinite bus is a power system that is so large that its voltage and frequency do

not vary regardless of how much real and reactive power is drawn from or supplied

to it. The power-frequency and reactive power-voltage characteristics are:



Consider adding a generator to an

infinite bus supplying a load.

The frequency and terminal voltage of all

machines must be the same. Therefore,

their power-frequency and reactive

power-voltage characteristics can be

plotted with a common vertical axis.

Such plots are called sometimes as house

diagrams.


39/49



If the no-load frequency of the oncoming

generator is slightly higher than the

systems frequency, the generator will be

floating on the line supplying a small

amount of real power and little or no

reactive power.

If the no-load frequency of the oncoming

generator is slightly lower than the

systems frequency, the generator will

supply a negative power to the system:

the generator actually consumes energy

acting as a motor!

Many generators have circuitry

automatically disconnecting them from the

line when they start consuming energy.



If the frequency of the generator is increased after it is connected to the infinite bus,

the system frequency cannot change and the power supplied by the generator

increases.

If the frequency of the generator is further increased, power output from the

generator will be increased and at some point it may exceed the power consumed by

the load. This extra power will be consumed by the load.

Summarizing, when the generator is operating in parallel to an infinite bus:

1. The frequency and terminal voltage of the generator are controlled by the

system to which it is connected.

2. The governor set points of the generator control the real power supplied

by the generator to the system.

3. The generators field current controls the reactive power supplied by the

generator to the system.


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Active and reactive power-angle

characteristics

P>0: generator operation

P


41/49


characteristics

PmPe, Qe

The armature current,

s

ATA

s

TAa

jX

jEVE

jX

VEI

sincos__

_

whereXs is the synchronous reactance per phase.

s

TAT

s

AT

s

TAT

s

AT

s

ATATaT

X

VEVQ

X

EVP

X

VEVj

X

EV

jX

jEVEVIVjQPS

2

2

*__

cos

&sin

cossin

sincos

87

VT


characteristicsPm

Pe, Qe

VT

s

TAT

s

AT

X

VEVQ

X

EVP

2cos&

sin

The above two equations for active and reactive powers hold

good for synchronous machines for negligible resistance To obtain the total power for a three-phase generator, the above

equations should be multiplied by 3 when the voltages are line-to-neutral

If the line-to-line magnitudes are used for the voltages, however,these equations give the total three-phase power

88


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90

91


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94

95


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96

97


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98

99


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Synchronous Motors

A synchronous motor is the same physical machine as a

generator, except that the direction of real power flow isreversed

Synchronous motors are used to convert electric power tomechanical power

Most synchronous motors are rated between 150 kW (200hp) and 15 MW (20,000 hp) and turn at speed ranging from150 to 1800 r/min. Consequently, these machines are used inheavy industry

At the other end of the power spectrum, we find tiny single-

phase synchronous motors used in control devices andelectric clocks

P, Q

Vt

Motor

Operation Principle The field current of a synchronous motor produces a steady-

state magnetic fieldBR

A three-phase set of voltages is applied to the stator windings of

the motor, which produces a three-phase current flow in the

windings. This three-phase set of currents in the armature

winding produces a uniform rotating magnetic field ofBs

Therefore, there are two magnetic fields present in the machine,

and the rotor field will tend to line up with the stator field, just

as two bar magnets will tend to line up if placed near each other.

Since the stator magnetic field is rotating, the rotor magnetic

field (and the rotor itself) will try to catch up

The larger the angle between the two magnetic fields (up to

certain maximum), the greater the torque on the rotor of the

machine101


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Vector Diagram

The equivalent circuit of a synchronous motor is exactly same as

the equivalent circuit of a synchronous generator, except that the

reference direction ofIa is reversed. The basic difference between motor and generator operation in

synchronous machines can be seen either in the magnetic field

diagram or in the phasor diagram.

In a generator,Ef lies ahead of Vt, andBR lies ahead ofBnet. In a

motor,Ef lies behind Vt, andBR lies behindBnet.

In a motor the induced torque is in the direction of motion, and in a

generator the induced torque is a countertorque opposing the

direction of motion

102

Vector Diagram

Ia

Vt

Ef

jIaXs

Ia

Vt

Ef

jIaXs

Bs

Bnet

BR

sync

Fig. The phasor diagram (leading PF: overexcited and |Vt||Ef|).103


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Application of Synchronous Motors

Synchronous motors are usually used in large sizes because in small sizes

they are costlier as compared with induction machines. The principal

advantages of using synchronous machine are as follows:

Power factor of synchronous machine can be controlled very easilyby controlling the field current.

It has very high operating efficiency and constant speed.

For operating speed less than about 500 rpm and for high-power

requirements (above 600KW) synchronous motor is cheaper than

induction motor.

In view of these advantages, synchronous motors are preferred for driving

the loads requiring high power at low speed; e.g; reciprocating pumps and

compressor, crushers, rolling mills, pulp grinders etc.

104

REFERENCES

1. Mohammed A. El_Sharkavwi, Electric Energy An Introduction, CRC

Press, 2013

2. Synchronous Machines: http://ee.lamar.edu/gleb/Index.htm by Gleb V.

Tcheslavski

3. http://elektro.fs.cvut.cz/en/SSem/2141025/Synchronous_Machine.pdf

2- hydro power and synchronous machines(1)

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