Raabe J. Hydro power - the design, use, and function of hydromechanical, hydraulic, and electrical еquipment

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Fig. 11.4.5. a) Longitudinal section of a fully water-cooled alternator at Tonstad, Norway (owner

Sira

Kvina Kraftselskap),

operating

satisfactorily since 1968. Contrary to plants like Wehr, needing

water-cooling, this

alternator

was equipped with it, to test the method. (Design Brown Boveri and

Cie, Baden, Switzerlar~d)

190 MVA;

375 rpm;

n,,

600 rpm; terminal voltage

kV;rated

current

9140 Amp; cos

0,85;

fly-wheel moment 3000 m2 tons; exciter current 2630 Amp;

exciter Voltage 220 V; total weight 480 tons. Relative losses: stator winding 43%; rotor winding

25%; bearing

13,9

stator iron 5%; water cooler 10%. From [11.76]. (Drawing courtcsy Brown,

Boveri

Comoany Ltd, Baden, Switzerland).

To observe

the required dimensional tolerances of the lamination segments, the contour, bolt holcs,

pole fixing grooves and keyways are punched out in one operation. Torque transmission between

laminated

rings and thc spider requires accurate keying, which ensures tangential guidance even with

floating ring.

The rings

of large low-speed machines are built up on the fully mounted spider in the power stations.

Laminations to a height of

few centimeters are rtpplicd only to chcck the spacing between the bolt

holes and pole securing grooves and to locate the keyways on the arms of the spider.

11.

Poles: Poles are solid or laminated depending on the mechanical stresses and the type

of operation. As a result of the relatively low ohmic resistance the additional losses in

solid poles, made of forged or cast steel, are higher than those in laminated poles. The

additional losses are essentially the eddy current losses caused

the higher field harmon-

Fiy. 11.4.5. b) Perspective sectional view of Itaipi! alternator, Rio Parani, Brazil, Paraguay (owner

Itnipu Binacional)

units fabricated by Consorcio Itaipu Electromeca~~ico (CIEM), comprising on

the electric

side the European firms, Alsthom Atlantique, France, Brown Boveri, West Germany and

S\\i~zerland, Siemens Limited, West Germany, and their Brazilian subsidiary firms Industria

Elkctrica l3rov.m Boveri S.A., and Siemens

S.A.

in Lapa. To date with resFect to output one of the

largest alternators for hydropower sets. Water-cooled stator. Data for the Paraguayan (Brazilian)

machine: frequency jO(60)

Hz,

ratcd capacity 823,6(766,0) MVA, power factor 0,85(0,95), rated

speed

90,9(92,3) rpm, runaway speed 170 rpm, rated terminal voltage 18

I_t

outside stator

diameter 19,7 m. stator bore diameter 16 m, thickness of stator bundle lamination 3,5(3,26)

Tllrust bearing load 4210(4300) tons, rotor weight 1961 (1945) tons, total weight 3343 (3342) tons.

S!arting operation 1983,'84. Rated head

115,4 m. Power transmission

50(60) Hz with

500

kV .4C,

600

DC (500 or 765 kV

AC).

Note skewed spidzr arms!

In this context. a comparison with the fully water-cooled alter~lators of Krasnoyarsk designed by

LhIZ,

SU.

and put into service

1966 may be of interest. Here rated head

101

rated

capacity 600

ILIVA.

93,8 rpm, stator bore diameter 16

rn,

rotor weight 2000 tons. axial thrust

3400

tons. From this

is secn that at nearly the same peripheral speed, rotor diameter, and rotor

weight,

the

electric output of an alternator with

water-cooled stator has been increased by about

O/b

compared with a fully water-cooled alternator built about 20 years carlier.

The gap clearance of

mm results in a gap/roior diameter ratio of

1,8

lo-'.

Bccausc of the radial

thermal

rspansion of the heated rotor working, this ratio has to be about

times larger than in the

rcnner seal of

the watcr turbine, where it is dictated by the clearance of the adjacent guide bearing

and the flexibility of the shaft. From

Gabler; Itaipli, Wasserkraftwerk der Superlative.

ETZ

103

(1982) no

10,

p. 522i528. (Drawing courtesy Siemens, Erlangen, West Germany.)

fig.

1.4.7.

Constr:~ction of the

solid

body:

cast or

lorw

liollo~~ body in cclltr;ll

section;

made

\evcml rolled steel

(liss

rotor):

I:IIII~II;I~C~

rllll rotor c~nSillin

spider :1nd onc or more rims

0vcrl;lpping

steel

Iam~n;~l~on

scsmQk

(Drawing from spccial issue

BBC

Fig.

1.4.3.)

damper bars in front of the rotor and the ring scgrnents permit compensation of varying

thermal

expansion of the individual bars.

Tile me~hod of securing the poles to the rotor body depends on thc stressing by the centrifugal

force

at rhe runaway speed. With low peripheral speeds and small bore

diameters

the poles arc bolted

to thc rotor rings or rotor rim from the air gap side. With larger bore diameters and

peripheral

speeds thc poles can bc bolted on from the hub side. The poles can then be dismantled axially

allow overhaul of thc pole or stator winding without dismantling the machine.

Single or multi-claw

mountings arc required for medium and high peripheral speeds, e.g., 2T-hcad

claws which engage in corresponding keyways in the rotor body. The claws are presscd against the

ro!or body by wedging at both ends. Carefully designed radii of curvature in notches

and

claws hclp

to rcducs the stress peaks causcd by the unavoidable notch elTect.

111.

Field coils: The individual turns of the field coils consist of hard drawn copper

sections cut

!o appropriate length and width and hard soldered at the joints. To increase

the cooling

siuface area individual turns may have a greater conductor width, so that the

outer coil surface forms

finned cooling element. The insulation between the turns

consists of glass asbestos or plastic films and is baked with the copper under pressure

(corresponding to the centrifugal force at the runaway speed).

The end \vindings are terminated by an insulating frame made of glass reinforced p!astics. In addition

high

mechanical strength this frame must have sliding properties to compensate for the relative

expal~sion of ths field coils relative to thc pole pieces. The winding is secured by steel frames bolted

the polc piece. Cup springs inserted in the frame keep the windings pre-stressed.

IV.

Pole changing: Occnsionally pump-turbine sets have two speeds, since optimum

hydraulic efficiencies are achieved if the pump speed is about

20%

above the turbine

speed. Pole changing can be achieved by the following methods:

a) The rotor poles are of identical construction and their number corresponds to the

lower speed. For the higher speed, poles are switched out or two adjacent poles are

energized in the same direction

(Fig.

11.4.8.1).

Poles with different widtns are distributed unevenly around the periphery

(Fig.

11.4.8.2)-

switching over groups of poles and possibly disconnecting individual

v~~5-?v~?vv

NSNNSNSS

SNS

NSN

34!,1;/

Fig.

11.4.8.

left: Pole arrangement and pole field pattern for changeover from 8 to

poles with

identical construction and uniform spacing of the poles. 2) Right: Pole arrangement and pole

ficld

pattern for changeover from

polcs with different co~~struction and irre_gular spacing of

the

poles. After

BBC

publication on "Synchronous machines for hydro-electric power plants". No

CH-T

130

082

poles, two different numbers of poles can be achieved, the smaller one giving a substantial

increase in the flux density of the fundamental wave. At the same

time the fields due to

other harmonics are weaker than in case a). Brown Boveri have combined the

advantases

of simple pole arrangement in a) with the ftinctional superiority of b)

making the pole

shoes asymmetrical and

displacii~g them in such a way that their flux distribution pro-

duces the best possible conditions for both speeds.

Moreover the stator of the

BHC

machines has two separate windings,

which evcry second coil of

one pole number is assigned to

the interposed coil of the other pole number.

With machines based on this concept, very smooth running is achieved up to the largest

outputs.

11.4.8.

Stator construction

Stator frame: This is a welded structure designed and dimensioned to withstand the

stresses

the rated torque, and

the case of a fault, e.g., short circuit, of the much greater

impulsive torque, of the dead weight of the stator, of the axial thrust on the rotating parts

(in the ctisc

vcrticcil set with thrust bc;lring on

the

hcnd covcr

th-

stat^$.:^

n~agnelic Sorccs and clue

thcrmal cxp:irlsion.

Stators

ill

horizontal niacliinos transmit these forces throi~gh

support in^

feet

the

plates into the foundatio~ls. The stators

vertic;~l machines arc supported on the

foundr.

tions by means

brackets or by bearer rings, particular attention bcing paid to the

involved. lIence, with rising temperature the stator can expand axially freely.

rsl

Single phasc machines ciluse

specinl problem with regard to stator support

of the torque b:ldly pulsating at twice the system frequency. Hcre n stator spring

sl,spcb

sion may reduce the transmissiol~

the jolting forces.

The siator

supported

spring units

each

comprising

number

leaf springs,

whore

ends

arc clamped in holders. The lower holder

connected to the stator

suspension

rib, the upper holder to the foundation

ring

via

supporting rib.

The

leaf springs

subjected to

the

stator weight with the result that the danger of buckling

diminat&

Thus

the stator can oscillate freely peripherally.

The concrete support of the stator has to be

reinforced

by stay wires which are tensioned in the axill

and peripheral direction, to avoid fatique effccts in the case of shcrt circuit. For this emergency

the stator frame has to resist any buckling by twist.

Fie. 11.4.6 shows different arrangements of stator.

11.

Stator iron: The active iron is formed of

0,5

mm thick laminations of low-carbon, silicon-alloyed

iron.

thin electrically insulating coating of varnish resistant to high temperatures is applied to both

s~des. R.1axirnum care is takcrl when installillg the laminations to prevent local ovcrhcating due

short-circuitcd laminations. The overlapping segments are guided by wedge-shaped bars of pris-

matic cross-section.

Thc type ofconnection bet~een the stator iron and frame ensures that the bore

tolerance

is observed

up to the

largest dismzters arid enables the stator frame to absorb the thermal expansion of the iron

core.

The

laminatioris are compressed by hydraulic jacks. On

completion

of the pressing operation,

pressure is applied to the

wcdge-shaped bars used for guidirl~ the laminations. Deslgncd as bolts

threaded

both

ends. thcsc ->:edge-shaped bars project beyond the cnd pliltcs and arc tcnsioned

nuts. Clamping fingers transfer the pressure to the stator tceth, so thilt tooth vibriition of the end

laminations and possible damage to the stator winding

insu!ation arc prevented.

For radial

flon of the cooling air the iron is subdivided into individual p;tckets of laminations in the

axial direction. The cooling ducts are formed by spacers of steel spot-welded to the end

lamination.

The number and arrangement of the spacers are selected so that the pressure is distributed as

uniformly as possible over the segment area and the resistance of the cooling air flow is moderate.

It is advantageous to glue the lamination segments of the stator iron for certain operation conditions

and

machines of

specific type.

In bulb turbines for example, this leads to mechanical bracing of

the stator. because the iron acts

an integral part of the frame. In pole changing machines the gluing permits the absorption

jolting forces caused by the upper harmonics.

111.

Stator ~viridi~l~: The reliability of the stator winding is determined primarily by the quality of

its insulation. An effective insulating system consists of

continuous tape insulation impregnated

with synthetic resin in vacuo and subsequently curcd.

glass fabric strip is used as

carrier material

for the insulating fleece and a special

solvent-free

epoxy resin is the impregnating irgent.

The larger machines usually have

double-layer bar in ope11 slots. The bar cross section consists of

several conductor elements transposed on the

Rochel

principle (Fig.

11.4.4.1).

These elements are

covered

with

glass filament and glued with cpoxy resin under prcssurc to form

compact bar.

The

insulation taping is subsequently applied to the bar strengthened in this way. Great care is taken to

ensurc firm permanent wedging of the conductors inside the slots and similarly

effective

support of

the end windings.

Some speculations on regulation and future tendencies: At reaction turbines with fixed runner blades

(to-date the majority of hydro turbines), an adaptation to the varying power demand causcs

changing angle of incidence of the relative flow and hence an inlct shock at runner inlet edgc. Th~s

decreases either the eficiency from its bep value with vanishing inlct shock or

creates cavitation

past the inlet edgc of the rotor vane. Moreover in such a machine a deviation from the bep causes

whirling flow past runner exit and hence draft tube surges.

Both troubles originate

from the velocity triangles in consequence of constant blade velocity due to

speed regulation.

Inlet shock and exit whirl disappear when the speed of the set is regulated

appropriately. Such a specd regulation may be implemented, if the alternator of the set supplies a

high voltage

power transmission line, provided its thyristors and wave filters for harmonics are

insensitive to the speed variation considered.

At heads up to

300

and constant gate, the turbind flow is not influenced by speed change. Thus

water hammer must not be accounted for. Nevertheless the range of speed regulation has to be

limited since some loss components,

e.g., disk friction and ventilation rise with speed. Moreover the

speed range has to

set so that, due to common practice at hydro power sets, the lowest critical

bending and torsion speed are above the upper limit of the working speed. Also material

strength

of the rotors limits the speed by stresses originating from centrifugal load. The large inertia of

alternator rotor due to its iron and conductor mass required and its long start-up

time is a

considerable hindrance to fast variable speed regulation. Material with higher magnetic

flux

density

and conductors with higher permissible current density (realized by applying cryogenic engineering)

could assist in overcoming these troubles.

Operations with continuously varying speed would require

appropriate regulation of generator

terminal voltage. The layout has to start from the ratio of upper and lower specd limit. On the

hydraulic side this depends on the turbine design and should not exceed the value of

1,5,

on the

electric side this could reach

The upper limiting speed results from the critical speed and material

strength at runaway with a glance at frequency-induced losses. The lower limit derives from too

largc

cross sectional areas the magnetic flux then requires.

To eliminate the inlet shock and exit whirl when the turbine works away from its bep, also an

operation with constant gate opening is thinkable. Ignoring the case with a number of sets

suficient

to work around the bep by switching them on and off, a constant flow operation could be imagined

when the excess of electricity generated, produces by-products, e.g., hydrogen electrolytically. I-low-

ever, this process, with about

90%

efficiency and its low voltage, can in no way compete with

up-to-date high voltage power transmission. Further the recuperation of energy thus

cl~ernically

stored in hydrogen is also linked to low voltage and hence uneconomical.

The high combustion temperature of hydrogen makes it useless for room

heating. Problems of

lacking material strength and resulting low service life also prohibit the application of hydrogen in

internal combustion machines. Hence to day the combustion of hydrogen could only be used for

welding or chemical processes. In future it may be used for power generation at highest temperature,

e.g., magncthydrodynamic process. At present, all this cannot compete with hydro-elcctric power

generation in respect to efficiency. Hence such a detour in energy transmission is only justified

energy is amuent and the by-products gained in this process are scarce thus increasing the resulting

economy of the installation.

Growing size of units, welded and hence more elastic construction, economic use of high yield point,

or wear-resistant material with

limited useful life, fatigue effects, transients like regulation, change

of operating mode, water

liammer and cavitation, will give priority to the prediction of the proto-

type's dynamic response and wear with respect to the phenomena mentioned above. This requires

rnodcl tests on the maclline, its components and their material.

Any prediction of this kind has to consider material properties of the set. its foundation including

the power house, its liquid-filled gaps in bearings and labyrinths, the properties

and geometry of its

piping, the properties of the working fluid as,

c.g., free air content (also that due to cavitation), the

modes

ol operating and regulating the set, and the power spectrum of exciting frequetlcies and their

response. This power spectrum, as a function of free air content, has been

rncasured, e.g., on

model

Francis turbine of high specific speed by Walter

[9.34].

llocevcr.

prctlic.tio~r

thc

dynamic

prototype phcnomcni~ occ11rl.ing

hydro.clectric

turbomachinc set,

with

its

kl~owri

material,

piping

and

clcctric

grid, will

rcmuin

task,

whose

\elution

rese:vcc!

the

fitturc.

List

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