flowchart procedure to assess the condition

8/18/2019 Flowchart Procedure to Assess the Condition

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5 6

IEEE Transactions on Energy Conversion, Vol.

5 No.

3, September

1990

FLOWCH RT PROCEDURE TO

ASSESS

THE CONDITION

OF TURBINE GENER TOR INSUL TION

G.C. Stone,

I.

C u l b e r t a n d H. Dhiran i

O n t a r i o H y d r o

Toronto

C a n a d a

KEY

WORDS: Rotating machines, Wind ing insulation, Life

ABSTRACT Maintenance planning and life extension programs require

that the condition o the insulation in the rotor and stator windings of

generators be assessed. Unfortunately the large number of insulation

deterioration processes w hich can aflict windings together with the lack

o

a universal diagnostic test which is sensitive to all these deterioration

processes m akes condition assessment dincult . Th b paper presents a

method using lowcharts which indicate a step-by-step procedure to assess

insulation condition o turbine generators. Information on likely

deterioration processes which arise fro m the insulation system

characteristics and operating practice is firs t collected. Then on-line

monitoring and off-line tests are done to determine ifproblems could be

occurring. Finally depending on the outcome

of

the previous steps

more

extensive tests and detailed inspections usually requiring a significant

outage are performed. Although the procedure itself is stmighrfonvard it

is best implemented by engineers with considerable experience in this

field. The lowcharts

w ll

indicate to nonexperts the complexity o reliably

assessing insulation condition.

I N T R O D U C T I O N

Planning maintenance for turbine generators requires that the condition of

the stator and

rotor

winding insulation systems used in

these

machines

be

known. Recently, the desire to extend the life of generating stations has

increased the need for insulation system condition assessment procedures.

Although general assessment procedures

are

available from machine

manufacturer maintenance manuals and IEEE uidelines [l] , many utilities

must depend on experts from machine manufacturers or other consultants

to help perform detailed insulation assessments on a particular machine.

In order to make assessment techniques more generally available to

utilities, and to indicate to maintenance engineers the complexity

of

the

task, flowcharts have been prepared

as

part

of

EPRI project

Rp

2577-1.

These flowcharts present a logical process for assessing machine

insulation condition. Although considerable experience is still required to

interpret information collected as part of the assessment procedure, the

flowcharts will

be

instructive

to

the nonexpert in understanding the wide

variety of information required before a realistic assessment can

be

made.

This paper presents flow charts for assessing the condition of the rotor and

stator winding insulation system s of large turbine generators and is largely

taken from Reference [2], which presents further details, and procedures

for other types of machines. Although space limitations preclude the

inclusion of examples of the use of these flowcharts here, four practical

examples are contained in [2].

The procedure for assessing insulation condition is not simple since

several dozen possible failure mechanisms may occur in stator and

rotor

windings [21. The large number of deterioration mechanism s results from:

90

W M

OQL 2 EC

b y t h e I EE E R o t a t i n g M a c h in e r y C o m mi t te e of t h e

IEEE P ow er E n g i n e e r i n g S o c i e t y f o r p r e s e n t a t i o n

a t

t h e IE EE /P ES 1 9 9 0 W i n t e r M e e t i n g , A t l a n t a , G e o r g i a ,

F e b r u a r y 4 - 8 1 9 9 0 .

A u g u s t

30

1 98 9; m ade a v a i l a b l e f o r p r i n t i n g

December

6, 1989.

A p per r e co m m e n de d a n d a p p r o v e d

Manuscript s u b m i t t e d

the wide variety of insulation systems dcveloped over the years,

which often respond quite differently to various thermal,

mechanical and electrical stresses.

the differing design philosophies of the major manufacturers on

the same

or

different insulations which result in different design

stress levels being placed on the insulation.

the different operating (for example base-load and peaking) and

maintenance (no maintenance to proactive maintenance) policies

of the utilities.

Assessing insulation condition is further complicated because there is no

single test

or

simple inspection which can easily track the condition of the

insulation under all possible deterioration mechanisms

[3]

When

combined, the abov e factors result in a large number of possible insulation

deterioration mechanisms, and the symptoms which are used to gauge the

insulation condition are often very diverse and difficult to measure.

ASSESSMENT SEQUENCE

The procedun: embodied in the flowcharts is based on collecting and

assessing information in a sequence which requires increasing levels of

effort and cost. The proposed sequence is:

collect information a) to determine likely deterioration

mechanisms and average lifetimes (b) from on-line monitoring

determine if deterioration is occurring

-

analyzc the above information

perform off-line tests and inspections which require only a min or

outage

-

do a detailed asscssment, if required, including an in-depth

inspcction and special tests. This requires a major outage.

Collecting background information is

a

critical step which allows the

maintenance enginecr to select the

tests

and inspections which will

be

most sensitive

to

the likely failurc mechanisms . The insulation systems

and operating environment in

a

particular machine should

be

identified,

since each generic t y p and operating mode can give r ise

to

particular

failure mechanisms

[2, 41

lndustry databases compiled by the NERC

Generator Availability Data System and Edison Electric Institute

committees can warn of generic problems and reliability of particular

machine types. By carefully reviewing background information, the

relevant tests and inspcctions can be selected from those presented in the

flowcharts to identify problem s specific to the machine being examined.

Oncc background information has been collected and assessed, the specific

procedurc to follow is shown in the flowcharts in Figures and 2. The

assessment techniques provide two altemative results which are

"Condition Acceptable" and "Correctivc Action is Required".

The

latter

refers to conditions where failure has occurred,

or

significant aging has

been detected. No attempt has been made to indicate how quickly

correctivc action should

be

taken. This will be dictated by the particular

circumstances under which the assessment is being made, the degree of

detcrioration, and the future operating mode of

the

machine.

Thc flowcham have thrcc subscctions (On-Line Monitoring, Off-Line

Tests, and Detailcd Asscssmcnt) which are cntered by making use of the

appropriatc logic lor ;I particular typc of asscssment. The assessment

0885-8%9/90~-0546$01.00 990 IEEE

I 7


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547

START

SSESSMENT

PARTUL

DISCHAROE

ESTS

S E E FE U R E l l b )

Figure

l a )

Sequence for collecting on-line monitoring and off line est results for .stator uindings.

process is relevant for turbine generators

as

well as motors

and

hydrogenerators.

This

procedures outlined in

the

charts should preferably be determined by

always commencing at the START point and following the logic

appropriate for a particular situation since this will ensu re that no test or

inspection technique has been ignored. Note that not all techniques may

be necessary to assess the condition of a machine and completion of all

possible tests will not necessarily result in a definitive assessment. The

general logic that determines which of the major subsections is entered

first is mainly influenced by inspection and maintenance strategies and

intcrprctations made from on-line monitoring results.

Many t a ts and inspcctions arc refcrcnced in the flowcharts. For the most

part, these tests and inspcctions can bc don e with commercially available

apparatus and have bccn dcmonstrdtcd to provide useful information.

Further details on wcll-establishcd tests and inspections can be found in

Rcfcrcncc

I 1

. Descriptions of morc modem tests, as wcll as a critical

rcvicw of the usefulness and applicability

of

all the

tests

referred to in

the

flowcharts, are contained in Reference

[2].

The proper evaluation of

winding condition requires correct interpretation of the results of tests and

inspections. IEEE Standard

56

[11 and other associated standards,

machinc manufacturer’s data, as wcll as Reference [2] can be used

to

aid

intcrprctation. Only with cxpcriencc can the maximum amou nt of

infonnatio n hc extracted from visual inspections.

Note that for experienced insulation cxperts, the best information on

insulation condition comes from a careful visual inspection

of

the

componcnts which makc up the insulation system. Unfortunately,

a

good

visual inspcction is intrusive and oltcn requires significant disassembly

of

thc stator or rotor winding, laking morc timc than most tests. Thu s when

suikible

tcsts

arc avail;rblc, (hey arc usually

less

costly and mo re desirable

tlim dctailcd inspcctions.


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548

E S T Y A T EEPNRS

R W R D

bNDO S T S

I

UROELUCS

W N D l f f i

SloE

C L E A R I N C E

bNDICY(TNES.5

C H E C K S

E S T U A T E

RPNRG

W R D

AND

O S T S

Figure

l b )

Detailed assessment procedurefor

stator

windings

ASSESSMENT OF S T A T O R W I N D I N G C O N D I T I O N

The genera l logic for the assessm ent of stator winding insulation systems

is shown in Figure 1. The detailed assessment procedures cover the

following subsystems for large turbine generators:

Groundwall and Phase Insulation

-

Slot Wedging

Endwinding Bracing

Groundwall an d Phase Insulat ion

Winding groundwall and phase insulation condition can be assessed, to

some extent, at any of the three levels indicated in the flow charts, i.e., by

On-line Monitoring, Off-line Tests and by the procedures defined in the

Detailed Assessment section. However,

a

Detailed A ssessment is usually

required to obtain a reliable evaluation

of

groundwall and ph ase insulation

condition, since existing on-line monitoring and off-line tests are not

comprehensive enough.

Only very general assessments can be made with On-Line Monitoring, but

the collected information allows decisions

to be

made on the need

for Off-

Line Tests,

or

Detailed Assessments. The amount

of

information collected

depends on the

type

number, loca tion and sophistication of the monitoring

devices fitted. Most generators are equipped only with voltage and current

relays, hydrogen leak detectors and embedded tem perature

sensors ore

extensive diagnostic information is available if the ge nerator is equipped

with condition

or

core) monitors (GCMs),

or

on-line partial discharge

testing is possible.


4/7

549

I

MEASURE

INSUL TION

RESISTANCE AND

PCi RIZITIOh

INDEX

DETNLED

ASSESSMENT

Sea Fgure qb)

SURGE TEST

IMPEDANCE

TEST

AN YO R AL L

SEOUENCE OPTIONAL

TURN AND

ACCEPTABLE

Figure

2 a) Sequence for collecting on-line monitoring

and

off-line test results fo r turbine generator rotor

windings

More

information on the groundwall insulation condition can be obtained

from off-line tests. Thcs e tcsts will indicate eithcr that the groundwall

insulation is generally in good condition, or that significant deterioration,

or failure, has occurred and a detailed assessment is required. One

or

more

of thc tests indicated in Figure 1 can be performed to provide such

information.

A

satisfactory insulation condition may be indicated by the

following rcsults:

withstanding an ac or dc or very low frequency hipot test

- high insulation resistance 100 M R or more) and, in gencral, a

polarization indcx of

2.0

or more

low partial discharge lcvels, and if prcvious partial discharge tests

wcrc performed, no significant increasc in partial discharge

activity

no significant incrcasc in dissipation factor and/or tip-up valucs

from those obtaincd in previous tests

A dctailcd asscssment may

bc

prudent if onc

or

a combination of the above

tcst results arc not achicvcd. Sincc of'[-line tests may not detect aging

during the ca rly svagcs or dcterioration, it is important to appreciate that

such tests cannot givc complete assurance

of

long term reliability. For this

reason, periodic (c.g., cvery 5 ycars) detailed assessmcnts should be m ade.


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550

NO

RETNNINO

0

R

MdMYENT

SLOT WEDGE

UID

P M I K

OR

CRICKS

C l

VISUALNSPECTION

FOR GING

INSULATION

RESISTINCE

INDEX

TESTS

AW EPTABL

4

lNSULATION

pq

EST

mO W VO L TME

mURGE

ESTIMATE

R E W IR E DEPAIRS

AND COSTS

ASSESS ALL

REPAIRSAND

OSTS

R E P U R W

E P U C E

Figure

2 b)

Detailed assessment procedure for rotor windings

As

implied in Figure 1, a detailed assessment requires that the rotor be

removed

to

permit

a

visual cxamination of the groundwall insulation

condition in the slot to dctermine if the insulation is puffy, discolored or

has been eroded. In the cndwinding

area,

there should be no signs of

clcctrical tracking,

or

insulation powdering

as

a result of partial

discharges. Partial discharge tests can help localize the most severe

dctcrioration sites. If partial dischagc tests can not be performed easily,

individual groups of bars can be isolatcd to permit dissipation factor tip-up

testing.

Slot Wedging

Where on-line partial discharge testing is fcasible,

a

comparison of

the

output signals obtained with various loads on the machine

[2]

makes it

possible, with expcricncc, to identify

loose

slot wedging

and

packing.

Whcn loose slot wcdging and/or packing is detectcd by such techniques, a

dctailcd assessment is required to determine the cxtcnt of such

deterioration and the necessary corrective action.

For a detailed assessment of the slot wedging condition, the machine must

bc

disassembled and carefully examined visually for abrasion, cracking,

migration, fretting, or powdery residues from relative movement betwecn

slot

components. In addition

to using the

visual symptoms, the following

tcsts [2]will check

for

coil tighincss in the slot for large machines:

stator wedge "tap"

t a t to

sec

i f

some wedges are loose

(commercial wcdgc tighincss

tcstcrs

could also be employed)

a side-clcnnncc

test

pcrfoonncd with a fceler gauge

to

determinc if

the bars arc tightly hcld against the sidc of the

slot

a rcsistance tcst to dcicrminc the contact resistance

of

thc bar to

thc iron.


6/7

The condition of the slot wedging and packing can he considered

;icccptahlc i f thcrc arc no visual symptoms

of

serious looseness, and the

test rcsults arc acccptahlc. Detailed assessments are recommended every

live years, or sooner,

i

on-line partial discharge monitoring indicates

cvidcncc of bar looscncss.

Endwinding Hrac ing

At present there are two available approaches for

a

of cndwinding bracing: (1) on-line endwinding vibra

gives

a

general indication,

(2)

impact resonance tests and a visual

inspection that require the machine

io be

disassemhlcd.

The k s t means of cffcctivcly assessing the condition of the cndwinding

bracing in an operating machine is to install vibration sensors on the

cndwindings and monitor their outputs. The present state-of-the-art

approach is to install a few strategically located non-metallic transducers

on the cndwindings of a machine with a bracing system that is in good

condition. Significant increases in the vibration levels over time may

indicate that thcrc is substantial dctcriordtion in the bracing system,

requiring a detailed assessment lo determine ihc cause.

Access to the cndw inding area is needed

to

perform

a

detailed assessment.

Although fairly sophisticated test equipment and expertise is required,

cndwinding vibration signature analysis is an effective means

of

detecting

dctcrioralion in large gcncl.ators. Signilican t changes in resonant

Ircqucncics bctwccn successive tcs ts can idcntily

the

prcscncc of aging in

its carly stiigcs.

The endwinding bracing sysicm is c;ipablc

of

providing acccptable support

to the stator windings for at least the next few years if the following

critcri;i arc met .

thcrc arc no signs of broken, cracked, loose or dislodged

components.

thcrc is no cvidcncc

of

dusting from abrasion or partial discharge,

or cracking 1 comp onent intcrCnccs.

. thcrc h as bccn no signilicanl change in endwinding structural

rcsonani frcqucncics and nonc of llicsc f'rcqucncics is close io the

predominant forcing lrcqucncics o f 120 Hz and rotational speed.

Signs of minor deterioration arc often difficult io assess, especially in thc

case of hairline cracks at component interfaces. Therefore, some

cxpcricncc is rcquircd

to dctcrminc

whcthcr any corrective action is

required when minor deterioration is

detected.

ASSESSMENT

OF

ROTOR INSULATION CONDITION

The importance

of

keeping turbine generator outages to

a

minimum has

led

to

the develo pment of fairly reliable on-line and off-line techniques to

detect faults in rotor windings. Unfortunately there are no effective on-

line or off-line assessment method s ihat will detect deterioration prior to

failure. The presen ce of aging in its early stages can therefore only bc

obtained by

a

direct visual examination of the insulation system.

In most

cases this requires machine disassembly and rotor removal. Figure 2

outlines the procedu re for assessing turbine generator rotor windings.

Both turn-to-turn shorts and ground insulation failures can be detected

from on-line monitoring. Rotor tum insulation failures are indicated by

the following symptoms:

variations in rotor shaft and/or bearing housing vibration levels

with varying excitation current, while holding the terminal voltage

approximately constant,

unusual magnetic flux patterns in

the

air gap, which can be

detected by air gap search coil measurements during normal

machine operation [2].

Thc most dclinitc indiciition of thc prcscncc and numhcr

of

short circuited

turns is given by iiir gap sciii-ch coil mc;isurcmcnts. A machine

fitled

with

such coils allows the rotor t u rn insulation to he monitored to determine

whcthcr the number

of

tum shorts is increasing. If an air gap search coil is

not l i l t ed or on-line search coil measurements are not conclusive, then it

may

bc

necessary to pcrlonn an off-line test to vcrify the prcscncc ofturn-

io- urn shon circuiis. The urgency with which corrective action should

be

taken depends on

th e

number

of.

short-circuited turns and ,

to some

extent,

the cause. If. only a few turns arc shon cd, it may he possible to run the

machine until the next scheduled outage, if no additional shorts develop

within

this

time and if the shaft and bearing vibration levels and the rotor

temperature continue to be satisfactory.

The prcscncc of rotor winding ground faults can

bc

detected by monitors

that arc fitted on most turbine generators. A single ground fault does not

usually result in a need for immediate shutdown unless

past

expcrience

indicates that this is required

to

reduce consequential damage.

Even if

immediate shutdown is not considered nccessary, the machine should be

made available for off-line tests and

a

possible detailed assessment of rotor

insulation condition at the earliest opportunity. The major concern is that

;I second ground Pault, with resulting severe rotor forging or endwinding

retaining ring dam age, may develop.

Rotor tum-to-turn short circuits and ground faults can often

be

verified by

off-line tests. Some of thcsc icsts may also detect general deterioration of

ground insulaiion. Off-line tests listed below and summarized in [21, are

available:

-

an open circuit

test to

mciisurc generator output voltage versus

licld

current

;I roior winding impcdancc test with rotor installed

- 3 iimc dom ain rcllcctomciry (TDR) surge test for shorted tums

an air gap llux test with

the

stator winding open- or short-

circuited.

f thc

rotor is

llttcd

with sliprings that allow the measurement of

impcdancc o r surge tcsring w hile

the

rotor speed is varied, then the imp-

cdancc

and

TDR

tests

will give more reliable resulis. Also, the use

of.

an

air gap search coil with the stator winding short circuited and excitation

applied

to

the licld winding

can

provide i more dcfiniic indication of

shorted turns. shorted turns arc confirmed by one or more of thcsc tests,

then

a

detailed assessment will be required to determine the likely cause

of

the

shorted

turns, their localion and the condition of the remaining turn

insulation . Note that

some types of tum

short circuits may disappear when

the machine is

shut

down.

I

the rotor winding Icads arc readily acccssihlc through sliprings, or by

disconncction, some g eneral indicators of the ground insulation condition

can bc obtained from insulation resistance, polarization index and hipot

tests.

When

one or all 1 he above tests indicate deterioration or failure of the

ground insulation, a detailed assessment of insulation condition should he

conducted. The rotor must he removed and ai least partiaUy disassembled

to allow sullicicnt access for a dchilcd assessment

of

turn insulation,

ground insulation,

slol

wcdging and cndwinding bracing condition.

f a failure o f thc tuin insulation

has

heen indicated hy on-line monitoring

and/or oll-line

tests,

then the location can he confirmed by one of the

following tests 121, and m a y also verily an unconfirmed turn fault.

low voltage ilc

test

low voltagc dc tcst

surge

test


7/7

552

The

ac

and dc tests measure the voltage drops across the various

turns,

and 2. Culbcrt,

I .M . ,

Dhirani, H., and Stone, G.C., "Handbook to

thus permit localization of any fault. If the repair of a fault and/or a

Assess thc Insulation Condition of Largc Rotating Machines",

detailed assessment

of tum

insulation condition is required, the rotor EPRI EL-SO36, Volum c 16, Junc 1989.

endwinding retaining rings and so me of the winding slot wedges must be

removed to allow a visual examination for signs of discoloration, burning, Stone, G.C., et

al.

"The Ability of Diagnostic Tests to Estimate

abrasion, buckling, copper

rub

marks

or

copper dust.

Removing the the Remaining Lifc of Stator Insulation", IEEE Trans EC, Dec

3 .

retaining rings may create new problems.

1988, p833.

The condition of the groundwall insulation can only be roughly assessed

with insulation resistance or polarization index tests.

A

thorough visual

examination for the signs of deterioration described above is required.

4.

Culbcrt, I., et al, "A Mcthod to Estimate the Insulation Condition

of High Voltage Stator Windings", Proc

1989

IEEE Electrical

Insulation Confcrencc, Chicago, October 1989.

CONCLUSION

Flowcharts have been presented which will indicate to the nonexpert the

information, testing and cxperience required to assess the condition of the

insulation systems used in turbine generator rotor and stator windings.

This process is intuitively followed by many experts. Exam ples of the use

of the flowcharts are in Rcferencc [2]. Although great progress has been

madc in developing diagnostic tests to evaluate insulation condition, there

is still no substitute for a carcful visual examination by an experienced

cngineer. This is especially important for rotor windings where few tests

arc available to assess condition. The flowc harts illustrated in this paper

may aid in distinguishing between good, slightly deteriorated and severely

deterioratcd windings. The prccision of the condition assessm ent depends

on the expertise and expcrien ce of the engincer. Unfortun ately, firm

cstimates of the remaining winding life are not yet possible due to the wide

range of insulation failure mechanisms which can occur, and the lack of a

diagnostic test sensitivc to all thcse mechanisms.

ACKNOWLEDGEMENT

This work was sponsorcd by EPRI project RP2577-1, Mr. B.S. Bemstein

projcct manager. The authors would like to thank J.C. Botts, D.

Harrington, J. K apler, L. Brdun, H. Sedding,

R.

Dal Mina, B. Lloyd and

many others who providcd useful comments on the flowchart procedure.

REFERENCES

1. IEEE Standard 56-1977, "Guidc for Insulation Maintenance of

Large Allcm aling Current Roldling Machines.

BIOGRAPHIES

G.C. Sto ne graduated from the University of Waterloo with a B.A.Sc.

and M.A.Sc. in electrical engineering in 1975 and 1978, respectively.

Since joining the Research Division

of

Ontario Hydro in 1975, he has been

active in developing test methods for insulation systems. Greg Stone has

participated in the creation of several IEEE Standards. He is presently

President

of the

IEEE Dielectrics and Electrical Insulation Society. He is a

registered professional engineer in Ontario, Canada.

I.

Culber t

He

received a B.S. Honour Degree in Electrical Engineering from Dundee

College of Technology in 1965. From 1966 to 1977 he worked as an

induction motor designer.

In this

time period he spent

7

years with Parson

Peebles in Edinburgh, Scotland and 4 years with Reliance Electric in

Stratford, Ontario, Canada. In 1977 he joined Ontario Hydro as a M otors

and Small Generators Specialist and his current title is Design Engineer

Specialist - Generators, Motors and Exciters. Mr. Culbert is a registered

professional engineer in Ontario, Canada.

was

bom in

Perth, Scotland on February 10, 1943.

H.

Dhirani was

born

n Tanzania in 1945 and received his B.E. degree in

Electrical Engineering from the University of Poona, India in 1969. He

came to Ontario Hydro in

1978 after having worked in the utility,

consulting and construction fields.

Mr.

H. Dhirani is currently involved

with the analysis and application of large generators for hydraulic, thermal

and nuclear stations. M r Dhirani is a registered professional engineer in

Ontario.

flowchart procedure to assess the condition

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