19. Design And Constructional Feature Of 500 MW Generator INTRODUCTION Turbo generator for 500MW units are two pole water and hydrogen gas cooled

generator. The primary water flows through the stator winding and picks up heat losses

arising in generator directly, which are transferred to the secondary cooling water. Core

losses, windage losses and rotor winding losses are picked up by circulating hydrogen

and transferred to Hydrogen cooling water in gas coolers. The stator frame is designed to

withstand the impact due to possible explosions inside. A brushless excitation system with

rotating diodes is provided to excite the machine.

Generator a. Type : THDF 115/59

B. Cooling Stator Winding : Directly Water Cooled

c. Stator core & Rotor : Directly Hydrogen Cooled

d. Apparent Power : 588 MVA

e. Active Power : 500 MW

f. Power Factor : 0.85 (lag)

g. Terminal Voltage : 21 KV

h. Stator Current : 16200 Amps.

i. Hydrogen Pressure : 4 Kg/cm 2

j. Short Circuit Ratio : 0.48

k. Class and Type of Insulation : Miscalastic

The two-pole generator uses direct water cooling for the stator winding, phase

connectors and bushings and direct hydrogen cooling for the rotor winding. The losses 227

in the remaining generator components, such as iron losses, windage losses and stray

losses, are also dissipated through hydrogen.

The generator frame is pressure-resistant and gas tight equipped with one stator end

shield on each side. The hydrogen coolers are arranged vertically inside the turbine end

stator end shield.

The generator consists of the following components

* Stator

Stator frame

End shields

Stator core Stator winding

* Rotor

Rotor shaft Rotor winding

Rotor retaining rings

Field -connections

* Hydrogen Coolers

* Bearings

* Shaft Seals

The following additional auxiliaries are required for generator operation

* Oil system

* Gas system

* Primary water system

* Excitation system

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STATOR

The 3 Armature winding of the generator is housed inside the stator which comprises

of a stator frame, end shields and the bushing compartment. Since an explosive gas

Hydrogen is used, the frame is made rigid enough to withstand a pressure of 10 bar.

The lower part of the frame near the exciter end houses the terminal box. The phase

and neutral leads of the 3 stator winding are brought out of the generator through six

bushings located in the terminal box. The hydrogen coolers are arranged vertically in

the turbine side stator end shield. The stator end shields contain the shaft seal and

bearing components.

STATOR FRAME

The stator frame consist of a cylindrical center section and two end shields which are gas-tight and pressure resistant.

The stator end shields are joined and sealed to the stator frame with an 0-ring and

bolted flange connections. The stator frame accommodates the electrically active parts

of the stator. i.e., the stator core and stator windings. Both the gas ducts and a large

number of welded circular ribs provide for the rigidity of the stator frame. Ring-shaped

supports for resilient core suspension are arranged between the circular ribs. The

generator cooler is sub-divided into cooler sections arranged vertically in the turbine

side stator end shield. In addition, the stator end shields contain the shaft seal and

bearing components. Feet are welded to the stator frame end shields to support the

stator on the foundation. The stator is firmly connected to the foundation with anchor

bolts through the feet. 229

230

STATOR CORE

The staler core is stacked from insulated electrical sheet steel laminations and mounted

in supporting rings over insulated dovetailed guide bars. Axial compression of the stator

core obtained by clamping fingers, pressure plates and non-magnetic through-type

clamping boits, which are insulated from the core. The supporting rings form part of an

inner frame cage. This cage is suspended in the outer frame by a large number of

separate flat springs distributed over the entire core length. The flat springs are

tangentially arranged on the circumference in sets with three springs each, i.e. two

vertical supporting springs on both sides of the core and one horizontal stabilizing

spring below me core. The springs are so arranged and tuned that forced vibrations of

the core resulting from the magnetic field will not be transmitted to the frame and

foundation.

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The pressure plates and end portions of the stator core are effectively shielded against 233

stray magnetic fields. The flux shields are cooled by a flow of hydrogen gas directly over no

assembly.

STATOR END SHIELDS

The ends of the stator frame are closed by pressure containing end shields. The end

shields feature a high stiffness and accommodate the generator bearings, shaft seals

and hydrogen coolers. The end shields are horizontally split to allow for assembly.

The end shields contain the generator bearings. This results in a minimum distance

between bearings and permits the overall axial length of the TE end shield to be utilized

{or accommodation of the hydrogen cooler sections. Cooler walls are provided in shield on

both sides of the bearing compartment for this purpose. One manhole in both the upper

and lower half end shield provides access to the end winding compartments of the

completely assembled machine.

Inside the bearing compartment, the bearing saddle is mounted and insulated from the

lower half end shield. The bearing saddle supports the spherical bearing sleeve and

insulates it from ground to prevent the flow of shaft currents.

The bearing oil is supplied to the bearing saddle via a pipe permanently installed in the

end shield and is then passed into the lubricating gap via ducts in the lower bearing

sleeve. The bearing drain oil is collected in the bearing compartment and discharged

from the lower half of the end shield via a pipe.

The bearing compartment is sealed on the air side with labyrinth rings. On the hydrogen

side, the bearing compartment is closed by the shaft seal and labyrinth rings. The oil for

the shaft seal is admitted via integrally welded pipes. The seal oil drained towards the

air side is drained together with the bearing oil. The seal oil drained towards the

hydrogen side is first collected in a gas and oil tight chamber below me Bearing 234

compartment for defoaming and then passed via a siphon to the seal oil tank of the

hydrogen side seal oil circuit.

The static and dynamic bearing forces are directly transmitted to the foundation via

lateral feet attached to the lower half end shield. The feet can be detached from the end

shield. -since the end shields must be lowered into the foundation opening for rotor

insertion.

Stator Winding

Stator bars, phase connectors and bushings are designed for direct water cooling. In

order to minimize the stator losses, the bars are composed of separately

insulated/strands which are transposed by 540C in the slot portion and bonded

together with epoxy resins in heated molds. After bending the end turns are likewise bonded together with baked synthetic resin fillers.

The bars consist of hollow and solid strands distributed over the entire bar cross-section so

that good heat dissipation is ensured. At the bar ends, all the solid strands are jointly

brazed into a connecting sleeve and the hollow strands into a water box from which the

cooling water enters and exits via teflon insulating hoses connected to the annular

manifolds. The electrical CQ Cnection between top and bottom bars is made by a bolted

connection at the connecting sleeve.

The water manifolds are insulated from the stator frame permitting the insulation

resistance of the water-filled winding to be measured. During operation, the water

manifolds are grounded. 235

Miscalastic High-voltage Insulation

High-voltage insulation is provided according to the proven Miscalastic system. With this

insulating system several halt-overlapped continuous layers of mica tape are applied to

the bars. The mica tape is built up from large area mica splitting which are sandwiched

between two polyester backed fabric layers with epoxy as an adhesive. The number of

layers, i.e., the thickness of the insulation depends on the machine voltage. The bars

are dried under vacuum and impregnated with epoxy resin which has very good

penetration properties due to its low viscosity. After impregnation under vacuum, the

bars are subjected to pressure, with nitrogen being used as pressurizing medium (VPI

process). The impregnated bars are formed to the required shape in MOULDS and

cured in an oven at high temperature. The high-voltage insulation obtained is nearly

void-free and is characterized by its excellent electrical, mechanical and thermal

properties in addition to being fully waterproof and oil-resistant. To minimize corona

discharges between the insulation and the slot wall, a final coat of semi conducting

varnish is applied to the surfaces of all bars within the slot range. In addition, all bars

are provided with an end corona protection to control the electric field at the transition

from the slot to the end winding and to prevent tie formation of creepage spark

concentrations.

Bar Support System

To protect the stator winding against the effects of magnetic forces due to load and to

ensure permanent firm seating of the bars in the slots during operation, the bars are

inserted with a side ripple springs, a hog-curing slot, bottom equalizing strip, and a top

ripple spring located beneath the slot wedge. The gaps between the bars. in the stator

end windings are completely filled with insulating material and cured after installation.

For radial support, the end windings are clamped to a rigid supporting of insulating

material which in turn is fully supported by 'the frame. Hot-curing conforming fillers

arranged between the stator bars and the support ring ensure a firm support of each 236

individual bar against the support ring. The bars are clamped to the support ring with

pressure plates held by clamping bolts made form a high-strength insulating material.

The support ring is free to move axially within the stator frame so that movements of the

windings due to thermal expansions are not restricted.

The stator winding connections are brought out to six bushings located in a

compartment of welded r end. Current transformers or metering and relaying purposes

can be mounted on the bushings.

STATOR WINDING (Corona Protection)

To prevent potential differences and possible corona discharges between 'the insulation

and the slot wall, the slot sections of the bars are provided with an outer corona

protection. This protection consists of a wear-resistant, highly flexible coating of

conductive alkyd varnish containing graphite. 1 Stator bar (slot end)

2 High-voltage insulatation

3 Outer corona protection

4 Transition costing

5 End corona protection

6 Glass tape-epoxy protective layer 7

Stator her (end winding)

Fig. No. - 44 TYPICAL BUILDUP OF CORONA PROTECTION

At the transition from the slot to the end winding portion of the stator bars. a

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semiconductive coating is applied. On. top of this, several layers of semi-conductive end

corona protection coating are applied in varying lengths. This ensures uniform control of

the electric field and prevents the formation of corona discharge during operation and

during performance of high voltage tests.

A final wrapping of glass fabric tapes impregnated with epoxy resin serves as surface

protection.

ROTOR SHAFT

The high mechanical stresses resulting from the centrifugal forces and short-circuit call for

a high quality heat-treated steel. Therefore, the rotor shaft is forged from a vacuum cast

steel ingot. Comprehensive tests ensure adherence to the specified mechanical and

magnetic properties as well as a homogeneous forging.

The rotor shaft consists of an electrically active portion, the so-called rotor body, and the

two shaft journals. Integrally forged flange couplings to connect the rotor to the turbine

and exciter are located outboard of the bearings. Approximately two-thirds of the rotor

body circumference is provided with longitudinal slots which hold the field winding. Slot

pitch is selected so that the two solid poles are displaced by 180.

Due to the non-uniform slot distribution on the circumference, different moments of

intertia are obtained in the main axis of rotor. This in turn causes oscillating shaft

deflections at twice the system frequency. To reduce these vibrations, the deflections in

the direction of the pole axis and the neutral axis are compensated by transverse

slotting of the pole.

The solid poles are also provided with additional longitudinal slots to hold the copper

bars of the damper winding. The rotor wedges act as a damper windings in the area of the

winding slots. 238

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Rotor Winding

The rotor winding consists of several coils which are inserted into the slots and

seriesconnected such that two coil groups form one pole. Each coil consists of several

seriesconnected turns, eachof which consists of two half turns which are connected

by brazing in the end section.

The rotor winding consists of silver-bearing de-oxidized copper hollow conductors with

two lateral cooling ducts. L-shaped strips of laminated expoxy glass fiber fabric with

Nomex filler are used for slot insulation. The slot wedges are made of high-conductivity

material and extend below the shrink seat of the retaining ring. The seat of the retaining

ring is silver-plated to ensure a good electrical contact between the slot wedges and

rotor retaining rings. This system has long proved to be a good damper winding.

Retaining Rings

The centrifugal forces of the rotor end windings are contained by single-piece rotor

retaining rings. The retaining rings are made of non-magnetic high-strength steel in

order to reduce stray losses. Each retaining ring with its shrink-fitted insert ring is shrunk

into the rotor body in an overhung position. The retaining ring is secured in the axial

position by a snap ring.

Field Connection

The field current is supplied to the rotor winding through redial terminal bolts and two

semicircular conductors located in the hollow bores of the exciter and rotor shafts. The

field current leads are connected to the exciter leads at the exciter coupling with

Multikontakt plug-in contacts, which allow for unobstructed thermal expansion of the

field current leads.

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Hydrogen Cooler

The hydrogen cooler is a shell and tube type heat exchanger, which cools the hydrogen

gas in the generator. The heat removed from the hydrogen is dissipated through the

cooling water. The cooling water flows through the tubes, while the hydrogen is passed

around the finned tubes.

The hydrogen cooler is subdivided intoidentical sections, which are vertically mounted in

the turbine-end stator end shield. The cooler sections are solidly bolted to the upper half

stator-end shield, while the attachment at the tower water channel permits them to move

freely to allow for expansion.

The cooler sections are parallel connected on their watersides. Shut off valves are

installed in the lines before and after the cooler sections. The required cooling water

flow depends on the generator output and is adjusted by control valves on the hot water

side. Controlling the cooling water flow on the outlet side ensures an uninterrupted

water flow through the cooler sections so that proper cooler performance will not be

impaired.

The sleeve bearings are provided with hydraulic shaft lift oil during startup and turning

gear operation. To eliminate shaft currents, alt bearings are insulated from the stator

and base plate respectively. The temperature of the bearings is monitored with

thermocouples embedded in the lower bearings sleeve so that the measuring points are

located directly below the babbitt. Measurement and any required recording of the

temperature are performed in conjunction with the turbine supervision. The bearings

have provisions for fitting vibration pickups to monitor bearing vibrations.

Shaft Seals

The points where the rotor shaft passes through the stator casing are provided with a

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radial seal ring. The seal ring is guided in the seal ring carrier which is bolted to the seal

ring carrier flange Hid insulated to prevent the flow of shaft currents. The seal ring is

lined with babbitt on the shaft journal side. The gap between the seal ring and the shaft

is sealed with hydrogen side and air side seal oil. The hydrogen side seal oil is supplied

to the seal ring via an annular groove in the seal guide. Inside the seal ring, this seal oil

is fed to the hydrogen side annular groove in the seal ring and from there to the sealing

gap via several bores uniformly distributed on the circumference. The air side seal oil is

supplied to the sealing gap from the seal ring chamber via radial bores and the air side

annular groove in the seal ring. To ensure effective sealing, the seal oil pressures in the

annular gap are maintained at a higher level than the gas pressure within the generator

casing, the air side seal oil pressure being set to approximately the same level as the

hydrogen side seal oil pressure. The oil drained on the hydrogen side of the seat rings

is returned to the seal oil system through ducts below the bearing compartments. The

oil drained on the air side is returned to the seal oil storage tank together with the

bearing oil.

On the air side, pressure oil is' supplied laterally to the seal ring via an annular groove.

This ensure free movement of the seal ring in the radial direction.

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Cooling System

The heat losses arising in the generator interior are dissipated to the secondary coolant

(raw water, condensate. etc.) through hydrogen and primary water.

Direct cooling essentially eliminates hot spots and differential temperatures between

adjacent components which could result in mechanical stresses; particularly to the

copper conductors, insulation, rotor body and stator core.

Hydrogen Cooling Circuit

The hydrogen is circulated in the generator interior in a closed circuit by one multi-stage

axial-flow fan arranged on the rotor at the turbine end. Hot gas is drawn by the fan from

the air gap and delivered to the coolers, where it is recooled and then divided into three

flow paths after each cooler.

Flow Path - I

Flow path-1 is directed into the rotor at the turbine end below the fan hub for cooling of

the turbine end half of the rotor.

Flow Path - II

Flow path-11 is directed from the coolers to the individual frame compartments for cooling of the stator core.

Flow Path-Ill

Flow path-Ill is directed to the stator end winding space at exciter end through guide

ducts in the frame for cooling of the exciter end half of the rotor and of the core end 243

portions.

The three (lows mix in the air gap. The gas is then returned to the coolers via the axialflow

fan.

The cooling water flow through the hydrogen coolers should be automatically controlled to

maintain a uniform generator temperature level for various toads and cold water

temperatures.

Cooling of Rotor

For direct cooling of the rotor winding, cold gas is directed to the rotor end windings at

the turbine and exciter ends. The rotor winding is symmetrical relative to the generator

center line and pole axis. Each coil quarter is divided into two cooling zones. The first

cooling zone consists of the rotor end winding and the second one of the winding

portion between the rotor body end and the mid-point of the rotor. Cold gas is directed

to each cooling zone through separate openings directly before the rotor body end. The

hydrogen flows through each individual conductor in closed cooling ducts. The heat

removal capacity is selected such that approximately identical temperatures are

obtained for all conductors. The gas of the first cooling zone is discharged from the coils

at the pole center into a collecting compartment within the pole area below the end

winding. From there, the hot gas passes into the air gap through pole face slots at the

end of the rotor-body. The hot gas of the second cooling zone is discharged into the air

gap at mid-length of the rotor body through radial openings- in the hollow conductors

and wedges.

Cooling of Stator Core

For cooling of the stator core, cold gas is admitted to the individual frame compartments

via separate cooling gas ducts. 244

From these frame compartrnents. gas then flows into the air gap through slots in the

core where it absorbs the heat from the core. To dissipate the higher losses in the core

ends, the cooling gas slots are closely spaced in the core end sections to ensure

effective cooling. These ventilating ducts are supplied with cooling gas directly from the

end winding space. Another flow path is directed from the stator end winding space past

the clamping fingers between the pressure plate and core end section into the air gap. A

further flow path passes into the air gap along either side of the flux shield.

All the flows mix in the air gap and cool the rotor body and stator bore surfaces. The gas is

then returned to the coolers via the axial-flow fan. To ensure that the cold gas

directed to the exciter end cannot be directly discharged into the air gap. an air gap

choke is arranged within the range of the stator end winding cover and the rotor

retaining ring at the exciter end. ,

Primary Cooling Water Circuit In The Generator

The treated water used for cooling of the stator winding, phase connectors and

bushings is designed as primary water in order to distinguish it from the secondary

coolant (raw water, condensate, etc.). The primary water is circulated in a closed circuit

and dissipates the absorbed heat to the secondary cooling water in the primary water

cooler. The pump is supplied with hot primary water from the primary water tank and

delivers the water to the generator via the coolers. The cooled water flow is divided into

two flow paths as described in the following paragraphs.

Flow Path-1

Flow path-1 cools the stator windings. This flow path first passes to a water manifold on

the exciter end of the generator and from there to the stator bars via insulated hoses.

Each individual bar is connected to the manifold by a separate hose. Inside the bars,

the cooling water flow through hollow strands. At the turbine end, the water is passed 245

through similar hoses to another water manifold and then returned to the primary water

tank Since a single pass water flow through the stator is used, only a minimum

temperature rise is obtained for both the coolant and the bars. Relative movements due

to different thermal expansions between the top and bottom bars are thus minimised.

Flow Path-11

Flow path-11 cools the phase connectors and the bushings. The bushings and phase

connectors consist of thick walled copper tubes through which the cooling water is

circulated. The six bushings and the phase connectors arranged in a circle around the

stator end winding are hydraulically interconnected so that three parallel flow paths are

obtained. The primary water enters three bushings and exits from the three remaining

bushings.

The secondary water flow through the primary water cooler should be controlled

automatically to maintain a uniform generator temperature level for various loads and

coldwater temperature.

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