Recommended Practice for Monitoring and Instrumentation of Turbine Generators

8/12/2019 Recommended Practice for Monitoring and Instrumentation of Turbine Generators

1/27

IEEEStd 1129-1992

IEEE Recommended Practice for Monitoringand Instrumentation of Turbine Generators

Sponsor

Electric Machinery Committeeof theIEEE Power Engineering Society

Approved J une 18, 1992

IEEE Standards Board

Abstract:

A basic philosophy a nd guidelines ar e established for the design a nd implementa-

tion of monitoring systems for cylindrical-rotor, synchronous turbine generators. Monitoringsystems a re used to display th e stat us of the genera tor and a uxiliary sys tems wh ile these sys-

tems are operating on line. The basic information needed to choose monitoring schemes best

suited for each application is provided. This standard does not specify actual equipment or

instrumenta tion, but it does indicate some crit ical a reas w here it is importa nt t o provide mon-

itoring capability.

Keywords:

cylindrical-rotor, synchronous turbine generat ors; tur bine genera tors

The Institute of Electrical and Electronics Engineers, Inc.345 Ea st 47th S treet, N ew York, NY 10017-2394, US A

Copyright 1992 by theInst itut e of Electrical and Electronics E ngineers, Inc.

All rights reserved. P ublished 1992Pr inted in the U nited Sta tes of America

IS B N 1-55937-233-8

No part of thi s publication may be reproduced i n any form ,

in an electroni c retr ieval system or otherwi se,

wi thout th e pri or wr itt en perm ission of the publi sher.

Authorized licensed use limited to: University of Witwatersrand. Downloaded on October 01,2013 at 20:41:28 UTC from IEEE Xplore. Restrictions apply.


2/27

IEEE Standards

document s a re developed wit hin t he Technica l Com-

mittees of the IEEE Societies and the Standards Coordinating Commit-

tees of the IEEE Standards Board. Members of the committees serve

volunta rily a nd w ithout compensat ion. They a re not necessa rily members

of the Institute. The standards developed within IEEE represent a con-

sensus of the broad expertise on the subject w ithin th e Institut e as w ell as

those activit ies outside of IEE E t ha t ha ve expressed an int erest in pa rtic-

ipating in t he development of the stan dar d.

Us e of an I EE E S ta nda rd is wholly volunta ry. The existence of an I EE E

Standard does not imply that there are no other ways to produce, test ,

measure, purchase, market, or provide other goods and services related to

the scope of the IE EE St an dar d. Furthermore, the viewpoint expressed at

the t ime a sta nda rd is a pproved a nd issued is subject t o cha nge brought

about through developments in the state of the art and comments

received from users of the sta nda rd. Every IEE E S ta nda rd is subjected to

review at least every fi ve years for revision or reaffi rma tion. When a docu-

ment is more tha n fi ve year s old an d ha s not been reaffi rmed, it is reason-

able to conclude that its contents, although still of some value, do not

wholly reflect the present sta te of the art . Users a re cautioned to check to

determine that t hey have the la t est edit ion of any I EE E S tanda rd.Comments for revision of IEE E S ta nda rds a re welcome from a ny inter-

ested party, regardless of membership affi liat ion with IE EE . Suggestions

for changes in documents should be in the form of a proposed change of

text , together w ith appropriat e supporting comments.

Int erpreta tions: Occasiona lly questions ma y a rise regard ing the mean-

ing of portions of standards as they relate to specific applications. When

the need for interpretat ions is brought to the a ttent ion of IEE E, the In sti-

tute will init iate action to prepare appropriate responses. Since IEEE

Standards represent a consensus of all concerned interests, it is impor-

tant to ensure that any interpretation has also received the concurrence

of a balance of interests. For this reason IEEE and the members of its

technical committees are not able to provide an instant response to inter-

pretation requests except in th ose cases w here the ma tt er ha s previously

received forma l considera tion.

Comments on standards and requests for interpretations should be

addr essed to:

Secretary, IEE E St anda rds Board

445 Hoes Lan e

P.O. B ox 1331

P iscat a wa y, NJ 08855-1331

U S A

IEEE Standards documents are adopted by the Institute of Electrical

an d Electronics Engineers wit hout regard to wheth er their adoption ma y

involve patents on articles, materials, or processes. Such adoption does

not assume any liability to any patent owner, nor does it assume any

obligation wha tever to part ies adopting the sta nda rds documents.



3/27

Foreword

(This foreword is not a part of IEE E Std 1129-1992, IEE E Recommended P ract ice for Monitoring an d Instr umenta-tion of Turbine G enera tors.)

This document is intended to establish a basic philosophy and guidelines for the design and

implementa tion of monitoring systems for large turbine generators. Monitoring systems a re

used to display the sta tus of the generat or and a uxiliary sy stems w hile on line. This docu-ment d oes not include aut omat ic protective devices or relay s.

At the t ime this standard was completed, the Working Group on Monitoring and Instru-

mentation of Turbine Generators had the following membership:

Ronald J . Corkins,

Chair

Robert F. G ra y J . V. P ospisil I . TrebincevicM. Lew is H . C. S a nderson S. D. U ma nsP. I . Nippes J . S piegl J . J . Wilkes

J. Timperly

The following persons were on the balloting committee that approved this standard for sub-

mission to the IEE E S tanda rds Board:

V. Aa re P. S. J ohrdo M. P ilot eJ . A. Ara dilla s G . Ka rolyi J . V. P ospisilM. B a la nson G . K . M. Kha n D. G . Ra meyF. C. B rockhurst J . L . Kir t ley, J r. S . Ra oG . W. B uckley S. B . Kuznet sov S. J . S a lonM. V. K. C ha r i D. La mbrecht H . C. S a ndersonR. J . Corkins P. R . H . La ndrieu M. S. S a rmaP. L . D a ndeno C. W. La w rence J . S pieglN. A. O. D emorda sh M. Lew is J . S t einJ . S . E dmonds T. A. Lipo J . F. S za blyaA. M. E l-S cra fi F. A. Lot t e J . TimperlyE . W. Fuchs J . A. Ma llick I . TrebincevicN. K . G ha i D. McLa ren S. D. U ma nsG . L. G odw in J . R. Micha lec P. D. Wa gnerB . E . B . G ot t S. H . Minnich T. R. Wa itR. F. G ra y T. W. Nehl D. L. Wa lkerD. R. G reen G . J . Neidhoefer P. A. Weya ntT. J . H a mmons N. E . Nilsson J . C. Whit eM. H . H esse P. I . Nippes E . C. Whit neyH . H . H w a ng D. W. Novotny J . J . Wilkes

J . A. Oliver

When the IEE E S ta nda rds B oard a pproved this sta nda rd on J une 18, 1992, it ha d the fol-

lowing membership:

Marco W. Migliaro

, Chair

Donald C. Loughry

, Vice Chai r

Andrew G. Salem

, Secretary

D ennis B odson Dona ld N. H eirma n T. Don Micha el*

P a ul L. B orr ill B en C. J ohnson J ohn L . Ra nkineClyde C a mp Wa lt er J . Ka rplus Wa lla ce S. Rea dD ona ld C. F leckenst ein Ivor N. Knight Rona ld H . ReimerJ a y Forst er* J oseph K oepfi nger* G a ry S. RobinsonD a vid F. Fra nklin I rving Kolodny Ma r t in V. S chneiderRa miro G a rcia D. N. J im Logot het is Terra nce R. Whit t emoreThoma s L . H a nna n La w rence V. McC a ll D ona ld W. Zipse

*Member Em eritus

Also included are the following nonvoting IEE E S ta nda rds B oard liaisons:



4/27

Sa t ish K. Aggarwa lJames Bea l lRichard B. EngelmanDa vid E. SoffrinSta nley Warsha w

Kris t in M. Dit tmann

IE EE Standards Project Editor



5/27

Contents

S E C TION P AG E

1. Scope a nd References .............................................................................................................. 1

1.1 Scope ................................................................................................................................ 1

1.2 References........................................................................................................................ 1

2. Definit ions ............................................................................................................................... 2

3. St at or Fra me an d Core .. . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . 2

3.1 Fra me............................................................................................................................... 2

3.2 Core .................................................................................................................................. 3

4. St a tor Winding ........................................................................................................................ 4

4.1 Electrica l Qua nt ities ....................................................................................................... 5

4.2 St a tor Winding Condit ions ............................................................................................. 6

4.3 B a r En d Section .............................................................................................................. 7

4.4 P ha se Connections .......................................................................................................... 84.5 Termina l B ushing s .......................................................................................................... 8

4.6 Flexible Lea ds ................................................................................................................. 8

5. Rotor ........................................................................................................................................ 8

5.1 Sha ft an d Forging .. . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . 9

6. Rotor Winding ......................................................................................................................... 9

6.1 Electrica l.......................................................................................................................... 9

6.2 Mechanica l..................................................................................................................... 10

7. Miscella neous Components .................................................................................................. 11

7.1 Fa ns ............................................................................................................................... 11

7.2 B earin gs ......................................................................................................................... 117.3 Hy drogen Sea ls.............................................................................................................. 12

7.4 P ermanent Magnet G enerator (P MG ) .. . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. 12

7.5 Collector Rings .............................................................................................................. 12

7.6 Hy drogen Cooler............................................................................................................ 12

8. Auxiliary Externa l Syst ems .. . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. . . .. 12

8.1 Hy drogen Sy stem .......................................................................................................... 13

8.2 Sea l Oil Sy stem ............................................................................................................. 14

8.3 St a tor-Cooling Wa ter Sy stem ....................................................................................... 17

Ind ex............................................................................................................................................. 19



6/27Authorized licensed use limited to: University of Witwatersrand. Downloaded on October 01,2013 at 20:41:28 UTC from IEEE Xplore. Restrictions apply.


7/27

1

IEEE Recommended Practice for Monitoringand Instrumentation of Turbine Generators

1. Scope and References

1.1 Scope.This document is intended to establish a basic philosophy and guidelines for thedesign and implementation of monitoring systems for cylindrical rotor, synchronous turbine

genera tors. Monitoring systems ar e used to display the sta tus of the generat or and a uxiliary

systems while these systems are operating on line. This document does not specify actual

equipment or instrum enta tion, but it does indicate some crit ical ar eas w here it is importa nt

to provide monitoring capa bility.

Generator-protection techniques are not discussed in this document. There is a fine line ofdist inction between instrumenta tion tha t is used for monitoring a nd instrum enta tion used for

protection, and there are ma ny instrument s tha t play a dua l role.

The purpose of monitoring is to provide information to the operator to guide appropriate

action. This a ction ma y be ma intenan ce planning, ma inta ining load , tr ipping the unit , or load

reduction. The key distinction between monitoring and protection is that with monitoring, the

action ta ken (if any ) is not a utomat ic but is init iat ed by th e opera tor. Some users ma y choose

to include some of the items listed here a s pa rt of the genera tor-protection scheme.

Monitoring of basic generator parameters is routinely performed on commercial genera-

tors. It is only recently, however, tha t t he economics of power genera tion ha s creat ed the need,

a nd a dva ncing technology provided the ability, to monitor near ly all a spects of generat or oper-

ation. This should allow the operation of large-capacity machines with increased reliability

an d a vaila bility an d w ith r educed downtime for outa ges. However, car e must be exercised to

avoid overmonitoring. While there is no doubt t ha t gr eat qua ntit ies of dat a may be useful toreview when (and if) time permits, the operator should not be subjected to an overload of

unessential da ta . The use of diagnostic systems ma y facilitat e handling of multitudinous data

to assist the operator.

This document provides the basic information needed to choose the monitoring schemes

tha t are best suited for each a pplicat ion. Not a ll items discussed in t his document a re neces-

sary for all generators. Some users may wish to add addit ional monitoring systems beyond

those presented in th is document . The user should refer to the ma nufa cturers monitoring r ec-

ommendations.

1.2 References. This recommended practice shall be used in conjunction with the followingpublications. When the following standards are superseded by an approved revision, the revi-

sion sha ll apply.

[1] ANSI C 50.13-1977, American Na tiona l St a nda rd R equirement s for Cylin drical-Rotor S yn-

chronous G enerators

1

.

[2] ANSI C50.14-1977, American National Standard Requirements for Combustion Gas Tur-

bine Driven Cylindrical R otor S ynchronous G enera tors.

1

ANSI publicat ions are ava ilable from the S ales Departm ent , American Nat ional Sta ndar ds Inst itut e, 11 West42nd Street, 13th Floor, New York, NY 10036, USA.



8/27

I E E ES t d 1129-1992 IE E E RE COMME ND E D P RAC TIC E FOR MONITORING

2

[3] ANSI C50.15-1989, American Na tiona l St a nda rd R equirement s for H ydrogen-Cooled C om-

bustion Gas-Turbine-Driven Cylindrical-Rotor Synchronous Generators.

[4] IEEE Std 67-1990, IEEE Guide for Operation and Maintenance of Turbine Generators

(ANSI).

2

[5] IE EE St d 421.1-1986, IEE E St an dar d D efinit ions for Excitat ion S ystems for Syn chronousMa chines (ANSI ).

[6] IEEE Std 492-1974 (Reaff 1986), IEEE Guide for Operation and Maintenance of Hydro

Generators (ANSI).

[7] IEEE Std C37.101-1985 (Reaff 1990), IEEE Guide for Generator Ground Protection

(ANSI).

[8] IE E E St d C 37.102-1987 (Rea ff 1990), IE EE G uide for AC G enerat or P rotection (ANSI ).

2. Definitions

monitoring.

The process of observing a syst em to verify t ha t its para meters a re within pre-

scribed limits.

particulate. A small part icle tha t is creat ed by th ermal decomposit ion of organic mat erialspresent inside the generator.

protection. The process of observing a sy stem, and aut omatically init ia t ing a n a ction t o mit-igate the consequences of an operating condition that has deviated from the established

acceptable performance criteria.

pyrolysate.

A product of t herma l decomposition.

stator bar.

A unit of winding on the sta tor of a m achine. Also:

bar; st at or coil.

3. Stator Frame and Core

Stator frames of turbine generators have been very reliable in making minimal contribu-

tions to forced outage rates. Some early-on problems with vibration and alignment have

occurred, usually in conjunction with foundation, soleplate, and grouting deficiencies. The

monitoring emphasis has been very low for this component of the generator. However, con-

cerns for internal components occasionally arise with respect to core overheating, local over-

heating due to short circuits between adjacent core laminations, and excessive tooth

vibration.

3.1 Frame

3.1.1 Presence of Liquids or Moisture.

The presence of liquid in the generator may be

evidence of a cooler leak, leak of a water-cooled stator winding component, or seal oil entry.

Moisture-laden hydrogen gas supply could also be a source of the water, as well as simply the

2

IEE E publicat ions are a vailable from the I nst itute of E lectrical a nd E lectronics Engineers, Service Center, 445Hoes La ne, P. O. B ox 1331, Piscat aw ay, NJ 08855-1331, US A.



9/27

I E E EAND INS TRU ME NTATION OF TU RB INE G E NE RATORS S td 1129-1992

3

operat ion of a u nit w ith a low cold ga s tempera tur e, wh ich may ca use condensa tion. Air-cooled

units that use ambient air may be susceptible to condensation under some conditions. The

biggest r isk ma y be when the unit is in cold shutdown, and wa rm moist a ir enters the sta tor.

Moisture entering the lubricat ing oil system from the turbine gland steam seals ma y a lso

be released into the generator from the hydrogen seal oil system.

Accumulation of liquids can seriously jeopardize the proper operation of the generator.

Leaking liquids can quickly fi ll the generator t erminal box, causing pha se or ground fa ults, orblock th e fl ow of cooling gas.

Generators are provided with several drain lines located at low spots in the generator

frame and stator core center region. These drain lines may be fitted with liquid level detectors

with alarms, and with sight glasses for visual observations. Drain lines must be arranged so

tha t the liquid reaches the detector.

The level of the units gas-borne moisture content may be monitored by electronic dew-

point or h umidit y monit ors, or by periodic sam pling. The dew point of th e inlet a nd outlet g a s

of the gas d ryer ma y be monitored in order to determine dryer effi ciency.

3.1.2 Frame Vibration.

Fra me vibra tion ma y be evidence of poor rotor ba lan ce, rotor mis-

alignment, unequa l rotor hea ting, mat erial loss or fracture of rota ting pa rts, improper bearing

loading or displa cement , poor grouting, or un even fra me foot load ing.

Fra me vibra tion ma y a lso be the result of excessive core vibra tion (see 3.2.7), unequa l oper-

ation of the coolers, or core vibration transferred to the frame. High frame vibrations are

sometimes observed due to fra me resonan t responses to rotor rota tiona l, or core ovalizing fre-

quencies. St ructural dam age t o frames a nd cores ma y occur.

Some ma nufacturers ha ve pedestal-mounted bear ings, so any rotor misalignment may not

affect fra me vibration.

A change in coupling between core and frame may also affect vibration.

Many stator cores are structurally isolated from the frame and foundation by heavy

springs. The adequacy of frame weight distribution on the foundation greatly affects frame

vibration. An indication of poor frame foot loading may be derived from seismic transducers

incorporated with journal-bearing vibration monitors. A means of monitoring this acoustically

is sometimes employed.

The monitoring instrument should be capable of detecting vibration at rated and doublefrequencies.

3.2 Core

3.2.1 General Overheating. A general increase in the core temperature beyond the nor-mal full-load temperature can be caused by an increase in the cold gas temperature, a load

output a bove the norma l full-load point, a reduction in ga s pressure, deteriora tion of hydr ogen

purity, or an increase in gas cooling water temperature. It may also be caused by a flow

restrict ion, such as dam per or cover plate, or a bypass a round the ga s fl ow circuit .

G enera l core overhea ting can be m easu red by t hermocouples (TC) embedded in th e core a t

several locations. Monitoring of the hot gas temperature and gas pressure also provides good

information about the core temperature condition.

3.2.2 Local Overheating. If the generator is being operated in the underexcited, leadingpower factor region of the capability curve, the resulting flux distribution may create higher

losses, particularly in the stator iron at the ends of the core. The higher losses in turn create

higher temperatures. The load should a lway s be limited to points w ithin t he capability curve

to ensure that the core temperature limit is not exceeded. Operation in the overexcited, lag-

ging power-factor region is preferred in minimizing stator core end temperatures. As a

backup, particularly for la rger ma chines, the sta tor core temperatur e ma y be m onitored. Core

temperatures should not be utilized as the sole basis for machine operation.



10/27


4

Low local temperatures on the stator core should not be used to permit operation beyond

the established limits of the generator capability curve. Core heating m ay vary with termina l

volta ge and frequency, and the boundaries of the capability curve are va lid only for the voltage

an d frequency for which the curve is dra wn.

The core temperature can be measured by a group of TCs strategically located around the

sta tor core in a nt icipa ted hot spot regions, and especially a t t he core ends. Core TCs a re some-

times used for init ial factory tests on prototype machines, or they may be installed during

ma nufacture of the sta tor core by special request . The temperature can then be displayed a nd

recorded. If the core temperature exceeds a preset limit , a n a larm ma y be a ctivated.

Severe localized overheating may be detected by a particulate detector. Pyrolysate collec-

tors, with tagging compounds, may provide a means of pinpointing the source of insulation

overheating.

3.2.3 Circulating Currents.

Local overhea ting of th e core could be a r esult of a sh ort cir-

cuit between ad jacent core lam inat ions a llowing axial (longitudina l) fl ow of current between

core lamina tions. Us ually t he insulat ion brea ks down, creat ing a n electrical short circuit due

to mechanica l dam a ge. Core circulat ing currents a re diffi cult to detect. Techniq ues for detect-

ing hot spots in the stator core include electromagnetic sensors, particulate detectors, scan-

ning t hermogra phic, or physical inspection.

3.2.4 End-Tooth Heating. Core end iron can overheat when the magnetic flux enters theends of th e sta tor core during un derexcited (lea ding power-fa ctor) opera tion of the genera tor.

The problem is worse wh en a un it is a lso operat ing a t ra ted MW wh ile underexcited. A par tic-

ula te detector may be used to detect overhea ting. Core-end teeth TCs m ay be used if th ey were

insta lled during m an ufacture of the generator.

3.2.5 Flux-Trap Heating. The flux trap, which acts as a flux shunt or barrier lesseningthe magnetic loading of the core end laminations, and structural elements of the core ends,

can also overheat during underexcited operation. Flux-trap TCs can be used if installed dur-

ing ma nufacture of the unit .

3.2.6 Core Vibration.

The core vibration of the turbine generators results from the

unequal magnetic pull in the air gap. The force is greater in the direct axis than in thequadrature axis. The rotating magnetic pull tends to deform the core, creating a double fre-

quency component of core vibration. Loose cores may respond dramatically to the 120

(100) Hz driving force from t he rotor. Keeping the core tigh t by t orquing t he a xial a nd circum-

ferentia l bolts (when incorpora ted) minimizes core vibrat ion. A loose core ca n result in high er

unit noise or, in extreme cases, th e breaka ge of lamina tions nea r t he bore surfa ce. A loose core

also can force vibration of the end turns and cause winding failure. The detection of a loose

core may be performed by a noise analyzer test and also by periodically measuring frame

vibration, particularly when the core is not spring-mounted.

4. Stator Winding

The sta tor winding is a n importa nt item to be monitored in gauging the hea lth of the gener-

ator. Operating load conditions can normally be correlated to temperature, vibration, and

other par a meters being m onitored to judge th e condition of the ma chine. The win ding ma y be

monitored for temperat ure rise of the copper stra nds a nd of the cooling medium. The ba r ends

may be monitored for radia l, axial, or ta ngential vibra tion at tw ice opera ting frequencies, an d

phase connection rings and termina l bushings may be instrumented for both temperature a nd

vibration.

Correlations normally are made of identical quantit ies at fixed operating conditions, held

constant for an adequate t ime period to establish steady-state conditions. However, some



11/27


5

occurrences rela te to cha nging conditions. In t hese ca ses, a n exact record a nd meth odica l var i-

a tion of conditions ma y be required for a proper diagn osis.

4.1 Electrical Quantities.

The electrical parameters that may be monitored include voltage,

current, frequency, power, rea ctive power, volta ge bala nce, a nd nega tive sequence current.

4.1.1 Stator Voltage.

The sta tor voltage ma y be monitored by potentia l tra nsformers. Dif-

ferent classes of potentia l tra nsformers ma y be employed for metering a nd relaying require-

ments. The stator voltage may be displayed and or recorded on the main control panel.

4.1.2 Stator Current.

P ha se current ma y be monitored and a larm ed.

The stator current may be monitored by current transformers. Different classes of current

transformers may be employed for metering and relaying requirements for each phase. They

are locat ed close to the generat or w inding a t the t erminal bushings. The a na log output from

the current tr an sformers, approximately 5 A at ra ted generator output current, may be dis-

played a nd/or recorded on t he ma in control pan el.

4.1.3 Frequency.

The frequency of the generator output voltage may be monitored from

the sa me potentia l tra nsformers t ha t m easure th e generat or output volta ge. The frequency is

usually displayed and recorded on the main control panel. A low-frequency alarm may be pro-vided. Alar ms a re often set for a period of time in a cert a in frequency ra nge. The restriction is

based on turbine blade da ma ge. Opera tion of the generator a t less tha n ra ted frequency, but

a t ra ted volta ge is also risky (see 4.1.6).

Prolonged or continuous operation at other than rated speed should be avoided. Operation

during subsynchronous resona nce that st imula tes sha ft t orsiona l resona nt frequencies can be

detected by use of a sha ft t orsional vibrat ion monitor. Operat ion below ra ted speed can lead t o

hydrogen seal da ma ge or w iped bearings d ue to higher vibra tions. Turbine blade dam age ma y

result from prolonged operat ion a t off-ra ted frequency (see 4.1.6).

4.1.4 Power Output. The pow er output, both rea ctive power (megava rs) and a ctive power(megawa tt s), may be monitored. The volta ge and current signals m ay be taken from th e same

potential and current transformers described above (see 4.1.1 and 4.1.2). This information

may also be displayed and recorded in the main control room.

4.1.5 Negative Sequence Current.

Negative sequence current is usually a result of

unbalanced load on the generator. Induced current will flow on the rotor surface, on the pole

face region, and in the coil slot wedges and teeth, causing overheating of the rotor body. This

may produce rotor vibrat ion a nd da ma ge if the condition is not cleared.

The negative sequence current is normally monitored to limit possible damage to the

machine due to unbalanced line currents. Whenever a generator is connected to an unbal-

an ced fault , or even an unba lanced load, a negative sequence current creates a fl ux fi eld in th e

genera tor tha t rotat es in th e opposite direction from t he ma in fl ux fi eld. This causes t he rotor

iron a nd slot wedges to heat a t ga ps and at high resistan ce points due to induced double fre-

quency current.

Negative sequence current may be displayed in the control room. Typically, an alarm may

be activated from negative sequence current. For specific acceptable values, refer to ANSI

C50.13-1977 [1]

3

or latest revision thereof. In some units, the mechanical torsional response

(particularly on turbine blades) imposes a greater restriction than to the electrical capability.

This should be checked wit h th e ma nufa cturer.

4.1.6 Volts per Hertz.

Magnetic saturation and result ing overheating of the stator core

ma y be possible if a n increa se occurs in t he volts-per-hertz ra tio due to overvolta ge or under-

3

The num bers in bra ckets correspond to t hose of the references in 1.2.



12/27


6

frequency operation. Limiters and relays are available to minimize exposure to this condition.

This monitoring ma y be part icularly importan t w hen the ma chine is off line.

4.2 Stator Winding Condition

4.2.1 Stator Winding Temperature. Damage caused by overheating of the copper con-

ductors in the stator winding can be extensive. For example, the magnetic forces on thestra nds in a coil are such tha t if there is deteriorat ion in th e bonds between a ny tw o or more

strands such that a strand can vibrate with the magnetic forces, then rapid mechanical wear

an d fat igue occurs. Thus it is importan t t o mainta in the proper temperat ures to maint ain t he

bond strengt h. Overheat ing of the copper ma y be the result of one of th e following:

(1) Higher than normal current densit ies due to overload

(2) Current redist r ibut ion due to broken st ra nds in the conductor

(3) Loss of cool ing

The overcurrent could be caused by a number of different problems. If abnormally high con-

ductor temperat ure is allowed to persist , the winding insulat ion ma y be dam aged. It is impor-

tant to note that these currents may not cause an alarm or tr ip, but they could st ill cause

dam age to the windings.

The temperature of the stator winding is normally monitored by resistance temperature

detectors (RTDs). The RTDs a re loca ted in t he slots in t he st a tor, between t he top an d bottom

ba rs. They a re dist ribut ed circumferent ially, often wit h one RTD in each slot. Sometim es only

six RTDs are insta lled, distributed uniformly a round the circumference, in t he separa te w ind-

ing groups. RTDs are typically located in an area of expected higher temperatures.

All stator-winding RTDs may be constantly monitored. Should the temperature increase

beyond the normal maximum an alarm may be activated, and an immediate investigation to

determin e the cause is necessar y. This ma y result in th e need to reduce generat or loa d.

A word of caution is in order, since large turbine generators are conductor-cooled. This

means that a cooling medium (oil, water, or hydrogen) is passed through the interior of the

sta tor w inding ba r, not over th e exterior. Therefore the hea t fl ow is inw a rd from th e conductor

to the coolant. An RTD embedded on the outside of the insulation, while useful, cannot give

the complete picture, and it may be necessary to monitor the coolant temperature as well.

RTDs do not sense h ot-spot tempera tur es. This is especially tru e for directly cooled ma chines.The main function of RTDs is to detect high stator bar temperatures due to overload condi-

tions, or loss of cooling capa bility ca used by fl ow rest riction or gas-pressure dr op.

4.2.2 Temperature DifferentialHottest to Coldest Bar.

B ar-coolan t temperature dif-

ferential on conductor-cooled machines may be indicative of problems, and should be mini-

mized among both t op bars an d bottom bars, and between top and bottom bars. Refer to the

manufacturers instruction book for guidelines on bar-coolant temperature differential that

should not be exceeded. On most hydr ogen- a nd a ir-cooled genera tors, only ba r group t emper-

at ures an d embedded temperatur e detectors are used.

As a genera l rule w ith w at er-cooled ma chines, sta tor-slot temperature a re not more tha n

10 C above the a verage of all s lot temperatures. Wat er discharge TCs are usually wit hin

5 C for each group; i .e., bottom ba rs, top bars, and bars tha t include pha se ring or lead con-

nections. Refer to the manufacturers recommendations for actual guidelines.

4.2.3 Electrical Discharges. Internal corona discharges can occur at voids aroundstra nds or voids within th e insulat ion.

E xternal discharge a ctivity in the form of corona , partia l discha rges, or slot dischar ges are

sometimes present. These can be dama ging to th e volta ge gradient systems used t o equalize

voltage stress on each stator bar. If not corrected, insulation damage may result . Radio fre-

quency (RF) monitoring ma y wa rn of an increa se in discharge a ctivity. Typical sensors include

air-core current transformers on the neutral or capacitance probes on the windings.



13/27


7

4.3 Bar End Section

4.3.1 Hydrogen Leakage into the Coolant.

On water-cooled units, leaks at the joints,

i.e., nipples, hoses, water box, and manifold can permit the entry of hydrogen into the stator-

cooling water and increase the hydrogen content that must be vented from the water tank.

Hydrogen may enter the coolant circuit through cracked strands that conduct the coolant

through the bars. The fatigue strand cracking, or cavitation damage, can occur due to exces-sive bar vibra tions, either in the slot or in t he end w inding. Excessive hydrogen leakage int o

the sta tor-cooling wa ter system ca n par tia lly or fully block bar wa ter fl ow, and ca n a dversely

affect wa ter conductivity. Hydrogen in-leakage can be detected using either a gas fl owm eter or

by measuring differential pressure on the stator bar water tank to measure the volume of

hydrogen vented from the tank (see 8.1.7). Hydrogen pressure is normally kept above stator-

cooling wa ter pressure.

Leaks at the gas tubes diverting gas to RTD on gas-cooled machines can result in art ifi-

cially low gas temperature readings; otherwise, leaks in the gas-conductor-cooled machines

are not norma lly a problem.

4.3.2 Coolant OvertemperatureGeneral. G eneral increases in the bar outlet gas t em-pera ture or wat er temperat ure indicat e an a bnormal condition such a s a high load condition,

increasing cold gas or cold water temperature, or a loss of cooling capability due to inter-rupted, or restricted fl ow, or drop in ga s pressure.

It is customary t o monitor bar ga s temperature by using RTDs rea ding bar discharge tem-

perature. RTDs reading the inlet hot gas temperature before the coolers may be monitored.

For water-cooled bars, the discharge water TCs mounted on the water hose connections to

the outlet w at er ma nifold an d th e TCs on the inlet wa ter ma nifold ma y be monitored.

4.3.3 Coolant OvertemperatureLocal.

A local indication of coolant over temperature

from a TC mounted at the nipple on the outlet manifold, or RTD at the end of the hose from

the coil, measuring the discharge coolant temperature, could be a result of an overheated bar

due to cracking of strands or shorting due to insulation breakdown or due to coolant flow

blocka ge in th e bar.

4.3.4 OvertemperatureGeneral. High temperatures should always have the causeidentified. Some possibilities include a high load condition, reduced coolant flow, or excessive

coolan t temperature.

4.3.5 OvertemperatureLocal. While it ma y be virtua lly impossible to monitor all localconditions, they can be reasonably monitored by t empera ture sensors and a part iculat e (burnt

insulation) detection inst rument.

4.3.6 Plugged Bars (Coils) or Strands.

One plugged cooling passage in a strand may

have a minimal effect on the bar performance and on its discharge temperature. A partly

plugged bar, possibly due to corrosion from incorrect w a ter chemistr y, could result in a par tia l

loss of coolant fl ow, which ma y result in a large rise in th e local discharge temperature rea d-

ing of a wa ter-cooled un it. Total blockag e of a wa ter-cooled ba r could result in a norma l (or

lower) tempera tur e reading a t t he dischar ge end. This is because the TC w ould be read ing th e

outlet m an ifold bulk wa ter temperat ure, since there would be no wa ter from the ba r itself .

A foreign object or subst a nce in the ga s tu bes of a ga s-cooled unit could result in a high or

low RTD read ing, depending on th e degree of blockage. This is because ga s-cooled un its ha ve

RTDs t ha t norma lly measure discharge gas diverted from the tube ends of the winding.

4.3.7 Strand Fracture. St ra nd fra cture can be a problem due to hydrogen embrit t lementor looseness of the ba r st ructur e, often resulting in r esonance or excessive vibra tion. Extr eme

load swings or load cycling may compound the problem, increasing the rate of failure and



14/27


8

decreasing the time to failure. Strand fracture for gas-cooled bars can be monitored indirectly

by using hot and cold coolant discharge temperature sensors. An algorithm may be employed

to correlate these readings with RF monitor data an d part iculat e detection da ta .

4.3.8 Vibration.

Vibrat ion of end w indings ha s become a gauge of stat or winding health. If

high winding vibrat ion is allowed to persist , the insulat ion system ma y be dama ged by abra -

sion. By charting the magnitude and responsiveness of vibration to load excursions, the pre-

diction of imminent problems is made evident. End-turn vibration can be monitored using

fi ber-optic vibra tion sensors or a ccelerometers, or by periodic visua l inspections.

4.4 Phase Connections

4.4.1 Coolant Leakage.

Coolant leaka ge from the pha se connections (par a llel rings) could

result in inadequate cooling of the ring assembly. This is usually determined from TC (or

RTD) readings on the rings measuring coolant temperature on the ring, by liquid level detec-

tors, or by gas fl ow into the st at or-cooling wa ter syst em.

4.4.2 Coolant Low Flow.Low fl ow of gas or wa ter in t he phase connections could result ina general elevation in the temperatures on the phase-connection temperature detectors for a

given coolan t inlet t empera ture. Where a separa te coolan t pa th is used, a low fl ow sensor maybe used.

4.4.3 Coolant Passage Blockage.

A blocka ge would prevent t he coolant from reaching t he

rings, quickly overhea ting t hem if opera ting a t or nea r full-load conditions. This can be moni-

tored by temperat ure sensors a nd a particulate (burnt insulat ion) detection instr ument.

4.5 Terminal Bushings

4.5.1 Coolant Overtemperature (Water).

Loss of cooling wa ter t o the termina l bushings

on machines with water-cooled bushings may cause overheating of the bushings. Generator

bushing cooling w at er discharge temperat ures ma y be m onitored using TCs. Where a separat e

coolan t pa th is used, a low fl ow sensor ma y be used.

4.5.2 Coolant Overtemperature (Gas).

Loss of cooling gas to the terminal bushings on

machines with gas-cooled bushings may cause overheating of the bushings. Accumulation of

liquids (water, seal oil, etc.) may flood the ventilation passages in the bushings. This may be

monitored using t empera ture sensors. Where a separat e coolant path is used, a low fl ow sen-

sor may be used.

4.5.3 CT Temperatures.

Current transformers and their cases can overheat but are usu-

ally not monitored for temperature. Thermovision or infrared scanners can be used to deter-

mine case temperatu res.

4.6 Flexible Leads. Breakage of the flexible leads used to isolate vibration and thermalexpansion may be detected by visual inspection, particulate (burnt insulation) detector, or by

RF monitors.

5. Rotor

The rotor usually has minimal instrumentation due to the rugged environment that the

sensor w ould ha ve to endure to ma inta in a ccepta ble integrity a nd a lso due to the diffi culty in

obta ining da ta because of the rota tion of the sha ft . Rotor sha ft unba lance, loose rotor pa rts,

and loose retaining rings may show up as increased rotor vibration, amplitude, or orbit



15/27


9

changes. Degraded blower blade performance may be detected as loss of acceptable differen-

tial fan pressure. Parameters that may be monitored include vibration, field current, field

volta ge, an d vibra tion for both t he exciter an d ma in fi elds.

5.1 Shaft and Forging

5.1.1 Torsional Vibration.

Torsiona l vibra tion of the rotor sha ft result s in loss of liferelated to the severity and duration of each incident incurred that is cumulative in nature

over t he life of th e unit (unless repa ired). This is n ot usua lly monitored, but m a y be minim ized

through t orsional modeling w ith appropriat e changes in construction, or equipment modifica-

tions in th e case of existing units. Several t ypes of torsiona l monitors, to evaluat e an d display

tra nsient events, are ava ilable.

Sh aft torsiona l vibration ma y be most severe during electrical fa ults. I t ma y a lso be caused

by any ra pid shift in the tran smission network power flow tha t , simply stat ed, creat es a tw ist-

ing effect on the t urbine genera tor sha ft . The tw isting w ill oscillate ba ck and forth for a t ime,

and should eventually decay to zero. The torsional effects can be amplified by unsuccessful

reclosure an d a re affected by power system st abilizers where fi tted.

5.1.2 Shaft Voltage. The presence of voltage on the shaft relative to ground can lead to

problems. See 7.2.4 for more details.

6. Rotor Winding

The rotor winding parameters may be monitored indirectly through the measurement of

field voltage and current. Brushless exciters lack slip rings, and the rotor winding may be

indirectly m onitored through measurement of the exciter para meters. Estima tes of genera tor

fi eld rotor qua ntit ies ma y be made using generat or V curves and exciter consta nt resista nce

load sat ura tion curves, perma nent ma gnet generator (P MG ) volta ges, and exciter fi eld voltage

and current.

6.1 Electrical

6.1.1 Excitation Current (Brushless). Main genera tor excita t ion current (fi eld current)may be monitored indirectly by supervising the main generator exciter field current, and

applying the proper correction curve to detect any degradation in the generator rotor winding.

For inst an ce, genera tor rotor sh orted t urns usua lly causes higher excita t ion current require-

ments for a given load output. Sometimes instrument slip rings ma y be used with a brushless

excitation system to give a positive method of monitoring excitation volts and rotor winding

integrity.

6.1.2 Excitation Voltage and Current (Slip Rings). Field voltage and current may bemonitored a t t he slip rings, displa yed, an d/or recorded on t he genera tor control pan el. These

qua ntit ies ma y be used directly to detect increased genera tor excita t ion current, th us shorted

turns.

6.1.3 Rotor Winding Ground Fault.

Rotor ground faults ma y be detected by a device tha t

continuously or periodically determines the insulation resistance of the rotor winding to

ground. The da nger inh erent in a ground is th e possibility of developing a second g round. This

could create a magnetic unbalance, and could result in large ground loop currents that could

dam age t he rotor winding an d forging extensively. Depending on t he insulat ion resista nce, an

ala rm m ay be init iated. Although it ma y be possible to detect the second ground, usually t he

dam age has a lready been done.



16/27


10

6.1.4 Rotor Winding Shorted Turns. Problems created by interturn short circuits are

rar ely cat ast rophic in na ture. The rotor ma y exhibit symptoms of therma l imbala nce result ing

in vibration due to the uneven thermal expansion caused by the uneven distribution of losses

in the field circuit . Generator capacity could be affected if there were a large number of

shorted turns requiring a significant increase in the excitat ion current at a particular load

point to compensate. A change in the rotor heating pattern could also develop shaft bowing

and high vibration. A more serious problem occurs if the interturn short develops into aground fault . Shorted turn s a re caused by failure or bridging of the insulation between turns

of the rotor winding. They may be caused or aggravated by centrifugal forces at full speed.

Since the current-carrying rotor conductors are rotating, they create a rotating magnetic

fl ux wa ve in the ma chine air ga p. The fl ux density in the ma chine air gap is proportiona l to

the ma gnitude of the current a nd to the a ctual number of conductor turns in each slot in t he

rotor. Therefore, measurement of the a ir-gap fl ux density using a tra nsducer such a s a Ha ll

effect probe, or sea rch coil, may indicat e possible shorted t urns.

6.1.5 Rotor Resistance. Rotor resistance is a function of field winding temperature. The

therma l t ime constant creat es a delay in the chan ge of resista nce. This is evident in n on-base-

loaded genera tors, part icularly during t ra nsient conditions.

6.2 Mechanical

6.2.1 Overheating.

Rotor overheating may, in some cases, be detected indirectly by an

increase in the wa rm ga s tempera ture.

The avera ge tempera ture can be calculated from th e winding resista nce obta ined from fi eld

current an d voltage, after a ppropriat e compensation for t he volta ge drop of the brush es an d

slip rings if applicable. The temperature may be displayed and recorded. A high temperature

ala rm ma y be provided.

Depending on the type of rotor cooling, winding hot-spot temperatures can exceed average

temperatures by a factor of 1.11.5. Alarm points may be set to recognize this as well as therotor insulation class.

6.2.2 Vibration. Shaft vibration may be a symptom of many different mechanical prob-

lems. Rotor imbalance, misalignment, cracks, hydrogen seal rubs, oil whirl, defective bear-

ings, uneven rotor heating, and damaged fans and mechanical st imulation through the

turbine a re only some of the causes of changes in sha ft vibra tion. Wha tever the cause, vibra-

tion is the sympt om of a problem tha t should be corrected before furt her da ma ge occurs. Bea r-

ing vibration on the exciter and turbine ends of the generator may be monitored to detect

abnorma lit ies in th e magnit ude, phase, and frequency of the vibrat ion a t va riable load condi-

tions. A frequency analysis may be required for detailed analysis of the vibration pattern.

Seismic and proximity vibra tion sensors, usually tw o sets 90 degrees apa rt a t ea ch bearing,

are used for monitoring.

The amplitude of the vibration may be monitored by either a shaft riding accelerometer,

bearin g h ousing mount ed a ccelerometer, proximity d etector, or by velocity detecting s ensors.

There ar e tw o a pproa ches to vibrat ion monitoring. The fi rst is t o simply measur e the am pli-

tude of the vibration and to ta ke action w hen the a mplitude reaches some preset ma ximum.

The second approach is to analyze the frequency spectrum of the vibration. The coast-down

vibration signat ure provides very useful informa tion since as the ma chine runs down it pa sses

through a wide ra nge of mecha nical excitat ion frequencies.



17/27


11

7. Miscellaneous Components

The monitoring of fans, bearings, seals, etc., has not been given high priority in the past.

However, the ma chine availa bility can be increased if early w ar ning is provided by signa ls on

the fa n differentia l pressure, bearing temperature a nd vibra tion, seal leaka ge, etc.

7.1 Fans

7.1.1 Gas-Differential Pressure. The differential pressure developed across the fan(blower) indicates w hether hydrogen purity h as changed, an a bnormality exists in th e ventila-

tion circuit (lost seals, baffle failure, or obstructions), fan blades have been lost, or adequate

pressure is being ma inta ined across the fa n. Using a reference purity fan to cancel or subtract

out purity/temperature chan ges aids in interpreting the da ta .

7.2 Bearings

7.2.1 Lubricating Oil Leak. Loss of lubricating oil can cause severe damage to the bear-ings and journals. One way to monitor oil leaks may be with liquid level detectors (see 3.1.1),

and by thorough inspection of the lubrication oil system.

7.2.2 Temperature.An increas e in bearing t emperat ure could be caused by one of severalfactors, such as a reduction or loss of lubricating oil, pitting of the babbitt due to shaft cur-

rents, or deterioration of the babbitt material. If any of these conditions continues, the bear-

ing ma y fail , causing extensive dama ge.

E ach bear ing temperatur e may be measured by TCs or RTDs. One or t wo sensors are t ypi-

cally located in t he bearing lower ha lf, very close to (but n ot in) the babbitt ma terial. If t he

bearing is insulat ed, the sensor must also be insulated from the bearing to prevent shorting

the insulat ion.

The temperature of the generator bearings may be monitored and recorded constantly. If

the temperature rises above normal, the cause should be determined and corrected. Excessive

bearing temperatures may init iate an alarm. An elevated bearing metal temperature at thebearing indicates a problem with a potential rub condition if the bearing vibration is also

higher than normal.

7.2.3 Vibration. Vibrat ion is norma lly monitored. An a larm ma y be init iat ed if limit va l-ues a re exceeded (see 6.2.2).

7.2.4 Shaft Currents. Shaft currents are a result of the shaft voltage being dischargedthrough t he bearings, seals, gear s, etc. , possibly dam aging them. Several conditions with in a

generator can result in a potential between the rotor shaft and ground. Shaft voltages above

50 V peak t o pea k can occur from t he electrost a tic char ges developed on low-pressure t urbine

blades. Generator sta tor ma gnetic asymmetries, residual ma gnetizat ion of the rotor or frame,

and high-frequency transients developed from thyristor excitat ion controls may also cause

shaft voltages that can damage bearings.

B earing current erodes th e bearing babbitt , result ing in a dull surface (frosting), and spark

tra cks or both, higher bearing temperat ures, an d ult ima tely bearing failure.

This usua lly does not ha ppen to a g enerat or main bearin g but ca n occur on one of the high-

pressure turbine bear ings, the governor mechanism, seals, or other sur faces tha t opera te w ith

a t hinner oil film t ha t can brea k down if sufficient voltage is present. G rounding of the sha ft

between the turbine a nd generat or through brushes, copper bra ids, or a n a ctive shaft ground-

ing system can be used to neutralize these currents and prevent conditions such as pit ted

bearings.



18/27


12

S ymmetrical electrical fi lters between th e exciter a nd slip rings a re sometimes employed

for st at ic excita t ion systems t o reduce sha ft voltage creat ed by t he solid-sta te rectifier excita -

tion system.

C ontinuous sha ft voltage monitoring with a dedicat ed monitor, or frequent m easurement

with an oscilloscope, may be desirable to verify proper operation of the shaft grounding sys-

tem.

7.2.5 Lubricating Oil Flow.The bearings must have a continuous flow of oil for lubrica-tion and cooling. A flowmeter can be installed on the main supply, or on each bearing supply

line to monitor the fl ow of lubrica ting oil. A low-fl ow a lar m ma y be provided.

7.2.6 Lubricating Oil Pressure.An a lternat e to monitoring the oil fl ow to each bearingwould be to monitor th e oil supply pressure for each genera tor bear ing. A low-pressure a lar m

ma y be provided.

7.2.7 Lubricating Oil Temperature. The temperature of the lubricating oil exiting thebearings gives an indication of the condition of the bearing. This temperature is typically

monitored an d recorded for each bearing, wit h a high-temperature a larm.

7.3 Hydrogen Seals

7.3.1 Shaft Seal Oil Leak.There is a minimum required flow of seal oil past the seal oilrings. An overpressure of the seal oil relative to the gas pressure, excessive seal ring clear-

ance, or excessive seal oil temperature can cause excessive flow past the seal oil rings. Exces-

sive flow past the seal oil r ings on th e genera tor shaft permits oil to enter th e genera tor frame

an d ma y deposit oil on t he windings or other live part s, creating a potentia l for surface-tra ck-

ing discha rge (a lso increa ses the potentia l for a dditiona l sta tor-w inding vibra tion). This condi-

tion can be detected by t he liquid-level detectors.

7.3.2 Seal Oil Flow. Very low oil flow to the seal rings can result in loss of sealing, thusallowing gas to escape from the generator. Excessively high oil flow can result in oil entering

the ma chinepossibly as a result of excess cleara nce in t he seal ga p. B oth low an d high fl owconditions can be detected by monitoring the seal oil pressure, an d a larming when it deviates

from a n a ccepta ble range.

7.3.3 Seal Oil Pressure. Low oil pressure can result in overheating, increased vibration,a nd/or possible rub condition. Hydrogen ma y escape past th e oil bar rier if the pressur e is too

low.

NOTE: Required seal oil pressure changes with hydrogen pressure.

The seal oil pressure at each bearing is t ypically m onitored, with an ala rm if t he pressure is

too low (see 7.3.2). If the machine is equipped with a differential pressure regulator to main-

ta in th e oil to gas-differential pressure, the differential pressure at the regulat or may be mon-

itored.

7.3.4 Seal Metal Temperature.B y monitoring the temperature of the meta l componentsof the hydrogen seals, it may be possible to detect seal rubs or misalignment.

7.3.5 Seal Oil Temperature.Oil tempera ture should be ma inta ined betw een the normaloperating limits as defined by the manufacturer. Unbalanced temperature can result in

increased vibration levels. Excessively high oil temperatures can result in increased oil flow

and can cause some oil to enter the generator. Excessively low oil temperatures can cause

reduced oil fl ow, a loss of clear a nce, a nd a possible rub w ith the r otor. The seal oil temperat ure



19/27


13

at each hydrogen seal may be monitored with an alarm if the temperature deviates beyond

acceptable limits.

7.4 Permanent Magnet Generator (PMG)

7.4.1 PMG Voltage. Permanent magnet generator voltage may be monitored for voltage

variat ion as one possible indication that a magnet in the PMG may have become demagne-tized.

7.5 Collector Rings

7.5.1 Air In/Out Temperature.Collector ring temperatures cannot be monitored directly;however, the difference in cooling medium temperature flowing through the rings and brush

rigging, and the actual discharge temperature, may be monitored. Typical limits are: maxi-

mum discharge tempera ture = 65 C, maximum tempera ture rise = 25 C.

7.5.2 Plugged Air Filters (where applicable). Monitor the pressure drop across the fil-ters to detect pluggage.

7.5.3 Hydrogen Leaks.A device to measure the presence of hydrogen in the air may belocated in the vicinity of the slip rings. This detects hydrogen escaping from the radial pins

connecting the upshaft lead to the slip rings.

7.6 Hydrogen Cooler

7.6.1 Hydrogen Cooler Leaks.If the hydrogen cooler leaks water from the coolers owncooling circuit , then m oisture ma y be a dded to the ga s, or st an ding w at er ma y collect under

the cooler frame. Liquid level detectors under the generator are useful for monitoring this

problem. In machines which operate under higher hydrogen pressure (60 or 75 psig), the

hydr ogen gas m ay enter int o the cooling circuit (depending on cooling w a ter pressur e), result-

ing in higher gas consumption.

A cooler ga s-ba ffl ing leak, w here hydr ogen is bypa ssing t he cooling circuit inside th e hydro-gen cooler, results in the warm gas being insufficiently cooled before being returned to the

generator, causing reduced cooling system efficiency. This results in a warmer exchange gas

for t he generator to tra nsfer its losses.

7.6.2 Air-Bound Coolers.Should the cooler vents become inoperative, air may collect inthe cooler, impeding its performance. The cooler vent lines may be monitored to ensure that

there is a continuous flow of water. The water and hydrogen TCs or RTDs may also identify

this problem.

8. Auxiliary External Systems

The externa l syst ems should be given special a tt ention. These syst ems a re critical to proper

operation of the generator a nd ma y be monitored to prevent misopera tion and ensure optimal

performance.

CAUTION

If cooler leaks do occur, hydr ogen can a ccumula te in th e cooler vent lines.



20/27


14

8.1 Hydrogen System.Hydrogen ga s is circulat ed under pressure through some ma chinesfor cooling of the rotor an d st a tor. Continuous m onitoring of the hydrogen can yield much use-

ful information. Those parameters that may be monitored include hydrogen dryness, purity,

pressure, temperat ure, consumption, loss, an d presence of par ticula tes.

8.1.1 Generator Humidity.A dryer ma y be used to mainta in the required gas dew point

by removing moisture from th e gas w hen th e generat or is both on a nd off line. Pr olonged oper-at ion w ith high dew point can result in electrical t ra cking. It can also lead t o stress corrosion

cracking of various components, such as the rotor retaining rings in the generator.

If th e temperat ure of the va rious meta l or insulated part s inside the ma chine falls below

the dew point of the ga s, there may be a possibility of forming a fi lm of surface moisture due to

condensation.

G as-coolan t h umidity m ay be monitored continuously by a dew-point indicat or and ma in-

ta ined at a value much lower th an the expected cooling wa ter t empera ture. A gas-coolan t d ew

point of below 0 C is typically acceptable. An alarm may be provided if the dew point rises

above the set point.

To minimize condensa tion, some ma nufacturers r ecommend tha t the generat or metal an d

insulated surfaces should be maintained at a higher temperature than the coolant gas. On

these ma chines, therefore, the sta tor-cooling w at er is t ypically ma inta ined a t least 5 C

warmer than the cold coolant gas. If this temperature difference falls below 3 C, an alarmmay be activated. Refer to the manufacturer for actual temperature differential limits (if

applicable).

8.1.2 Hydrogen Purity. A gas analyzer may be provided to monitor the concentration ofhydrogen gas in the generator. Hydrogen from the generator may be continuously passed

through th e ana lyzer. The output from t he an alyzer ma y be displayed as hydrogen purity in

the control room. If the purity (concentration) should fall below the manufacturers recom-

mendat ion, an a larm may be activated; the operator should investigate a nd init iat e corrective

action.

The purity of the hydr ogen ga s is norma lly ma inta ined a bove 92%purity. For effective t her-

mal performance, a purity above 98%is typically preferred. Low hydrogen purity results in

increased w indage losses a nd lower efficiency, and ma y a lso raise the stresses on t he ventila-

tion sy stem (blower). Pure hydrogen will not support combustion. However, when mixed with air, hydrogen is

explosive. At atmospheric pressure, hydrogen concentrations from 4%to 74%are dangerous.

For this rea son the concentra tion of hydrogen in the generator must be mainta ined at a high

level.

Additional monitoring for safety purposes may be required during purging operations.

8.1.3 Hydrogen Pressure.The heat removal capability of hydrogen is determined by itspressure inside the genera tor. The capabilit y curve limits a re relat ed to the hydrogen pressure

of the generator. At full load, the hydrogen pressure should be maintained at rated design

pressure. At reduced load, the genera tor is more effi cient if t he pressure is just s lightly h igher

tha n needed as determined by the capa bility curves.

Pressure regulators are used to reduce the supply pressure of the hydrogen gas to the

required operating pressure of the generator. Proper generator cooling depends on maintain-

ing proper hydrogen pressure. If adequa te pressure cannot be ma inta ined, the load capability

of the generator will be affected.

The hydrogen pressure inside the generator may be monitored and displayed in the control

room. In a ddit ion, a low-pressure a larm may be provided and set below the norma l operating

pressure. The existence of a low-pressure alarm indicates that a hydrogen leak exists.

A differential-pressure indicator and alarm may also be provided to monitor the pressure

across the rotor fan blades (blowers). This would provide an indication of restricted flow,

chan ges in purit y, or cha nges in moisture. This ca n a lso be monitored periodica lly.



21/27


15

A third point to monitor is the pressure difference between the hydrogen and the stator-

cooling water system (if applicable). The hydrogen pressure is normally higher than the sta-

tor-cooling water pressure to prevent ingress of water into the stator in the event of a leak in

the sta tor-cooling w at er syst em.

8.1.4 TemperatureCold Gas.Cold gas temperature may be monitored and maintained

between upper and lower design limits. In general, the temperature of the cold gas suppliedby each ga s cooler is typically bala nced to w ithin 2 C in genera l with only short excursions t o

an unbala nce of up to 5 C. Normal recommended cold ga s temperat ure can be obtained from

the manufacturers instruction book.

The cold gas temperature ma y be measured by RTDs or TCs locat ed in the ga s fl ow pat h a t

the discharge of the gas coolers. Usually a sensor is provided for each cooler. A temperature

indicat ion a nd high a larm m ay be provided.

8.1.5 TemperatureHot Gas. The temperature of the hot gas returning to the coolersreflects the heat absorbed by the gas. The gas temperature rise may be monitored and is

typically maintained at less than maximum recommended by the manufacturer. Excessive

temperature rise can indicate abnormalities such as low purity, or pressure in case of

hydrogen-cooled machines.

The gas t empera ture ma y be measured by RTDs or TCs locat ed in the gas fl ow path at theinlet t o the ga s coolers. Usua lly a sensor is provided for ea ch cooler. A tempera tur e indicat ion

an d high ala rm ma y be provided.

For those machines that have direct hydrogen-cooled windings, the temperature of the

hydr ogen leav ing selected coils ma y be meas ured by individu a l RTDs or TCs.

8.1.6 Hydrogen Consumption.Hyd rogen consumption may be monitored to ensure tha tthe consumption rate is not increasing, and t ha t hydr ogen leaka ge rates a re at acceptable lev-

els. Excessive levels of lea ka ge are both da ngerous an d expensive. P urge an d fi ll (vent a nd

add) operations may be closely watched and recorded. A gas-totalizing flow meter on the inlet

(ma keup) line may provide indicat ion of the t otal m a chine hydrogen consum ption.

8.1.7 Hydrogen Loss to Stator-Cooling Water.Hydrogen gas leakage into the statorwa ter system through the stat or bar hollow str an ds or wa ter hoses at t he wat er manifold can

lead t o blockage of the cooling pat h, becoming hyd rogen bound d ue to a ga s bubble. This ma y

result in overheating of the bars. For water-cooled stators, a gas flow measurement system

may be installed on th e vent line of the sta tor wa ter ta nk to monitor excessive gas leakage.

8.1.8 Hydrogen Loss to Hydrogen Coolers. A hydrogen cooler leak may also result inhydrogen entering the cooler water system. This condition, while difficult to monitor, may

eventually be detected by high hydrogen consumption.

8.1.9 Hydrogen Loss to Oil. Hydrogen gas leakage into the seal oil system is possiblewhere hydrogen an d oil come into conta ct . For syst ems wit h dua l fl ow, i .e. , separa ted a ir a nd

hydrogen side supply, the hydrogen side can be considered a closed loop and no appreciable

hydrogen losses are incurred. For single-flow systems, which typically have a vacuum cham-

ber a nd pump, const a nt hydr ogen losses occur. They a re included in tota l consum ption.

8.1.10 Particulates.An indica tion of genera l or loca l overhea ting m ay be provided by a ga spar ticula te d etector commonly called condition monit or or core monit or. This d evice monitors

a continuous sa mple of generator hydr ogen. If a ny overheating of th e genera tor interna l com-

ponents occurs (stator winding, laminations, etc.), small particles of organic material are

released into the gas stream due to thermal decomposition. The gas-particulate detector may

sense these part iculat es and ma y provide an a larm at a preset level.



22/27


16

In addition, a pyrolysate collector can be incorporated as part of the gas-particulate detec-

tor. The collector will automatically collect a sample of the gas whenever a gas-particulate

detector ala rm is a ctivated. The sam ple can t hen be ana lyzed to determine the a ctual source

of the part iculat es.

8.2 Seal Oil System. The seal oil syst em is designed t o prevent lea kag e of hydr ogen from t he

genera tor to the a tmosphere. With some ma chines, the system is s plit into tw o partsthe a irside and the hydrogen side. The air-side seal oil is pumped to the shaft seals through coolers

and fi l ters. I t t hen flows through the sea ls via a nnular gaps between the shaf t a nd the sea l

rings. Finally, it drains to the air-side seal oil storage tank for recirculation.

The hydrogen-side seal oil system is a single-flow system and is similar to the air side

except that the oil flows into a separate seal oil tank where the entrained hydrogen escapes

from the oil. The hydr ogen-side seal oil supply pressure is ma inta ined higher t ha n th e hydro-

gen ga s pressure, an d slight ly lower or equa l to the a ir-side seal oil supply, if so equipped.

8.2.1 Differential PressureFilters.The seal oil filter differential pressure may be mon-itored to ensure proper opera tion of the fi lter. A high d ifferentia l pressure ma y be a result of a

clogged filter or excessively high oil flow. A low differential pressure can be an indication of

low oil flow.

The differentia l pressure a cross the sea l oil syst em fi lter ma y be monitored (or periodica llychecked) with a differential-pressure indication and a high-pressure alarm.

8.2.2 Differential PressureGas to Oil. The differentia l pressure betw een the hyd rogengas and the supply oil must be maintained at a safe margin (approximately 10 psig) to pre-

vent the hydrogen from escaping from the generator at the seal r ings. Oil pressure must

alw ays be higher th an hydrogen pressure when there is hydrogen in t he genera tor.

The differential pressure between the sea l oil a nd t he hydrogen gas ma y be monitored w ith

a differential-pressure indicat ion an d a low-pressure alar m.

8.2.3 Differential PressureAir Side to Gas Side.The differentia l pressure of the sealoil betw een the air-side seal oil system a nd t he hydr ogen-ga s-side seal oil system ma y be mon-

itored w ith a differential-pressure indication an d a high-pressure alarm .

8.2.4 Air-Side Seal Oil Pressure.The pressure of the sea l oil on t he a ir side of the sy stemmay be monitored, if applicable, with a low-pressure alarm.

8.2.5 Hydrogen-Side Seal Oil Pressure.The pressure of th e seal oil on the h ydrogen sideof the system ma y be monitored with a low-pressure ala rm.

8.2.6 Seal Oil Pump Discharge Pressure. The discharge pressure of the seal oil pumpmay be monitored with a low-pressure alarm.

8.2.7 Seal Oil Tank Level.Vacuum detr ain ta nks w ith h igh oil levels could ba ck up intothe defoam ing ta nks a nd eventua lly into the generat or. Low oil levels indicate a n oil leak from

the syst em.

The level of oil in th e seal oil tan ks ma y be monitored wit h high- and low-level ala rms.

8.2.8 Seal Oil System Temperature. Measure the temperature in th e seal oil system atthe follow ing point s: seal oil leaving t he coolers (high tempera tur e ala rm); cooling w a ter leav-

ing t he coolers; a nd cooling w a ter in let t o the cooler.

8.2.9 Seal Oil System Flow.Measu re the sea l oil fl ow to each sea l (if possible) on both t heair side and the hydr ogen side of the sea l oil system.



23/27


17

8.3 Stator-Cooling Water System. Direct cooling of conductors using deionized water maybe the most effective way to achieve high generator capacity. The cooling water is passed

thr ough hollow conductors or t ubing ins ide the w inding. To achieve reliable operat ion, contin-

uous monitoring and periodic sampling may be provided to determine the water conductivity,

temperature, pressure, and content of oxygen, hydrogen, copper, and pH value. Overheating

can cause therma l degrada tion of the insulat ion, physical da ma ge to the insulation due to rel-

at ive movement, and boiling of the wa ter, producing ga s pockets a nd impa ired wa ter fl ow.

8.3.1 Conductivity. The demineralized water may be monitored for acceptable conductiv-ity. High conductivity may be dangerous in the stator bar environment of the water system.

Demineralizer beds should be functional and effective during system operation. High conduc-

tivity can result in electrical tracking to ground. Typical conductivity levels are between

0.1 /in a nd 1.0 /in. An a lar m ma y be initia ted w henever the conductivit y is outside

acceptable limits a s esta blished by t he ma nufacturer.

8.3.2 Differential PressureInlet to Outlet.The differential pressure across the statorwinding from inlet w at er man ifold to outlet wa ter ma nifold may be monitored to ensure ade-

quate cooling water flow, and to maintain the effectiveness of the cooling system. Increased

differential pressure at a given fl ow ma y be an indicat ion of increased fl ow resistance due to

fouling or plugging.

8.3.3 Inlet Temperature. Inlet stator cooling water temperature may be monitored forstable operation at acceptable levels as described in the manufacturers instruction book. An

ala rm ma y be provided at the ma ximum permitted tempera ture, such a s 50 C. Refer to ANSI

C50.13-1977[1].

8.3.4 Outlet Temperature.Outlet st at or cooling wa ter t empera ture ma y be monitored forsta ble operat ion at a cceptable levels as described in the ma nufa cturers instruction book. This

may include an alarm at a temperature above that expected with normal operation, such as

above 75 C.

8.3.5 Oxygen Content.The dissolved oxygen content of the wa ter in closed syst ems is n or-

ma lly controlled to prevent corrosion of hollow copper str a nds, w hich could block off the cool-ing pas sa ge through t he bar s. Norma l level is typica lly less tha n 50 ppb (par ts per billion) for

hydrogen-saturated systems. There is essentially no limit for open ventilated systems. See the

ma nufa cturers inst ruction book for requir ements. Oxygen content m a y be continuously moni-

tored.

8.3.6 Pressure.The wa ter pressure of the st at or cooling wa ter syst em is typically ma in-ta ined at the proper levels to ensure a dequat e high-pressure wat er supply to the inlet w at er

manifold and (if applicable) that the stator-cooling water pressure is always less than the

hydrogen ga s pressure.

8.3.7 Copper and Iron Content.Copper and iron concentration may also be monitored.Normal levels are typically less than 20 ppb.

8.3.8 Hydrogen Content (see 8.1.7).The hydrogen content of the water system typicallyis minimal if no leaks into the system are present. Excessive venting from the relief valve on

the sta tor wat er tank ma y be an indication tha t hydrogen is entering the system.

8.3.9 pH Value. The pH value of the water may be determined to ensure stable waterchemistry according to th e ma nufacturers r ecommenda tions.



24/27

19

A

Abrasion, 4.3.8Accelerometer, 4.3.8, 6.2.2

Acoustic, 3.1.2

Active power, 4.1.4

Air bound coolers, 7.6.2

Air-cooled, 3.1.1, 4.2.2

Air fi lters, 7.5.2

Air-ga p fl ux d ensit y, 3.2.6, 6.1.4

Air t emperat ure, 7.5.1

Air-to-ga s pr essur e, 8.2.3

Alignment, 3.0, 3.1.2, 6.2.2, 7.3.4

Axial bolts , 3.2.6

B

Babbitt damage, 7.2.2, 7.2.4

Baffle damage, 7.1.1, 7.6.1

Balance, 3.1.2

B a r, 4.2.1, 4.2.2, 4.3.1, 8.3.1

B a r ga s t emperat ure, 3.2.1, 4.3.1, 4.3.2

B ar leaks, 3.1.1

B a r outlet w a ter t emperat ure, 4.3.1, 4.3.2, 4.3.6

B a r plugged , 4.3.1, 4.3.3, 4.3.4, 4.3.6, 8.1.7, 8.3.5

Bearing, 3.1.2, 4.1.3, 6.2.2, 7.2.1, 7.2.2, 7.2.3,

7.2.4, 7.2.5, 7.2.6, 7.2.7B earing current, 7.2.4

Bearing loading, 3.1.2

B lad e, tur bine, 4.1.3

B locked cooling, 4.3.3, 4.3.6, 4.4.4, 8.3.5

B locked pa ssa ge, 4.3.3, 4.4.3, 4.3.6, 8.3.5

B lower (fa ns), 6.2.2, 7.1.1, 8.1.2, 8.1.3

B oiling w a ter, 8.3

B ottom ba rs, 4.2.1, 4.2.2

B rush less exciters, 6.1.1

Brushes, 6.2.1, 7.5.1

Burnt insulation, 4.3.5, 4.3.7, 4.4.3, 4.6

B ush ings , 4.1.2, 4.5.1, 4.5.2

C

Ca pabilit y curve, 3.2.2, 8.1.3

Ca pacita nce probe, 4.2.3

Ca vitat ion, 4.3.1

Circulating currents, 3.2.3

Circumferent ial bolts, 3.2.6

Coast-down vibration, 6.2.2

Cold gas temperature, 3.1.1, 3.2.1, 4.3.2, 8.1.4

Cold shut down, 3.1.1Cold wa ter t empera ture, 4.3.2

Collector rings, 7.5.1

Condensation, 3.1.1, 8.1.1

Condition monitor, 8.1.10

Cond uctiv ity, 4.3.1, 8.3.1

Conductor-cooled, 4.2.1

Conductor temperature, 4.2.1

Copper in wa ter, 8.3.7

Coolant , 8.3

Coolant flow, 3.2.1, 4.2.1, 4.3.1, 4.3.2, 4.3.3,

4.3.4, 4.3.6, 4.4.2, 4.5.1, 4.5.2, 8.3.2

Coola nt lea k, 4.3.1, 4.4.1, 4.5.2

Coolant temperature, 3.2.1, 4.2.1, 4.3.2, 4.3.3,4.3.4, 4.3.7, 4.4.1, 4.5.1, 4.5.2, 8.1.1

Cooler lea k, 3.1.1, 4.4.1, 7.6.1

Cooler operation, 3.1.2

Cooling s yst em, 3.1.2, 4, 7.6.2, 8.3

Coupling, 3.1.2

Core-end region, 3.2.2, 3.2.4, 3.2.5

Core heating test, 3.2.3

Core la mina tions, 3.2.3, 3.2.6

Core loop test, 3.2.3

Core m onitor, 8.1.10

Core temperature, 3.2.1, 3.2.2, 3.2.3, 3.2.4,

3.2.5, 4.1.6Core tigh tness, 3.2.6

Core vibration, 3.1.2, 3.2.6

Corona , 4.2.3, 8.1.1

Corrosion, 4.3.6, 8.1.1, 8.3.5

Cracked strand, 4.2.1, 4.3.1, 4.3.3, 4.3.7

Cr itical frequency, 4.1.3

CT temperat ure, 4.5.3

Cu rr ent , 4.1.2, 6.1.1, 6.1.2

Current imbalance, 4.1.5

Cur rent, nega tive sequence, 4.1.5

Current transformers, 4.1.2, 4.1.4, 4.2.3, 4.5.3

Cy cling, 4.3.7, 5.1.1

D

Demin eralizer, 8.3.1

Dew point, 3.1.1, 8.1.1

Differential pressure, 4.3.1, 7.1.1, 8.1.3, 8.2.1,

8.2.2, 8.2.3, 8.3.2

Differential temperature, 4.2.2

Index



25/27

20

Direct a xis force, 3.2.6

Direct-cooled, 4.2.1, 8.1.5

Double freq uency, 3.2.6, 4.1.5

Drains, 3.1.1

Dryer (gas), 8.1.1

E

Electrical discharges, 4.2.3, 7.3.1, 8.1.1, 8.3.1

Electroma gnetic sens or, 3.2.3

Electrostatic charge, 7.2.4

End-tooth heating, 3.2.2, 3.2.4

En d t urns, 3.2.6, 4.3.8

End winding, 4.3.8

Excitation, 3.2.2, 3.2.4, 3.2.5, 6.1.1, 6.1.2, 6.1.4,

6.2.1, 7.2.4

Exciter fi eld, 6.1.2

F

Fans (blower), 6.2.2, 7.1.1, 8.1.2, 8.1.3

Fault , electr ical, 5.1.1

Fiber optic, 4.3.8

Field amps, 6.1.1, 6.2.1

Field resistance, 6.1.5, 6.2.1

Field volts, 6.1.1, 6.2.1

Filter, 7.2.4, 7.5.2, 8.2.1

Flashover, 4.2.3, 7.3.1, 8.1.1, 8.3.1

Flexible leads, 4.6

Flux density, 6.1.4Flux tra p heat ing, 3.2.5

Fra me foot loa ding, 3.1.2

Fra me vibra tion, 3.1.2, 3.2.6

Frequency, 3.2.2, 3.2.6, 4.1.3, 4.1.6

Frostin g, 7.2.4

G

G as an alyzer, 8.1.2

G a s bu bble, 8.1.7, 8.3

G a s-cooled, 4.3.1

G a s-differentia l pressur e, 7.1.1, 8.2.3

Gas dryer, 3.1.1, 8.1.1

Gas flow, 3.2.1, 4.2.1, 4.3.1, 4.3.6, 4.4.1, 8.1.3,

8.1.6

G a s pr essur e, 3.2.1, 4.2.1, 4.3.1, 4.3.2, 8.1.3

Gas temperature, 3.1.1, 3.2.1, 4.3.1, 4.3.2, 4.3.6,

4.3.7, 6.2.1, 7.5.1, 8.1.1, 8.1.4, 8.1.5

G a s-to-a ir pr essure, 8.2.3

G a s-to-oil pressu re, 7.3.3, 8.2.2

G a s-to-wa ter pressu re, 8.1.3

G land st eam sea ls, 3.1.1

G overnor, 7.2.4

G round fa ult, 6.1.3, 6.1.4

Grounding brush, 7.2.4

G routing problem, 3.1.2

H

Recommended Practice for Monitoring and Instrumentation of Turbine Generators

Documents