Modern Power Station Practice,VolumeC,chapter6 (the generator)

British Electricity International

Modern Power Station Practice Third Edition incorporating Modern Power System Practice

TURBINES, GENERATORS AND ASSOCIATED PLANT. Volume C Only Ch 6: The Generator

British Electricity International .· .·•· ·

.!

. Modern . . . Power Station · Practice Third Edition incorporating Modern Power System Practice

TURBINES, GENERATORS AND ASSOCIATED PLANT. ,

Volume· c . .

8~rgamon '~i' ..

Press

·

MODERN POWER STATION PRACTICE Third Edition

Incorporating Modern Power System Practice

British Electricity International, London

·volume C

Turbines, Generators and Associated Plant

PERGAMON PRESS OXFORD

.

NEW YORK

.

SEOUL

. TOKYO

'

-..ain Editorial PaneJ :

~

w

Littler, BSc, PhD, ARCS, CPhys, FlnstP, CEng. FlEE (Chairman)

::-:"essor E. J. Davies, DSc, PhD, CEng, FlEE -

E Johnson

= ( rkbyf

=3 "

BSc/ CEng, MIMechE, AMIEE

Myerscough, CEng, FIMechE, FINucE

••.right, MSc, ARCST, CEng, FlEE, FIMechE, FlnstE, FBlM

Volume Consulting Editor 7 =-:"'essor E. J. Davies, DSc, PhD, CEng, FlEE

Volume Advisory Editor

= Hambling,

CEng, MlMechE

Authors :- ::::~--s 1 & 2

G. F. Hunt, BSc(Eng), CEng, MIEE

_ -::::~" 3

M. Douglass, CEng, MIMechE

-~--""

4

-- :::::;" 5 - -::::~"

6

A. R. Woodward, BSc(Eng) D. L. Howard, BSc, CEng, MIMechE E. F. C. Andrews, CEng, MlMechE, ABTC B. J. Beecher, BSc, CEng, MlMechE ffi J. J. Arnold, BSc, CEng, MIEE J. R. Capener, BSc, CEng, MIEE

Series Production P. M. Reynolds H. E. Johnson =;;s:_"::;es and

::-:-:·ration

T. A. Dolling J. R. Jackson

U.K. U.S.A.

Pergamon Press pic., Headington Hill Hall, Oxford OX3 OBW, England Pergamon Press, Inc., (395, Saw Mill River Road,) Elmsford, New York 10523, U.S.A.

SEOUL

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Copyright © 1991 British Electricity International Ltd

All Rights Reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic tape, mechanical, photocopying, recording or otherwise, without permission in writing from the copyright holder. First edition 1963 Second edition 1971 Third edition 1991

Library of Congress Cataloging in Publication Data Modern power station practice: incorporating modern power system practice/ British Electricity International.3rd ed. p. em. Includes index. 1. Electric power-plants. I. British Electricity International. TK1191.M49 1990 62.31 '21 - dc20 90-43748 British Library Cataloguing in Publication Data British Electricity International Modern power station practice.- 3rd. ed. 1. Electric power-plants. Design and construction I. Title II. Central Electricity Generating Board 621.3121. ISBN 0-08-040510-X (12 Volume Set) ISBN 0-08-040513-4 (Volume C)

Printed in the Republic of Singapo;e by Singapore National Printers Ltd

Contents CoLOUR PLATEs

VI

FOREWORD

Vll

PREFACE

CoNTENTs oF

ix ALL

VoLUMEs

Chapter 1 The steam turbine

XI

1

Chapter 2 Turbine plant systems

124

Chapter 3 Feedwater heating systems

241

Chapter 4

323

Condensers, pumps and cooling water plant

Chapter 5 Hydraulic turbines

422

Chapter 6 The generator

446

INDEX

563

(

Foreword G. A. W. Blackman, CBE, FEng Chairman, Central Electricity Generating Board and Chairman, British Electricity International Ltd

FoR oVER THIRTY YEARS, since its formation in 1958, the Central Electricity Generating Board (CEGB) has been at the forefront of technological advances in the design, construction, operation, and maintenance of power plant and transmission systems. During this time capacity increased almost fivefold, involving the introduction of thermal and nuclear generating units of 500 MW and 660 MW, to supply one of the largest integrated power systems in the world. In fulfilling its statutory responsibility to ensure continuity of a safe and economic supply of electricity, the CEGB built up a powerful engineering and scientific capability, and accumulated a wealth of experience in the operation and maintenance of power plant and systems. With the privatisation of the CEGB this experience and capability is being carried forward by its four successor companies National Power, PowerGen, Nuclear Electric and National Grid. At the heart of the CEGB's success has been an awareness of the need to sustain and improve the skills and knowledge of its engineering and technical staff. This was achieved through formal and on-job training, aided by a series of textbooks covering the theory and practice for the whole range of technology to be found on a modern power station. A second edition of the series, known as Modern Power Station Practice, was produced in the early 1970s, and it was sold throughout the world to provide electricity undertakings, engineers and students with an account of the CEGB's practices and hard-won experience. The edition had substantial worldwide sales and achieved recognition as the authoritative reference work on power generation. A completely revised and enlarged (third) edition has now been produced which updates the relevant information in the earlier edition together with a comprehensive account of the solutions to the many engineering and environmental challenges encountered, and which puts on record the achievements of the CEGB during its lifetime as one of the world's leading public electricity utilities. In producing this third edition, the opportunity has been taken to restructure the information in the original eight volumes to provide a more logical and detailed exposition of the technical content. The series has also been extended to include three new volumes on 'Sta~ion Commissioning', 'EHV Transmission' and 'System Operation'. Each of the eleven subject volumes had an Advisory Editor for the technical validation of the many contributions by individual authors, all of whom are recognised as authorities in their particular field of technology. All subject volumes carry their own index and a twelfth volume provides a consolidated index for the series overall. Particular attention has been paid to the production of draft material, with text refined through a number of technical and language editorial stages and complemented by a large number of high quality illustrations. The result is a high standard of presentation designed to appeal to a wide international readership. It is with much pleasure therefore that I introduce this new series, which has been attributed to British Electricity International on behalf of the CEGB and its successor companies. I have been closely associated with its production and have no doubt that it will be invaluable to engineers worldwide who are engaged in the design, construction, commissioning, operation and maintenance of modern power stations and systems.

March 1990

~.

-

Preface The increase in generating capacity of the Central Electricity Generating Board (CEGB) during the last thirty years has involved the introduction· of new 500 MW and 660 MW turbine-generator plant for a variety of operational duties from base load to that of flexible two-shift operation. These plants have been installed in nuclear, coal and oil fired. power stations. The early operational experience of the 500 MW units provided important data for the design development of the 660 MW turbine-generator plant. These latter machines benefited from the high quality approach to the design of major components by UK manufacturers using their developed analysis techniques in the areas of aerodynamics and stress analysis. The soundness of this approach has been demonstrated by the improved reliability and performance of the later plants. The Third Edition of Modern Power. Station Practice gives a detailed account of experience obtain

Synchronou:; generator theory

2.1

Electromagnetic induction Speed, frequency and pole-pairs 2.3 Load, rating and power factor 2.4 MMF, flux an.d magnetic circuit 2.5 Rotating phasors 2.6 Phasor diagrams 2.6.1 Rated voltage, no stator current, open-circuit conditions 2.6.2 Rated voltage, rated stator current and rated power factor 2.7 Torque 2.8 Three-phase windings 2.9 Harmonics: distributed and chorded winding 2.2

1.

l

Introduction General description of static diode rectifier equipment Rectifier protection Static thyristor rectifier schemes 6.4 The voltage regulator 6.4.1 Historical review 6.4.2 System description 6.4.3 The regulator 6.4.4 Auto follow-up circuit 6.4.5 Manual follow-up 6.4.6 Balance meter 6.4.7 AVR protection 6.4.8 Thyristor converter protection 6.4.9 Fuse failure detection unit 6.4.10 The digital AVR 6.5 Excitation control 6.5.1 Rotor current limiter 6.5.2 MVAr limiter 6.5.3 Overfluxing limit 6.5.4 Speed reference controller 6.6 The power system stabiliser 6.6.1 Basic concepts , 6.6.2 Characteristics of GEP 6.6.3 System modes of oscrllation 6.6.4 Principles of PSS operation 6.6.5 The choice of stabiliser signal 6. 7 Excitation system analysis 6. 7.1 Frequency response analysis 6. 7.2 State variable analysis 6.7.3 Large signal performance investigations 6.3.1 6.3.2 6.3.3 6.3.4

3 Turbine-generator components: the rotor 3.1

Rotor body and shaft Rotor winding 3.3 Rotor end rings 3.4 Wedges and dampers 3.5 Sliprings, brushgear and shaft earthing 3.6 Fans 3.7 Rotor threading and alignment 3.8 Vibration 3.9 Bearings and seals 3.10 Size and weight

3.2

1

4 Turbine-generator components: the stator 4.1 4.2 4.3 4.4 4.5 4.6 4.7 4.8 5

Stator core Core frame Stator winding End winding support Electrical connections and terminals Stator winding cooling components Hydrogen cooling components Stator casing

7

Running-up to speed Open-circuit conditions and synchronising The application of load 7.4 Steady state stability 7. 5 Capability chart 7.6 Steady short-circuit conditions, short-circuit ratio 7.7 Synchronous compensation 7.8 Losses efficiency and temperature 7.9 Electrically unbalanced conditions 7.10 Transient conditions 7.11 Neutral earthing 7.12 Shutting down

Cooling systems Hydrogen cooling Hydrogen cooling system 5.3 Shaft seals and seal oil system 5.3.1 Thrust type seal 5.3.2 Journal type seal 5.3.3 Seal oil system 5.4 Stator winding water cooling system 5.5 Other cooling systems

5.2

Excitation Exciters 6.1.1 Historical review 6.1.2 AC excitation systems 6.1.3 Exciter transient performance 6.1.4 The pilot exciter 6.1 .5 The main exciter 6.1.6 Exciter performance testing 6. 1. 7 Pilot exciter protection

8

6.1

446

,. ~!i

Generator operation 7.1 7.2 7.3

5.1

6

Telemetry systum

6.2.4 Instrument slipnngs 6.2.5 Rotating rectifier protection 6.3 Static rectifier excitation equipment

Mechanical considerations

8.1 8.2 8.3 8.4 8 ,, 9

Rotor torque Stress due to centrifugal force Alternating stresses, fretting and fatigue 'Slip-stick' of rotor windings r·Joise

Electrical and electromagnetic aspects 9.1

Flux distribution on load

Introduction 9.2 9.3 9.4 9.5 9.6

Control and calculation of reactances The cause and eff~ct of harmonics Magnetic pull Shaft voltage and residual magnetism Field suppression Voltage in the rotor winding Stator winding insulation

9.7 9.8

1 0 Operational measurement, control, monitoring and protection

10.1 Routine instrumentation 10.1.1 Temperature 10.1.2 Pressure 10.1.3 Flow 10.1.4 Condition monitoring 10.1.5 Electrical 10.1.6 Vibration 10.2 Logging and display 10.3 Control 10.4 On-load monitoring, detection and diagnosis 10.4. 1

10.4.2 10.4.3 10.4.4 10.4.5 10.4.6

1

1.1

Air gap flux coil Core or condition monitor Insulation discharge Rotor winding earth fault indication Shaft current insulation integrity Stator winding water analysis

Introduction

Types of generator

The CEGB transmission systertl operates at a frequency of 50 Hz: so do all the: generators connected synchronously to iL The larger _generators ate almost all directly driven by steam turbines rotating at 3000 r/min; a few operate at 1500 r/min. These high speed generators are commonly known as turbine-generators, or cylindrical rotor generators; in this chapter, such machines are implied unless otherwise stated. The CEGB has for many years standardised on generating units of 500 and 660 MW electrical output_ At these ratings, there have been six different designs of generator, each design incorporating minor changes as time progressed. However, they are all sufficiently similar for a generalised description to be applicable. Where a design departs radically from that being described, this will be noted (see Fig 6.1). The bulk of this chapter deals with generators of this size; the theory applies to all synchronous generators. Brief descriptions of other types of generator in use on the CEGB system will be found at the end of this chapter.

1.2

Historical background

fhe advantages of AC over DC as a means of electricity distribution were established towards the end

10.5 Protection 10.5.1 Class 1 trips 10.5.2 Class 2 trips 11

Maintenance, testing and diagnosis

11 .1 11.2 11 .3 11 .4 11.5 11.6

Maintenance Maintenance outage Maintenance Maintenance Reassembly Diagnosis

and tests during operation and tests when shut down for a short during a longer outage and tests with the machine dismantled

12 Future developments 12.1 Extension of present designs 12.2 Extension of water cooling 12.3 Slotless generators 12.4 Superconducting generators 12.5 wer rating Water turbine d'riven salient pcie synchronous generators 13.2.1 Excitation and control 13.2.2 Other features 13.3 Diesel engine driven salient-pole generators 13.4 Induction generators

of the 19th century, and the rapid growth of AC systems led to a demand for AC generators. At first, these were slow speed machines driven by reciprocating engines but, by 1900, generators driven directly by high speed steam turbines were being introduced .in what are recognisably the forerunners of modern :machines, the benefits being principally in the prime :mover. The early, turbine-gel).erators were made both in vertical and horizontal shaft configurations. The vertical shaft design required a large thrust bearing, and was quickly abandoned. The development of horizontal shaft machines was rapid; unit outputs had risen from a few hundred kW to 20 MW by 1912 (see Fig 6.2). The rate of increase in output slowed subsequently, but unit outputs had risen to 50 MW by the 1930s. The 60 Hz frequency standardised in the USA required the speed of American two-pole generators to be 3600 r /min, and the losses caused by air friction at this speed made the much-less-dense gas hydrogen attractive as a cooling medium. In the UK, hydrogen cooling was used on 3000 r /min units of 50 MW and above from about 1950. Later, the search for increasingly effective means of heat (loss) removal led first to the use of hydrogen at higher pressure, then insulating oil, and finally, pioneered in the UK, water in direct contact with the winding conductors. By these means, generators with the increasing outputs demanded were able to be manufactured, transported and installed in a power station as single units, which was both economically and operationally attractive (see Fig 6.3). 447

..,...,.

--1 ::r

CXl

-----Cr I .~------------

~~~,

li

li

FLEXIBLE HOSE

AXIAL EXTENT OF PISTOYE SLOTTING

INSULATING SLEEVE

I

,• MAIN FLUX

CURRENT INDUCED BY AXIAL FLUX

SUPPORT SADDLE

J

PISTOYE SLOT

AXIAL VIEW OF STATOR TEETH

FIG. 6.33 Pistoye slots in stator teeth

Slots must therefore accommodate six similar winding circuits, differing only in phase displacement; and 42, 48 or 54-slot arangements are commonly used. A two-layer arrangement is adopted, in which a winding progresses from a top conductor (bar nearest the bore) in a slot, bending in two planes after it emerges from the core to span nearly a quarter of the circumference. At this point it is connected to a similar bar which continues the span but on a larger conical diameter, and re-enters the core as a bottom conductor almost opposite the first (not exactly opposite because of short-pitching). This bottom bar is then connected, at its other end, to the top bar in 478

the slot next to the previous one, and the winding continues in this manner until one-sixth of the slots are filled. Because of short-pitching, some slots. contain a top bar of one phase and a bottom bar of a different phase. A 776 MVA, 23.5 kV generator has a rated RMS current of 19 080 A, i.e., a current of 9540 A per bar. By cooling with water in contact with the conductor, a current density of 8 A/mm 2 of cross-sectional area can be achieved. With a slot width of about 45 mm, and allowing for insulation, the effective conductor width is restricted to about 30 mm. Sufficient area must be allocated for satisfactory water flow, and

Turbine-generator components:

the stator

a:

CJ

>

>-

'-'

0

~

2,

:::0

0 a:

I

v;

z CJ ;:::

""-o

>-

M

(.)

w

0

(f)

.:

>-

a:

it

(f) . 6.37 Core end-plate and screen

design dimensions, while the curved ends are consolidated using heat-shrinkable tape. Tests are carried out to ensure that the insulation is properly canso-

D D D

SLOT CONTAINING IDENTICAL CONDUCTORS

lidated and free from significant voids, and electrical tests confirm the integrity of the insulation. The insulation is very hard and the insulated bar has little flexibility. The slot length of each bar is treated with semiconducting material to ensure that bar-to-slot electrical discharges do not occur, and a high resistance stress grading finish is applied to the ends to control surface discharge, particularly during high voltage tests. Bars carrying such large currents experience large forces; in the slots these are directed radially outwards towards the bottom (closed end) of the slot, and alternate at 100 Hz. The closing wedges therefore are not required to restrain the bars against these forces, but it is important that the bars do not vibrate, and the wedges are arranged to exert a radial force, either by tapered packers or by a corrugated glass spring member. Some designs use a corrugated glass spring packer in the slot side to provide sideways restraint. Packers of insulation material, separators and drive strips, and layers of conformable thermo-setting dough are also used in the slot fill (see Fig 6.40). Support of the end windings and the arrangement of connections are dealt with in later sections. The electrical loss due to the stator winding is traditionally separated into the 12 R loss, using the measured DC resistance of the winding phases at the operating temperature, and the 'stray' loss, in which are included components due to: • AC resistance being greater than DC resistance (skin effect). • Eddy currents, as already noted.

DISTANCE FROM SLOT BOTTOM. X

(a)

(b)

CURRENT BELOW X MMFAT X

(a)

Ib)

FLUX LINKAGE AT X EDDY VOLTAGE AT X

FIG. 6.38 Illustrating the variatioll" of eddy currents in stator conductors

482

DISTRIBUTION OF EDDY CURRENT

FIG. 4.24

Heysham 2 condenser - modular construction

FtG. 6.31

Flux test on completed core

-

Ftu. 6.41

View of a 660 MW generator ;;tatur end-windings

I

I I

u.. ...

'""'"

FrG. 6.90

C't!h'dltion ninnitor (NEI Parsons Ltd)

FIG. 6.97

Dinorwig motor-generator during site winding

Turbine-generator components:

the stator

FIG. 6.39 Roebel transpositions

• Currents induced in core end plates, screens, and end teeth. • Harmonic currents induced in the rotor and end ring surfaces. • Currents induced m frame, casing, endshields, fan baffles, etc. These individual losses have to be assessed so that the appropriate cooling medium is directed to their sources, in order to a~oid~ unacceptable localised hot spots.

4.4

End winding support

In the end windings, bands of conductors are arranged side-by-side, all carrying the same current although not all in phase, and considerable electromagnetic

forces are produced, both at rated load and particularly when large current peaks occur during fault conditions. The end turns must be strongly braced to resist the peak forces and also to minimise the 100 Hz vibration. The MMF produced in the end winding region by the combined effect of the stator and rotor end windings produces a considerable magnetic flux in the end regions. Paramagnetic material would tend to concentrate the flux into itself, and electrically-conducting material would have eddy currents induced in it, causing both additional loss and potential hot spots, Metallic inserts and fastening devices can be caused to vibrate and loosen, or wear away their surrounding medium. Consequently non-metallic components are used, mainly moulded g!assfibre. Substantial support brackets are bolted to the core end plate and provide a support for a massive glassfibre conical support ring. The outer layer of end turns is pulled onto a bedding of thermosetting con483 li

'i

Chapter 6

The generator

EACH BAR COMPRISES 2 GROUPS OF 2 STACKS OF STRAPS

BOTTOM INSULATION PACKING STRIP AND CONFORMABLE DOUGH

CROSSOVER INSULATION

ALTERNATE SOLID AND HOLLOW COPPER CONDUCTORS

GROUP BINDING TAPE

RIPPLE SPRING ASBESTOS FINISHING TAPE

INSULATION PACKING STRIPS AND CONFORMABLE DOUGH BETWEEN COILS

GROUP VERTICAL SEPARATOR

ALL HOLLOW COPPER CONDUCTORS

MAIN INSULATION WRAP

STRAP INSULATION

TOP COIL

PROTECTIVE DRIVING STRIP CLOSING WEDGE

OPPOSED TAPER WEDGES APPLYING RADIAL RESTRAINT

FIG. 6.40 Stator slot

484

r------------------------------------------------------------------------------------------------------------~

Turbine-generator components:

formable material between i! and the support cone, and packers between the bars arch-bind the structure circumferentially. The inner layer is treated similarly, with a ring of blocks pulled down onto the cone by through-bolts, completing the very rigid structure. Some designs use sheets of insulation material to enclose any spaces and prevent the accidental ingress of any foreign material. Magnetic material is particularly unwelcome, since it can be caused to vibrate and abrade, or be heated by eddy currents and degrade the adjacent insulation (see Fig 6.33). Vibration of the end windings must be minimised, since it can promote fatigue cracking in the winding copper. This is particularly serious if it occurs in a water-carrying tube, since hydrogen will leak into the water circuit. Resonances close to 100 Hz must be avoided, since both the core ovalising and the winding exciting force occur at this frequency. Accelerometers in the end winding structure allow any increase in vibration due to support slackening to be monitored. Vibration amplitude is highly current dependent. Any looseness developing after a period in operation can be corrected by tightening the bolts, by inserting or tightening wedges, and/or by pumping a thermosetting resin into rubber bags located between conductor bars. Figure 6.41 shows the stator core and end windings for a 660 MW generator.

4.5

Electrical connections and terminals

Electrical connections between one conductor bar and the next in series are made differently in different designs. In one, a common electrical and water connector is formed by a copper tube bent into a U-shape, and brazed onto small copper waterboxes into which all the bar subconductors are brazed. In another, the electrical joint is made by a solid copper bolted joint, with the water connections separate. It is common practice to insulate the joint or to enclose it in a rubber housing. The conductor bars at the high voltage end (line end) and the low voltage end (neutral end) of a phase band are electrically connected to tubular connectors which run circumferentially behind the end windings at the exciter end, to the outgoing terminals, usually with line terminals at the bottom and neutral terminals at the top, although other arrangements do exist. These connectors are internally water cooled, and must be insulated for line voltage. Terminal bushings (Fig 6.42) are proprietary paperinsulated items, with internal water cooling from the stator winding water system. Their insulation must be capable of withstanding the hydrogen pressure in the casing, with no perceptible leakage. It is common practice to flange-mount the terminals on a plate of non-magnetic material, and to arrange for a terminal to be withdrawable from outside the casing. Current transformers for instrumentation and protection sig-

the stator

nals are housed on the external stems of the bushings. The connections from the generator terminals to the generator transformer are described in Volume D. Phase isolated connections are always adopted, so that an electrical fault at the connections must start as a line-to-earth fault, which is much less damaging to the generator than a line-to-line fault.

4.6

Stator winding cooling components

Water is the best of the commonly available media for cooling the stator winding, and imposes only one condition that would not also apply to other fluids: it must be pure enough to be effectively non-conducting (electrically). It is continuously degassed and treated in an ion exchanger, with the following target values being aimed for: Conductivity:

100 JLS/m

Dissolved oxygen:

200 j.tg/litre max (in some systems > 2000 is acceptable)

Total copper:

150 JLg/litre max.

pH value:

9 max.

At these levels, no aggressive attack on the winding copper has been noticed after very many years' experience. Any erosion of copper is detected by the monitoring equipment. Water is passed into one or more inlet manifolds, which are copper or stainless steel pipes running circumferentially around the core end plate. From the manifolds, flexible PTFE hos~s are connected to all the water inlet ports on the stator conductor joints. In a two-pass design, water passes through both bars ~n parallel and is transferred to the two connected bars at the other end, returning through similar hoses to the outlet manifold which adjoins the inlet manifold. This design minimises the number of hoses, but requires a larger pressure head of water across the winding (see Fig 6.43). In a single-pass arrangement, hoses connect both ends of a bar to the manifolds, which are located at opposite ends. Thin metallic-sleeved components are crimped inside and outside the ends of the PTFE hoses, and these are then attached to bosses on the manifolds and winding connectors, using screwed-up olives, 0-rings or brazed joints. The casing hydrogen pressure is everywhere greater than the water pressure in the winding circuit, so that any leakage is of hydrogen into· water, rather than the reverse, which would be damaging to the winding insulation. The loss input into the water circuit at rated load is designed to raise the water temperature by less than 30°C. With an inlet temperature of 40°C, there is plenty of margin before the\temperature at which boiling would occur, 115-120°C at the working pres485

The generator

Chapter 6

Ftu. 6.4! View of a 660 MW generator ;tator end-windings (see also wlour photograph between pp 4~2 and 483)

486

Turbine-generator components:

the stator

GAS SIDE

CONNECTION PALM

2 GAS/WATER

·o·

RING SEALS.

SEAL

OUTER STATOR FRAME

SEAL

ALUMINIUM TERMINAL PLATE

CLAMP FLANGE

TUBULAR COPPER CONDUCTOR

MAIN INSULATION

EXCITER END

SEAL

CONNECTION PALM

AIR SIDE

FIG. 6.42 Generator terminals

487

The generator

Chapter 6

TO NEUTRAL TERMINAL ____\ FROM INLET MANIFOLD

OUTLET FROM MAIN TERMINAL

INLET TO NEUTRAL TERMINAL

- INLET MANIFOLD EXCITER END

'- OUTLET MANIFOLD

, MAIN TERMINAL TO OUTLET MANIFOLD

PTFE HOSES PHASE RINGS

\

I

~

DIAGRAMMATIC SECTION OF STATOR COIL- TO-PHASE RING CONNECTIONS

COIL-TO-COIL CONNECTIONS

FIG. 6.43 Stator winding water cooling system components

sure. Monitoring the temperature of each bar by thermocouples, either in the slots or in the water outlets, enables a reduction or stoppage of water flow in a bar to be detected.

4. 7

Hydrogen cooling components

The advantages of hydrogen cooling, and its parameters, are described in Section 5 of this chapter. Hydrogen enters the generator casing through an axially-oriented distribution pipe at the top, carbon dioxide for scavenging being admitted through a similar pipe at the bottom. The rotor fans circulate hydrogen over the end windings and through the stator core, while a parallel flow passes through the rotor. At rated load, the hydrogen temperature increases by about 25°C during the few seconds taken to complete this circuit. Two or four hydrogen coolers are located vertically or 488

horizontally inside the casing; they consist of banks of finned or wire-wound tubes through which water flows in one or two passes while hydrogen flows over them. The coolers are arranged so that their headers are accessible (for tube cleaning) without degassing the casing. The tubes and the cooler frame must be supported so as to avoid resonances close to the principal exciting frequencies of 50 Hz and 100 Hz. It is most important that moisture does not condense on the stator end windings, since electrical breakdown may then occur. The dewpoint of the hydrogen (at casing pressure) must be at least 20°C lower than the temperature of the cooled hydrogen emerging from the coolers, and this is continuously monitored by a hygrometer. In normal on-load operation, the stator winding water maintains the winding temperature above 40°C; if condensation occurred it would be on the hydrogen coolers first. During run-up, however, the stator winding water is likely to be cold, and it is either pre-heated electrically, or

.'' Turbine-generator components: irculated for a lengthy period, to .warm the winding fore the generator is excited.

8

Stator casing

.. e casing contains the stator core and core frame, .nd must resist the load and fault torques. It must :o provide a pressure-tight enclosure for the hyogen. Historically, casings have been made strong nough to withstand the pressure developed by an '"1ition of the most explosive mixture of hydrogen d air, without catastrophic failure. Because any mixture of hydrogen and air within he explosive range is not allowed to occur, attain~nt of explosion pressure is not a credible condition, ___ d to specify the casing on the basis of withstanding uch a pressure without leaks, as would be required BS5500, is unrealistic. Consequently, the full retirements of the pressure vessel code are not invoked, hough some of them are applied. This pragmatic mproach has been justified by worldwide experience over 'ty years. Casings are fabricated steel cylinders of up to ~5 mm thickness, reinforced internally with annular rings td axial members which strengthen the structure and •rm passages for the flow of hydrogen (see Figs 6.44 and 6.45). Internal spaces are provided with

runners to accommodate the hydrogen coolers. At the ends, thick rings provide facings for the separate end shields. Internal supports for the core frame, in the form of horizontal footplates or spring plate fixings, are provided, and external feet support the complete assembly. Lifting trunnions are usually made detachable. The design of the welded joints is carefully controlled to avoid the presence of unfused lands wherever possible. The main welds have to be leak-tight against hydrogen at 4 bar, which is a very exacting requirement. The complete casing may be too large to be stress-relieved in an annealing oven, in which case it must be assumed that stresses up to yield. stress exist il). the welds. In one design, the casing is constructed in two halves, which are stress-relieved before being welded together. The end shields are thick circular fabricated steel plates, ribbed to withstand the casing pressure with minimal axial deflection. They house the shaft seal stationary components and, in some designs, the outboard bearing. Leak-free sealing of the end shield/ casing joints against the hydrogen pressure, as with all other casing joints, is effected by gaskets, 0-rings and sealing compounds injected into grooves. The completed casing assembly is hydraulically pressure tested, and finally must be demonstrated to be leak-tight to a level corresponding to a fall from

COOLER ENCLOSURE POCKET

~

FRAME RIB PLATES

TURBINE END

JACK SUPPORT BRACKET COOLER SEAL BARS,

the stator

ROTOR COOLING GAS DUCTS

~~ ~ ~

;:~~'-~\

~'

END PLATE

\, JACK SUPPORT BRACKET MAIN TERMINAL ENCLOSURE

FIG. 6.44 Outer stator casing

489

The generator

Chapter 6

FIG. 6.45 Core frame being inserted into casing

490

Cooling systems rated hydrogen pressure of not more than 0.035 bar in 24 h. ' Some of the core vibration is' transmitted to the casing, and rotor vibration is transmitted through the end shield and the foundations. The casing assembly must be designed to avoid resonances in the range of these exciting frequencies. Drains are arranged so that any oil or water collecting in the bottom of the casing is piped to liquid leakage detectors, which initiate an alarm. Distribution pipes for hydrogen and C0 2 are built-in; a temperature sensor at the C0 2 inlet initiates an alarm if the incoming gas has not been adequately heated and could chill the fal;>ricated casing locally. Electrical heaters are fitted in the lower half of the casing to maintain dry conditions during outages. The casing is bolted down to the supporting steelwork on packing plates which are machined after trial erection to provide the correct alignment. Axial and transverse keys prevent subsequent movement. The weight of the casing, complete with core frame, coolers and water, is up to 450 tonnes.

5

Cooling systems

ductivity and specific heat of hydrogen, the effect is that heat removal from heated surfaces is up to ten times more effective, resulting in lower temperatures. Coolers can also be considerably smaller. • The use of hydrogen imposes the need for hermetic sealing and condition control, which helps to ensure that the original electrical clearances are maintained.

• More importantly: the degradation of insulation by oxidation processes cannot occur in a hydrogen atmosphere. The disadvantages are: • Since concentrations of from 4~o to 76~o of hydrogen in air are explosive, hydrogen must not be allowed to escape from the stator casing and its associated pipework in significant quantities and become trapped in potentially explosive pockets. The casing and end shields have to be of rugged construction and leak proof, demanding meticulous welding techniques. Penetrations such as the rotor shafts, and all outgoing connections, must be positively sealed, the former requiring a sophisticated sealing system.

A generator of this type has an efficiency of about

98.51Jio. Even though the. losses are low in terms of the output, they amount to some 10 MW, all of which must be removed by the cooling systems; the heat lost by convection and radiation from the casing is not significant. In some stations, most of the generator (and exciter) losses are transferred into the boiler feedwater system by using condensate in the heat exchangers. While such an arrangement can be economic, there is a penalty in the form of added complication, and the most modern stations do !not have this feature.

5.1

Hydrogen cooling

Hydrogen has several advantages over air as a means of removing heat from turbine-generators: • The density of hydrogen is the lowest of all gases and is one-fourteenth that of air. Even at the rated pressure (4 or 5 bar) and with the allowable level of gaseous impurities, it is still only half as dense as air at normal temperature and pressure (NTP). The large loss due to the gas being churned by the rotor, and to its circulation through the fans and cooling passages, is minimised by the use of hydrogen as a coolant. • The heat transfer cap~bility of hydrogen is up to twice that of air in similar conditions, though, as with all gases, it increases with increasing pressure. Together with the several times higher thermal con-

• A comprehensive gas control system is required. For generators rated much above I 00 MW, air cooling is not practical; more than half the total loss would be due to fan and rotor windage. At 500 and 660 MW, hydrogen pressures of 4 or 5 bar are economic; higher pressures than this have little or no advantage. The only practical alternative at these ratings is complete wat~r cooling including the rotor, which has not been adopted in the UK, and only rarely elsewhere, because of leakage problems at the very high water pressures produced by the rotation.

5.2

Hydrogen cooling system

It is necessary to ensure that potentially explosive mixtures of air and hydrogen do not occur when filling the casing with hydrogen, or when emptying it. The usual method is to use carbon dioxide as a buffer between the two other gases, in a process known as scavenging, or simply gassing-up and degassing. Carbon dioxide, stored as a liquid under pressure, is expanded to a suitably low pressure above atmospheric. It is also heated, because the expansion causes it to cool and it would otherwise freeze. With the rotor stationary, C0 2 is fed into the bottom of the stator casing through a long perforated pipe, and because it is more dense than air it displaces air from the top via the hydrogen inlet distribution pipe to atmosphere outside the station. Some mixing of gases occurs at the interface. A gas analyser is used to 491

The generator

monitor the proportion of C0 2 in the gas passing to atmosphere; when tllis is sufficiently high, the C0 2 inlet is closed (see Fig' 6.46). High purity hydrogen from a central storage tank or electrolytic! process is then fed through a bus main at about 10 bar to the gas control panel, where its pressure is reduced before being fed to the casing through the top admission pipe (Fig 6.47). Being very much lighter, it displaces the C0 2 from the bottom of the casings via the C0 2 pipe to atmosphere, again with some degree of mixing. When the proportion of C0 2 in the vent is low enough, the proportion of air left in the casing will be very low, and if the casing is then pressurised with hydrogen to its pperating pressure (say 4 bar), the proportion of air will be reduced to a quarter of this low value. The complete process normally occupies a few hours. Separate procedures are followed to ensure that other components, such as tanks, are properly scavenged, so that dangerous mixtures do not occur. The reverse of the foregoing procedure, using C0 2 and then dry compressed air, is followed to remove hydrogen from the machine for inspection or for a prolonged outage. In one design of 500 MW generator, air is removed from the casing by drawing a vacuum, using the pump normally used to degas the seal oil. The shaft seals are arranged to seal effectively under this unusual operating condition. When the vacuum is as low as can be achieved, hydrogen is admitted, the resulting purity when pressurised being sufficiently high. Normally, hydrogen purity remains high, since air cannot leak into the pressurised system. Some air may, however, be released from the shaft seal oil flowing into the casing hydrogen space. Replacement hydrogen to make up for leakage is usually sufficient to maintain the required purity. The differential pressure developed across the rotor fans is used to circulate a sample of casing hydrogen continuously through a katharometer-type purity monitor, which initiates an alarm if the purity falls below a p'reset value, typically 97o/o. The purity monitor (and the gas analyser) can be calibrated with pure gases from the piped supplies. A check on the purity is also possible by monitoring the differential pressure developed by the fans, which responds markedly to the change in density produced by air impurity. A pressure sensitive valve admits hydrogen from the bus main if the casing pressure falls below a predetermined level, while a spring-loaded relief valve is set to release hydrogen to the outside atmosphere if the pressure becomes excessive. It is important that these two 1 pressures are not set so close that wastage occurs, particularly as the gas temperature and pressure changes when on-load cycling. Monitoring of the hydrogen consumption is a recently introduced feature on some units (see Fig 6.48). The temperature of the hydrogen is normally moni492

Chapter 6

tored by several thermocouples, whose readings should be averaged, at the inlets to and outlets from the hydrogen coolers. Typically, hydrogen is circulated at 30 m3 Is which, with a full-load loss input of about 5000 kW, results in a temperature rise of the order of 30°C. The cooled gas should not be hotter than 40°C, so the temperature of the gas entering the coolers should not exceed 70°C. Water cannot normally leak into the casing from the stator winding water circuit or the hydrogen coolers, since the water pressure is lower than the gas pressure in both circuits. It can be released from the shaft seal oil, particularly if the oil is untreated turbine lubricating oil which has picked. up water from the turbine steam glands. It is important that the moisture content of the casing hydrogen be kept low enough to prevent condensation occurring on the coldest component, which may be the water cooled winding. The differential pressure is used to circulate a flow of hydrogen continuously through a dryer, typically of the twin-tower type, using activated alumina, with automatic changeover and regeneration. A motor-driven blower maintains the flow through the rotor when the rotor is not running at speed (see Fig 6.49). Continuous monitoring of the humidity of the casing gas is provided by means of a hygrometer. The maximum permissible dewpoint is not less than 20°C below the cold gas temperature, measured at casing pressure. It is important that this caveat is observed, particularly if the dewpoint is being compared with that of a sample drawn from the casing and measured at atmospheric pressure. Hydrogen is circulated by the fans through the stator core and end wiQdings, the precise paths being different in different designs. The rotor acts virtually as its own fan, hydrogen being drawn through the windings and exhausted into the airgap, again differently in different designs. The hydrogen removes the electrical loss in the rotor winding, the 'iron loss' in the stator core, the windage loss produced by the rotor and fans, and most of the electrical losses generated in the frame and end winding structures. Because it is impractical to ensure that potentially explosive mixtures of hydrogen and air never occur in the small bore instrumentation pipework, those instruments and devices containing electrical circuits in contact with the gas, such as katharometers, must be intrinsically safe in such mixtures. This means that a sudden break in an electrical circuit must not be capable of providing enough spark energy to ignite the gas. It is impossible to ensure complete freedom from leakage of hydrogen over the lifetime of the plant, and the areas near to potential leakage sources are classified into zones of differing degrees of hazard, described in detail in CEGB Code of Practice 098/34: 'Code of Practice for the Design Principles relating to the use of Hydrogen in Large Generators'. Zones 0

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Cooling systems

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displacing air with C0 2

493

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Chapter 6

The generator

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I FIG. 6.85 Finite element mesh for tooth-wedge stress calculations

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543

The generator

Chapter 6

8.3

Alternating stresses, fretting and fatigue

A stationary rotor sags under its own weight, causing a compressive stress in the outermost fibres at the top and at the axial centre of about 15 MPa, with a corresponding tensile stress at the bottom of the same magnitude. As the rotor rotates, each fibre experiences a compressive/zero/tensile/zero/ compressive stress cycle once per revolution. Since a rotor operating at 3000 r/min accumulates 1.5 x 109 cycles in a year, alternating stress due to bending has to be considered in the design. Though its magnitude is small, it is superimposed on the high steady stresses in the rotor and wedges identified above, and can promote the growth of cracks by high cycle fatigue. One source of crack initiation may be fretting. If a once per cycle movement can occur, say at the gap between two short slot wedges, the resultant localised damage may be sufficient to intensify the local stress field at a minute 'crack-tip', from which the alternating bending stress can propagate. Such features are avoided wherever possible, and particularly near the axial centre where alternating bending stresses are highest. The concepts of fracture mechanics are used to study such crack tip stress intensification.

8.4

FIG. 6.86 End ring lug area -

finite element mesh and stress contours

of all the other conductors in the slot which is of concern, particularly where the copper area is reduced by ventilation grooves and slots. Some creep of the copper may be observed at such slots after many yeqrs in operation. 544

'Slip-stick' of rotor windings

One effect not mentioned in Section 3.8 of this chapter is the behaviour of the rotor winding during a loading cycle. The rotor is run up from cold, and though the windings and rotor body are warmed by gas friction, there is little differential in thermal effect at this stage. At speed, the winding conductors are locked together and to' the wedge by the centrifugal force, unless an axial force can overcome the friction between them. When current is applied to the rotor winding, it reaches a higher temperature than the rotor body, and as the coefficient of thermal expansion of copper is nearly twice that of steel, the conductors experience an axial force directed outwards from the axial centre. As the differential temperature increases, the axial forces increase, until slippage occurs at a point where the build-up of axial force is able to overcome the friction. Because the 'bottom' conductor experiences the least centrifugal load, it is most easily able to overcome friction, and a shorter length of it rem_ains frictionally-locked than those of coils further up the slot. Slippage in most windings appears to occur in small steps, apparently randomly, though possibly repeatably, so that the release of the axial forces does not result in sudden changes in vibration of sufficent magnitude to be significant. In some rotors, however, due to higher frictional restraints having to be overcome, the release of much larger axial forces appears to cause the bending moment to change significantly, resulting in a noticeable sudden change in vibration.

-. ,....

Electrical and electromagnetic aspects

One feature of this is that th~ rotor must usually be run down in speed before the changed vibration disappears, when the cycle can be repeated. Once the rotor is at speed and temperature, it does not tend to suffer from high cycle effects. It is more vulnerable to effects promoted by relative movement, such as abrasion, when running at lower speeds and \\'hile barring, when the centrifugal locking-up is absent.

8.5

Noise

. The generator rotor, with its fans, generates very ' high noise energy at speed. The spectrum is wide, but contains peaks at frequencies related to the number · of fan blades and slots. The other main source of noise is generated by . the stator core when magnetically excited. As previously noted, the magnetic forces 'ovalise' the stator , core, causing vibration and noise at 100 Hz and multiples. The main component of magnetic noise, however, arises from distortions on a much smaller scale, that of the magnetised iron crystals, in the phenomenon known as magnetostriction, at 50 Hz and multiples thereof. The robust stator casing acts as an effective sound attenuator, and little can be achieved to reduce the - transmitted noise further, for example, by the acoustic treatment of the inside surfaces. In practice, the major sources of high noise intensity tend to occur in the driven components such as exciters, which have fans operating in air and no heavy steel surround. Sometimes the complete line of driven units is housed under an acoustic cover to attenuate these sources. Access doors and windows must be provided, and these can reduce the effectiveness of the covers. A sound power level of 93 dBA is specified at - 1 m distant from the plant. Legislation may require this to be reduced for new plant in the future.

9

Electrical and electromagnetic aspects

__ Some electrical and magnetic aspects of generators, not previously considered, are dealt with in this section.

9.1

second increases the required flux magnitude; both increase saturation, the effects of which are highly non-linear (see Fig 6.87). One result of this is that overall iron losses will be higher than those calculated for no-load conditions, and their distribution will differ. Another is that the calculation of the required MMF (rotor current) required for any load condition cannot be accurately based on the simple phasor diagram. Since the rotor is necessarily designed with little margin, accurate calculation of the rotor current needed for rated conditions is essential.

Flux distribution on load

When, in previous sections, magnetic flux densities ·have been mentioned, operation at rated voltage, no-load has been assumed, where the load angle is zero and the rotor operates in the 'direct axis'. In . practical load situations, the load angle is 40-50° ,md the effective f1ux level must be large enough to overcome the leakage reactance voltage drop. The first effect distorts the flux pattern markedly; the

FiG. 6.87 Flux distributioq on load

A method previously used took as its basis the simple no-load unsaturated phasor diagram, and defined an imaginary reactance, the 'Potier' reactance, empirically derived, which was used to define a 'Potier' voltage drop, IXP, for the given load conditions. An internal voltage required to overcome this voltage drop, the 'Potier voltage' was thus established. The MMF difference between the airgap and open-circuit characteristics at the 'Potier voltage' was then phasorially added to the unsaturated MMF phasor. In this way, the increasing and non-linear effects of saturation were taken into consideration (see Fig 6.88). Present methods use finite element calculations, which can be reduced to two dimensions for the central part of the machine. Even so, the detailed geometry and non-linear magnetic characteristic make the calculation complex. In the end regions, a three-dimensional approach is almost essential, although various schemes have been devised in which simplifications can be made. In addition to the difficulties already noted, the thick conducting plates in which non-linear eddy currents 545

The generator

Chapter 6

STATOR VOLTAGE ADDITIONAL ROTOR CURRENT REQUIRED FOR POTIER REACTANCE DROP mi

I

v

ROTOR CURRENT

FtG. 6.88 Potier construction for on-load excitation current

are induced, and other conducting components,. must be included in the modelling. It has reached the stage of refinement where detailed changes, say, in the thickness of magnetic screens, can be modelled in order to optimise the design, and to indicate where potential hot spots may occur due to unwanted flux concentrations.

9.2

II

The reactance of an inductive circuit determines its voltage/current relationship. In a generator, different reactances are identified in order to model or describe voltage (or flux)/ current relationships under different circumstances. The synchronous reactance, Xct, relates the armature reaction MMF (proportional to stator current) to the MMF needed for rated flux in the air gap. For a given design of machine, increasing the radial length of the air gap proportionately reduces Xct and improves steady state stability. This results in a larger outside diameter, and a higher rotor current at full load. The stator leakage reactance, Xf, is not a specified quantity, and its value is a matter of economic design. The transient and sub-transient reactances, Xct' and Xct", are specified. They describe the flux/current relationships during transient changes, and under these circumstances, ·the amount of flux encircling the stator slots, rotor slots and end windings are of impor-

II. .1'

II

l II

Control and calculation of reactances

I

546

tance. If higher values are required than the 'natural' design produces, the leakage reluctance can be reduced by making the slots narrower, and/or sinking them deeper into the core. Again, this is extravagant and results in a larger design. If lower values are required, it is not usually sufficient to manipulate the slot geometry, and a more basic change to the design might be needed. Using computer programs similar to those mentioned in the previous section, more accurate representation of the reactances can be made, over the range of load conditions, than is possible by simple calculation.

9.3

·~

! ~~

The cause and effect of harmonics

As explained earlier, stator winding distribution is designed to minimise the generation of harmonic voltages and currents. The stator winding is invariably star connec.ted, so that triple harmonics cannot occur in the line voltage or current. Since one pole of the rotor is identical with the other, it cannot produce second-order flux harmonics, which would make the two halves of the flux wave dissimilar. The only harmonics of significant magnitude which will appear are those of order S, 7, 11, 13, etc., with diminishing amplitudes, and those near to the rotor slot pitch, e.g., 41 and 43 for a 42-pitch rotor slotting. The no-load rated voltage wave must not contain a greater total harmonic

] l

i

Electrical and electromagnetic aspects content than that specified in RS5000, in which certain ranges of frequency are more highly weighted than others because of their effect (in the transmission system) on communications lines. This now rather outdated concept is still accepted as an agreed and useful criterion, since high harmonic levels can induce high local losses in parts of t~e generator, e.g., the rotor surface. In practice, harmonics are generated by the connected loads, a recent trend being the even-order harmonic requirements of equipment using thyristors. This must be supplied by the generators and must therefore appear in the flux wave, causing rotor surface losses similar to those produced by unbalanced load conditions. Rotor windings occasionally develop short-circuits between adjacent turns in a coil, and while this is not usually of great concern, the difference in flux pattern from the two poles is detectable, using a small flux coil mounted in the airgap. When the signal from one pole is offset against the signal from the other, differences reveal any abnormality. Another method which has been suggested uses the presence of second harmonics in the stator current, as noted above, but this has to be able to reject those imposed by the load requirements.

9.4

Magnetic pull

If the rotor is exactly centred in the bore of the stator, the magnetic pull between one pole of the rotor and the stator will be exactly balanced by that of the other. If not centred, there will be an unbalanced pull acting as an attractive force on the pole with " the smaller air gap. However, the air gap of these large machines is so large (80 to 130 mm), in order t to achieve the required synchronous reactance, that J centring the rotor to a readily achievable accuracy does not impose a magnetic pull at all comparable 1 with the gravitational force on the rotor. Similarly, the net axial magnetic force on the rotor is Lcro if it is axially centred in the stator, and this is the condition normally achieved at rated load with l the rotors at their normal temperatures. With the i usual axial offset which occurs with the rotors cold, the axial magnetic pull is only of the order of a few t thousand Newtons and is not a significant additional f load on the thrust bearing.

source impedance, and can circulate significant current through bearings, seals, etc., causing eventual break-up of white-metalled surfaces. Voltages of the same frequency as the shaft-driven excitation machines can be measured on the generator shaft, but these are capacitively coupled, have a high sour.ce impedance and will not sustain a large current. The steam turbine rotors may develop a voltage due to the electrostatic action of steam and water droplets on the blades, and one function of the shaft earthing brushes is to ensure that this is discharged. A phenomenon which has occurred (rarely) on turbines is that, where a rotor or rotors have a degree of permanent magnetism and there are contacts of low resistance between shaft and earth at suitable axiallyseparated locations, the small generated voltage can circulate a small current through the turbine casing, which, in certain designs, can act as a partial 'turn' of a winding encircling the shaft. This then produces an MMF and therefore a higher shaft voltage, the whole process building up until many thousands of amperes circulate, causing damage at the contacts. It is therefore important to ensure that deliberate contacts, such as earthing brushes, have a resistance (say, 1 ohm) deliberately included in series, and that heavily magnetised shafts are de-magnetised (see Fig 6.24). The residual magnetism of a generator rotor will normally produce a voltage of several hundred volts at speed, even without external excitation; and access to terminals, connections, etc., must not be allowed.

1

i

f

1

9.5

Shaft voltage and residual magnetism

The production of a voltage (predominantly at 50 Hz) , from one end of the generator rotor to the other oc-

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·= . ~ .::> .... ,!"!e ,,ator. or some d1fference m mag;·ro;:-~~\\\)11~ L>'i 'Ct:>1'e \I) core bars, or damaged core bar insulation, where • this is fitted. Some core back burning, and some fretting products (e.g., 'cocoa dust') seems to be innocuous. The core frame can be inspected for ob\ious signs of damage, and patches of overheated paint or metal should be investigated. E\ery stator slot wedge should be checked for tightn-:'~~ along its whole length, using a tightness tester de1 eloped in 1985, or by tapping with a coin or simil~n l>biect to observe the expected 'ringing', indicative ,,r .1 tight slot. Airgap flux coils can be fitted or :en ell eel at this stage. C.,r,nor end windings can be more thoroughly checked ~han a-; described in the previous section. Signs of lc'O.,cncss of packings, fretting, slack fastenings, etc., a: c all indicative of movement. If there are unfilled bags between coils, these can be filled with epoxy resin at this stage. The surfaces should be cleaned using a proprietary cleaner suitable for electrical windings, but it is not recommended that repainting is under'1.11!' taken without the manufacturer's advice. If a 'worm'~1 \ lwlt:' (made by small conducting particles) is found, the particle should be removed and the insulation patched rather than left in, possibly to break through 'lllj'·i: intL' the copper. ~~Iii The state of the hoses and their connecting joints should be checked. A leakage test on the stator windin.;:. using vacuum or pressurised air with a tracer gao. 1\ ill reveal any significant leaks. It may be con.,idered prudent to renew all the rubber 0-rings, bl'th :n these locations and elsewhere, if they have b~cn in sen ice for several years. Care must be taken lL' · ,, ilo11 the assembly instructions meticulously, as 01 ercJghtening may damage the joints. The opportunity should be taken to clean the stator casing. particularly at the bottom, noting if water ha~ collected, and checking that the flow to the leaka£e detector is unobstructed. lnsulat1on res1stance tests should be carried out on the rotor winding, using a 500 or 1000 V megger, and on excitation windings. An RSO test could also

l 1

be performed on the rotor winding, with slipring brushes lifted. If any hot spots in the stator core are suspected, or as a reassurance exercise, a core flux test can be carried out. This may take the form of an hour long test with about rated flux in the core, using about 10 turns of 11 kV cable wrapped around the core and fed from a suitable 11 kV source, and using an infra-red camera to scan the bore to monitor its temperature. Easier, but less positive, is a low flux test using one turn of light current cable and a magnetic imperfection detector. It is not easy to ensure that the stator winding is dry enough to make an insulation resistance test meaningful, though a technique of applying a vacuum to the winding has been used. A 2 or 2.5 kV motorised megger should be used, monitoring one phase at a l1me, ana mamtam'mg fhe test tor l 0 minutes so that the polarisation index can be obtained. It is not normal to apply a high voltage test, the only exception being after some damage has occurred, possibly with partial replacement of the winding, when an agreed HV test on the remaining bars gives some reassurance.

11.5

Reassembly

With the rotor reassembled, mechani~·a! checks such as alignment, axial clearances and concentricity of couplings, and of the rotor in the stator, arc carried out, and that all locking plates and other cle\ ices are properly assembled. All jointing materials, 'uch as gaskets, 0-rings, jointing compound, etc., should be renewed, and the appropriate leakage tests 'carried out. It is so important that small metallic items do not fall into, or get left inside, the generator, where they could be drawn into the windings, that a strict accounting system for such items is recommended. Several expensive failures have occurred a short time after a major maintenance outage, due to this cause.

11.6

Diagnosis

If the reading of any instrument has been outside its

expected limit, or caused concern in other ways, it is sensible to investigate its possible causes during an outage. It may be tempting to extend the operating regime beyond its normal level, before such an outage, in order to observe the effects, but this is not recommended, since a 'stable' fault hch been known to become 'unstable' during such operation. causing problems when the unit is recommissioned. Specialised techniques, some in their de1 elopment phase, may be a\ ailable to assist in ,uspected fault location, and up to date advice should be sought. Sometimes readings of more than one type may be high, though not so high as to be alarming in 557

•-

1

!

Maintenance, testing and diagnosis

' amination is made. Defects greater than 2 mm should - : be ground out, blending in the .ground area so that ' there are no discontinuities. Finally, the ring must · be re-treated with its protective finish before being i refitted. The whole operation requires great skill and \ experience, and though it can be carried out at site, . it is better done at the manufacturer's works, fol' lowing which the rotor can be subjected to overspeed !: and balancing runs. These comments also apply to 1 exciter end rings. · '· Examination of the stator core can be carried out ' . by inspecting the bore for loose areas, which can t be tightened by insertions of hard insulation, or by treating with an epoxy-based liquid having low surface tension which will penetrate between the laminations. Ventilation ducts should be inspected for :·debris, blockages and broken spacer bars. The back :, of the core will reveal excessive welding of core to core bars, or damaged core bar insulation, where this is fitted. Some core back burning, and some ·fretting products (e.g., 'cocoa dust') seems to be :tinnocuous. The core frame can be inspected for obvious signs of damage, and patches of overheated paint or metal should be investigated. Every stator slot wedge should be checked for tightness along its whole length, using a tightness tester developed in 1985, or by tapping with a coin or similar object to observe the expected 'ringing', indicative of a tight slot. Airgap flux coils can be fitted or . renewed at this stage. Stator end windings can be more thoroughly checked than as described in the previous section. Signs of . .looseness of packings, fretting, slack fastenings, etc., .are all indicative of movement. If there are unfilled :bags between coils, these can be filled with epoxy resin at this stage. The surfaces should be cleaned using a proprietary cleaner suitable for electrical windings, but it is not recommended that repainting is under·. taken without the manufacturer's advice. If a 'wormhole' (made by small conducting particles) is found, the particle should be removed and the insulation patched rather than left iu, possibly to break through ;'into the copper. ;, The state of the hoses and their connecting joints .'·should be checked. A leakage test on the stator wind, 1ng, using vacuum or pressurised air with a tracer . gas, will reveal any significant leaks. It may be :::considered prudent to renew all the rubber 0-rings, ~.both in these locations and elsewhere, if they have .~peen in service for several years. Care must be taken ji'tb follow the assembly instructions meticulously, as yovertightening may damage the joints. :~} The opportunity should be taken to clean the stator casing, particularly at the bottom, noting if water :'has collected, and checking that the flow to the leak:age detector is unobstructed. Insulation resistance tests should be carried out 'on the rotor winding, using a 500 or 1000 V megger, ,and on excitation windings. An RSO test could also

be performed on the rotor winding, with slipring brushes lifted. If any hot spots in the stator core are suspected, or as a reassurance exercise, a core flux test can be carried out. This may take the form of an hour long test with about rated flux in the core, using about 10 turns of II kV cable wrapped around the core and fed from a suitable II kV source, and using an infra-red camera to scan the bore to monitor its temperature. Easier, but less positive, is a low flux test using one turn of light current cable and a magnetic imperfection detector. It is not easy to ensure that the stator winding is dry enough to make an insulation resistance test meaningful, though a technique of applying a vacuum to. the winding has been used. A 2 or 2.5 kV motorised megger should be used, monitoring one phase at a time, and maintaining the test for I 0 minutes so that the polarisation index can be obtained. It is not normal to apply a high voltage test, the only exception being after some damage has occurred, possibly with partial replacement of the winding, when an agreed HV test on the remaining bars gives some reassurance.

11.5

Reassembly

With the rotor reassembled, mechanical checks such as alignment, axial clearances and concentricity of couplings, and of the rotor in the stator, are carried out, and that all locking plates and other devices are properly assembled. All jointing materials, such as gaskets, 0-rings, jointing compound, etc., should be renewed, and the appropriate lekkage tests 'carried out. It is so important that small metallic items do not fall into, or get left inside, the generator, where they could be drawn into the windings, that a strict accounting system for such items is recommended. Several expensive failures have occurred a short time after a major maintenance outage, due to this cause.

11 .6

Diagnosis

If the reading of any instrument has been outside its

expected limit, or caused concern in other ways, it is sensible to investigate its possible causes during an outage. It may be tempting to extend the operating regime beyond its normal level, before such an outage, in order to observe the effects, but this is not recommended, since a 'stable' fault has been known to become 'unstable' during such operation, causing problems when the unit is recommissioned. Specialised techniques, some in their development phase, may be available to assist in suspected fault location, and up to date advice should be sought. Sometimes readings of more than one type may be high, though not so high as to be alarming in

-

The generator

--

themselves. Wheh judged jointly, clues may be obtained which individual' ~eadings might not have revealed.

-

12

12.1

Future developments

Extension of present designs

The choice of 3000 or 1500 r/min for future turbinegenerators is made almost entirely from considerations of the steam turbine and its steam cycle. ln general, if a two-pole generator can be designed and manufactured at a particular rating, then so can a fourpole generator, its overall dimensions will be a little larger. The present UK designs with water cooled stator windings and hydrogen cooled stator core and rotor can be extended to .at least 1300 MW by extrapolation. Increases of the order of lOOJo to the rotor and casing diameters, electrical loading (ampere conductors per metre of circumference), magnetic densities and voltage, and perhaps 25% on length over the present designs, would be envisaged (see Fig 6.3). The increased diameter and length of the rotor result in the critical speeds and alternating bending stesses being similar to those of the present machines. A judgement would have to be made about the number of parallel paths in the stator winding of a two-pole machine. If only two paths are used, the number of slots and bars is low, but the bar forces become very large; if four are used the circuits cannot be exactly balanced, and circulating currents and losses are generated. Parameters, such as reactances and efficiencies, would not be very different from those of the present machines.

12.2

Extension of water cooling

Since water cooling has been used so effectively for the stator winding, it may be wondered why it is not used in the rotor winding where space is at such a premium. Water cooled rotor windings have been successfully operated; in the UK in a 500 MW unit with an experimental rotor for a few months, and internationally in a few units commercially. The more intensive cooling provided by water means that smaller winding copper sections can be used, but this increases the resistance and therefore the l 2 R loss. In a hydrogen cooled 660 MW rotor, this loss is about 2.5 MW at rated load, so a worthwhile reduction in section brings an expensive loss penalty. There are difficult problems to be solved in feeding the water into and out of the rotating rotor, but the major concern is that the centrifugal force imposes very high pressures (20 MPa) in the water circuit, 558

Chapter 6 which the plumbing and insulated connections have to withstand with no detectable leakage. Stainless steel pipes, with some welds having to be made in situ, were found to be necessary in the UK experience. Nevertheless, water cooling the rotor winding and other parts, for example, the stator core, may be an answer if unit ratings much above 1300 MW are envisaged. One difficulty, that of aqueous stress corrosion of rotor end rings, has been removed with the advent of 18/18 rings. A major advantage is that in an all-water-cooled generator, hydrogen is no longer necessary, and the casing can be of much lighter construction. The rotor can operate in a partial vacuum to reduce windage losses.

12.3

Slotless generators

The very large radial dimension of the air gap in the 660 MW design appears to be a waste of space, and prompted much activity in the 1970s into the design of generators with slotless stators and even slotless rotors. In a slotless stator, winding conductors occupied a radial dimension of about half the stator slot depth, and since there were no teeth, could occupy twice the circumferential distance. This is economical on outer core diameter, and because the conductor bars are not embedded in iron slots, a more economical design of insulation should be possible. The idea has not been pursued, largely because it was overtaken by the superconducting generator concept, which promised greater economies of size, better efficiency and the prospect of much larger unit ratings than any other design.

12.4

Superconducting generators

The phenomenon of superconductivity can be applied to DC circuits, but cannot sensibly be used with the rapidly changing fluxes and currents involved with 50 Hz (see Fig 6.95). It is therefore used only in the rotor windings, where it has two advantages: • The rotor I2 R loss is reduced to zero. • The rotor current and MMF can be very large, so that higher levels of flux density can be used than are permitted by iron saturation. The need to maintain the rotor winding at a temperature of 10 K means that only that amount of heat which can be removed by the refrigerant can be allowed to pass into the rotor, so that elaborate heat shields are necessary. Liquid helium is used as the refrigerant, the windings being made of a niobium-tin alloy embedded in a copper matrix. The rotor body is made from a stainless steel forging.

-

Other types of generator

-

LAMINATED IRON CORE CONCRETE STATOR

-

STATOR WINDING DRIVE END

NON-DRIVE END.

OUTER ROTOR INNER ROTOR WITH SUPERCONDUCTING WINDING

TAIL BEARING

FIG. 6.95 Prototype superconducting 500 MW generator

At the higher flux densities envisaged, an iron core offers no advantages and the disadvantage of the magnetic core loss, so a cast 'concrete' core is envisaged. Some form of outer environmental screen around the core is necessary to prevent leakage flux from inducing currents in support steelwork, etc., this can take the form of an annular magnetic or conductin,g copper scr.een. Many problems remain to be solved, and development is ongoing in seyeral countries. If the technique reaches the stage where reliability is as good as for conventional machines, it offers the possibility of up to 5000 MW in one generating unit, a prospect not available through any other known technology.

chines from an established design achieve a settled reliability of better than 990Jo, and operate at an efficiency of better than 98.5%. Those breakdowns which do occur are generally due to lapses in quality control, or if in old machines, to practices long since overtaken. Thus the impetus for embracing new materials and technologies is not great.

13

Generators, other than the 500 and 660 MW turbinegenerators and direct coupled AC exciters for turbine-generators, described in the previous sections, m operation by the CEGB include:

e 12.5

Other types of generator

Turbine-generators of lower rating.

Auxiliary systems

The most likely other areas for new developments are those of instrumentation, control and diagnosis. New techniques are continually being investigated for instrumentation, and in the environment of a generator, the means of communicating the signal nonelectrically in order to avoid the pick-up of spurious electromagnetic signals and noise are very well worth pursuing. Here, fibre optics are expected to be prominent. Also, the use f microprocessors to relate one parameter to others, as previously noted, will become more common. Perhaps automatic diagnostic techniques will reach a stage where they can be used with confidence, and selective recording of non-standard signals will be introduced more widely. It should be recognised that generator design and manufacturing techniques are old-established. Ma-

'

• Water turbine driven salient-pole synchronous generators. • Diesel engine driven salient-pole generators. • Induction generators. A very brief survey of these groups follows.

13.1

Turbine-type generators of lower rating

Virtually all the steam turbine driven turbine-generators now in operation are hydrogen cooled. At the lower end of the range, machines of 60 MW have a rated pressure of 0.1 bar, i.e., just above atmospheric. Above 200 MW, water cooled stator windings are used, 559

The generator though there are some units in which higher pressure hydrogen is blown through the hollow conductors of the stator winding. ' In other respects, the generators are very similar to the larger, more modern units, except that they are less intensively rated. In some cases, a degree of refurbishment has been carried out to extend their operating lives beyond the 25 years or so already achieved. There are also a number of gas turbine driven generators intended for peak load and synchronous compensation duty. These have ratings up to 70 MW, and are usually air cooled. The single-piece stators are of lighter construction than is necessary in hydrogen cooled units, and the auxiliary systems are minimal. In some cases they were designed for unmanned stations, so manual monitoring equipment and sophisticated logging is minimal. Brushless excitation is universal, for reasons of minimum maintenance, and even the fuses protecting the excitation diodes have been omitted. A noteworthy feature of the most recent of these units is the facility to disengage the prime mover, or, in the case of the Quad-Olympus units (Fig 6.96) in which the generator is driven at both ends, both prime movers. Then, after a period of peak load generation, the synchronous clutches are disengaged, leaving the generators operating as synchronous compensators, with excitation controlled to suit the requirements of the system. When peak load or emergency generation is next required, the gas turbines are runup to speed and the clutches moved into engagement.

13.2 Water turbine driven salient-pole synchronous generators There are only a few of these on the CEGB system, but the most recent, the pumped-storage units at Oinorwig, rate a brief description to complement the water turbine section in Chapter 5. The six generators are each rated at 330 MW, 0.95 power factor, 18 kV, 500 r/min, and have a motor rating slightly lower when operating in the reverse direction. The very onerous requirements included: • Full speed, no-load to full-load, in I 0 s. • From rest to full-load in 100 s. • From full-load pumping to full-load generating in 90 s. • 5000 stop/start cycles per year.

• Multiple load cycling from 5007o to 100% for system frequency regulation. • Availability of 9807o and mode change reliability of 99%. 560

Chapter 6 The comparatively low speed meant large diameters, and 011-~itt:: assembly of the stators was essential (see Fig 6.97). Air cooling was adopted, mainly for reasons of reliability. Partly on account of this, the stator winding bars were unusually deep, with a large number of subconductors, necessitating a 540° Roebel transposition. The core was stacked in situ, being compressed with hydraulic jacks at intervals, and bonded together for mechanical stability. A fabricated steel spider surrounds the forged steel shaft and carries the keyed-on laminated rim and poles. Great care was taken to ensure the integrity of the welds, which are subject to an unusual amount of cyclic stressing. Ventilation is provided by motor-driven fans blowing cooled air onto the stator end windings top and bottom, with some booster fans for the centre of the core. Water cooled heat exchangers are mounted at the outside diameter of the core. The thrust bearing has an arduous duty, having a load of 510 tonnes and requiring larger thrust pads, at the specific loading, than had previously been used at the specific loading and speed. Each pad rests on a 'mattress' of coiled springs, and is arranged to pivot centrally to allow for rotation in both directions. Lubrication is by oil bath and natural oil circulation, with an immersed water cooled heat exchanger.

13.2.1 Excitation and control Two variable-frequency starting equipments are provided for the station, each rated at 14.8 MV A, consisting of air cooled thyristor rectifier/ AC connector/ inverter banks. On starting as a pump, the stator winding is fed with low frequency AC from the starter, using forced commutation at speeds below I 0% and natural commutation thereafter. It is run to just above 500 r/min and is synchronised as it runs down through synchronous speed. There are also arrangements for starting one unit as a pump from another, being driven up to speed by its turbine. Excitation power is taken from the generator terminals, through a transformer to a thyristor bridge, whose output is controlled by the A VR, and then to the sliprings which are located at the top end of the rotor shaft. The synchronous operation of such machines follows very closely that of steam-driven turbine-generators. The electromagnetic loading is considerably less, leading to a smaller radial air gap. The very different magnetic path presented by a pole centre line and an inter-pole gap results in marked differences in direct axis and quadrature axis synchronous reactances, compared to a turbine-generator in which they are almost identical; this is the 'saliency' effect. By applying excitation in the reverse direction to normal, an increase in the steady state stability can

• •

AUTOMATIC DRY ROLL TYPE AIR INTAKE FILTERS

GAS GENERATOR AIR INTAKE FILTER HOUSE

BYPASS DOORS A C GENERATOR AIR INTAKE FILTER HOUSE

AIR INTAKE SPLITIERS

GAS GENERATOR ACOUSTIC CELL TURBINE AND GENERATOR LUB. OIL PACKAGE POWER TURBINE ACOUSTIC SCREEN

0

~

A C GENERATOR POWER TURBINE EXHAUST DUCTING

MAIN GENERATOR CONNECTIONS

::r

(1)

....

-<

'0 BRUSHLESS EXC ,TEA

OLYMPUS GAS GENERATOR GAS GENERATOR INSTRUME PANEL

LUTCH AND BEARING ASSEMBLY POWER TURBINE ASSEMBLY

GAS GENERATOR LUB. OIL FUEL VALVE CABINET Ul (J)

c.v

CORNER BEND

U1

m

FIG. 6.96 Quad-Olympus generator

(1)

(J)

0._.., (Q (1)

::J (1) ....

Ill

~

0 ....

The generator

Chapter 6

,, r ·'

FIG. 6.97 Dinorwig motor-generator during site winding (see also colour photograph between pp 482 and 483)

be gained, i.e., operation further into the leading reactive regime becomes possible.

13.2.2

Other features

Other features peculiar to these machines include the continuing integrity of stator bar insulation in an air environment, the continuing stability of the bonded stator core and the built-up rotors, the vacuum extraction of dust from the shaft brakes, and the very high overspeeds possible; e.g., a transient value of 1.5 for Dinorwig.

13.3 Diesel engine driven salient-pole generators These machines, with ratings of a few MW, are in562

stalled in a few stations for emergency duty. The generators are standard industrial units with proven high reliability. The need for sudden run-up after long periods at standstill means that brushless excitation and casing heaters are essential.

13.4

Induction generators

These machines, rated usually at less than I MW, are used in remotely controlled run-of-the-river hydro plants, and in wind generators on an experimental basis. Such machines do not operate synchronously, but have a characteristic similar to induction motors except that they run at above synchronous speed. A greater input from the prime mover increases the power output. Like all induction machines, they draw their magnetising current from the system and therefore do not require an excitation supply.