IEC 60034 27 4 2018

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Rot at ing elect rical machines –

Part 27-4: Measurement of insulat ion resist ance and polariz at ion index of

w inding insulat ion of rot at ing elect rical machines

Machines élect riques t ournant es –

Part ie 27- 4: Mesure de la résist ance d’ solement et de l index de polarisat ion sur

le syst ème d’ solat ion des enroulement s des machines élect riques t ournant es

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R ot at ing elect rical machines –

Part 27-4: Measurement of insulat ion resist ance and polariz at ion index of

w inding insulat ion of rot at ing elect rical machines

Machines élect riques t ournant es –

Part ie 27- 4: Mesure de la résist ance d’ solement et de l index de polarisat ion sur

le syst ème d’ solat ion des enroulement s des machines élect riques t ournant es

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FOREWORD 5

INTRODUCTION 7

1 Sco e 8

2 Normative ref eren es 8

3 Terms an def i ition 8

4 In ulation resistan e – comp nents an influen e f actors 10 5 Polarization in ex 10 6 Me s rement 1

6.1 Influen es on the me s rement of the in ulation resistan e 1

6.1.1 General 1

6.1.2 Win in temp rature cor ection 1

6.2 Me s rin eq ipment 12 6.3 Test o ject an me s rin circ it 13 6.3.1 General 13 6.3.2 Thre -phase stator win in s 13 6.3.3 Other win in s 14 6.4 Me s rin voltage 15 6.4.1 Type an mag itu e 15 6.4.2 Polarity 15 6.5 Me s rin time 15 6.6 Safety 15 6.7 Me s rement proced res 15 6.7.1 Stan ard proced re 1

5 6.7.2 Sp cial proced res 16 7 Interpretation of me s rement res lts 16 7.1 General 16 7 2 Suita i ty for testin an o eration 1

6 7.3 Tren in of in ulation con ition 17 7.4 Comp rison b twe n mac ines or b twe n phases 17 7.5 Ef fects at very hig values of in ulation resistan e 17 7.6 Limitation of the in ulation resistan e test 17 8 Recommen ed l mits of in ulation resistan e an p larization in ex 18 8.1 General 18 8.2 In ulation resistan e 18 8.3 Polarization in ex 18 9 Test re ort 19 9.1 Op rational aged win in s 19 9.2 New windin s 2

An ex A (informative) Comp nents of the direct c r ent 21

A.1 Total c r ent I T 21

A.2 Ca acitive c r ent I C 21

A.3 Con u tion c r ent I G 2

A.4 Polarization c r ent I P 2

A.5 Sur ace le kage c r ent I L 2

An ex B (informative) Gra hical estimation of the slo e p rameter X for temp rature

cor ection fom me s rement data 2

An ex C (inf ormative) Examples of test res lts of s nthetic resin based hig voltage win in s 2

C.1 Mac ine with dry an cle n s race of the in ulation 2

C.2 Mac ine with a wet an contaminated s r ace 2

C.3 Mac ine with contin ou stres control layers in galvanic contact with hig voltage con u tors 2

C.3.1 Stres control co tin c r ent I S 2

C.3.2 Ef fects on in ulation resistan e an p larization in ex 3

C.3.3 Examples of test res lts 3

An ex D (inf ormative) Me s rement of le kage c r ent to as es interphase in ulation resistan e 3

An ex E (informative) Other DC tests 3

E.1 General 3

E.2 Dielectric a sorption ratio (DAR) 3

E.3 Monitorin c arge an dis harge c r ents 3

E.4 Hig voltage DC tests 3

E.4.1 General 3

E.4.2 Uniform-time voltage ste test 3

E.4.3 Graded-time voltage ste test 3

E.4.4 Ramp d-time voltage ste test 3

E.5 Wet in ulation resistan e me s rement 3

Biblogra h 3

Fig re 1 – Eq ivalent circ it diagram of win in in ulation in a DC voltage test 10 Fig re 2 – Con ection for testin of the entire win in 14 Fig re 3 – Con ection for phase-to-e rth me s rement 14 Fig re A.1 – Relation hips b twe n dif ferent c r ents an time 21

Fig re B.1 – Gra hical estimation of the slo e p rameter X in a semi-logarithmic diagram 2

Fig re C.1 – Total c r ent vers s time on a cle n an dry in ulation The s ales are logarithmic 2

Fig re C.2 – In ulation resistan e vers s time on a cle n an dry in ulation 2

Fig re C.3 – Total c r ent vers s time on a wet an contaminated in ulation 2

Fig re C.4 – In ulation resistan e vers s time on a wet an contaminated in ulation 2

Fig re C.5 – Total c r ent vers s time on a dry an cle n s rf ace with a contin ou stres control co tin 3

Fig re C.6 – In ulation resistan e vers s time on a dry an cle n s r ace with a stres control co tin 31

Fig re D.1 – Con ection f or phase-to-phase me s rement The test in trument s al b flo tin with resp ct to e rth Other phase to phase combination are p rmited 3

Fig re D.2 – Me s rement of interphase le kage c r ent with a me s rement in trument eq ipp d with a g ard con ection 3

Fig re D.3 – Me s rement of interphase le kage c r ent with a me s rement in trument not eq ip ed with a g ard con ection 3

Fig re E.1 – Me s rement of c r ent an in ulation resistan e that res lts in a DAR

Fig re E.2 – Charge an dis harge c r ents af ter a ste voltage of 2,5 kV f or the

thre -phase win in s of a 5 MVA h dro-generator: 3

Ta le 1 – Values of the p rameter X for the temp rature cor ection 12

Ta le 2 – Guidel nes for DC voltage mag itu es to b a pl ed d rin the in ulation

resistan e me s rement 15

Ta le 3 – Recommen ed minimum in ulation resistan e values at a b se temp rature

of 4 °C 18

Ta le 4 – Recommen ed minimum values of p larization in ex f or hig voltage

in ulation s stems 19

Ta le B.1 – Example data fom in ulation resistan e me s rements at diff erent

win in temp ratures 2

INTERNATIONAL ELECTROTECHNICAL COMMISSION

_

ROTA TING ELECTRICA L MACHINES –

Part 27-4: Measurement of insulation resistance and polarization

index of winding insulation of rotating electrical machines

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International Stan ard IEC 6 0 4-2 -4 has b en pre ared by IEC tec nical commite 2:

Rotatin mac inery

The text of this International Stan ard is b sed on the folowin doc ments:

Ful inf ormation on the votin for the a proval of this International Stan ard can b f ou d in

the re ort on votin in icated in the a ove ta le

A l st of al p rts in the IEC 6 03 series, publs ed u der the general title Rotat in el ectric l

ma hines, can b f ou d on the IEC we site

NOT A ta le of cro s-ef ere c s of al IEC TC 2 p blc tio s c n b f ou d in th IECTC 2 d s b ard o th

IEC we site

The commite has decided that the contents of this doc ment wi remain u c an ed u ti the

sta i ty date in icated on the IEC we site u der "htp:/ we store.iec.c " in the data related to

the sp cific doc ment At this date, the doc ment wi b

• with rawn,

• re laced by a revised edition, or

• amen ed

IMPORTANT – The 'colour inside' logo on the cov r pa e of this publ c tion in ic te

that it contains colours whic are consid re to be us f ul f or the cor e t

understa din of its conte ts Us rs s ould th ref ore print this doc me t using a

colour printer

This doc ment provides g idel nes for me s rement of the in ulation resistan e an the

p larization in ex on stator an rotor win in in ulation of rotatin electrical mac ines The

doc ment also des rib s typical in ulation resistan e c aracteristic , the ef fect of influential

f actors whic imp ct or c an e these c aracteristic , an how these c aracteristic in icate

win in con ition It recommen s minimum ac e ta le values of in ulation resistan e for AC

an DC rotatin mac ine win in s Interpretation wi de en on the nature of the in ulation

materials – sp cifical y if the in ulation is of the thermoset or thermo lastic typ

In ulation resistan e me s rement has b en recommen ed an u ed f or over 5 ye rs to

evaluate the con ition of electrical in ulation It is recommen ed to trac p riodic

me s rements, ac umulated over month an ye rs of service or in con ection with servicin

an overhaul of rotatin mac ines

Empirical l mits verif ied in practice can b u ed as a b sis for evaluatin the q al ty of stator

win in in ulation s stems in man facturin Furthermore, tren evaluation, e.g diag ostic

tests as p rt of the f un tional evaluation of in ulation s stems or in con ection with servicin

an overhaul of rotatin mac ines, can also provide information on agein proces es,

p s ible re air o tion an the recommen ed time interval b twe n tests These

me s rements give no in ication of local we k p ints in the in ulation s stem an the tren

evaluation can ot b u ed to predict the time to faiure of the win in in ulation

ROTA TING ELECTRICA L MACHINES –

Part 27-4: Measurement of insulation resistance and polarization

index of winding insulation of rotating electrical machines

This p rt of IEC 6 0 4 provides recommen ed test proced res for the me s rement of

in ulation resistan e and polarization in ex of stator an rotor win in in ulation of

rotatin electrical mac ines

This doc ment recommen s minimum ac e ta le values of in ulation resistan e an

p larization in ex of win in in ulation val d for f ul y proces ed low an hig voltage AC an

DC rotatin electrical mac ines with a rated p wer of 7 0 W or hig er

The f ol owin doc ments are ref er ed to in the text in s c a way that some or al of their

content con titutes req irements of this doc ment For dated referen es, only the edition

cited a ples For u dated ref eren es, the latest edition of the ref eren ed doc ment (in lu in

• IEC Electro edia: avai a le at ht p:/ www.electro edia.org/

• ISO Onl ne browsin platform: avai a le at htp:/ www.iso.org/o p

3.1

rate volta e

<f or an electric mac ine rated lne-to-l ne voltage for a thre -phase AC mac ine, l n

e-to-e rth voltage for a sin le phase mac ine an rated direct voltage for DC mac ines or f ield

<f or an electric mac ine ca a i ty of the electrical in ulation of a win in to resist direct

c r ent an is determined by the q otient of the a pl ed direct voltage divided by the total

c r ent acros the mac ine in ulation, taken at a sp cified time t f rom start of voltage

a pl cation

Note 1 to e try: Th v lta e a plc tio time is u u ly 1 min (R

i1) a d 10 min (R

i10); h we er oth r v lu s c n b

u e Unit c n e tio s: s b cript v lu s of 1 thro g 10 are a s me to b in min te , s b cript v lu s of 5 a d

gre ter are a s me to b in s c n s

c r ent res ltin fom p larization proces es, whic decay with time of DC voltage

a pl cation at a decre sin rate f om an initial value to es ential y zero

Note 1 to e try: Th p lariz tio c re t is als c le a s rptio c re t

ohmic c r ent that is con tant with time an p s es over the s race of the en win in s of

the stator win in or b twe n exp sed con u tors an the rotor b d in in ulated rotor

win in s if there are de osition of con u tive materials, e.g moisture or contamination

3.7

c pa itiv c r e t

I

C

<for an electric mac ine c r ent of comp ratively hig mag itu e an s ort d ration

( ypicaly < 1 s), whic decay exp nential y with time of DC voltage a plcation

3.8

stre s control coating c r e t

I

S

ohmic c r ent that is con tant with time, flowin in p ral el to the s r ace le kage c r ent

throu h a contin ou stres control co tin on the s race of the en win in in ulation

b twe n con u tor an e rth

in ulation resistan e meter le d are reversed

Note 1 to e try: This is o s rv d wh n h midity is pre e t in th in ulatio It is c u e b a p e ome o k own

4 Insulation resistance – components and influence f actors

The in ulation resistan e of a rotatin mac ine win in is a f un tion of the typ an con ition

of the in ulatin materials, the in ulation s stem desig an the tec niq es u ed to

man facture the win in

The in ulation resistan e is me s red with DC voltage The me s rement of the resistan e

over time provides inf ormation on c r ent comp nents cau ed by diff erent ph sical

mec anisms Fig re 1 is a s hematic s owin the dif ferent direct c r ent comp nents

Inf ormation on the variou c r ent comp nents is provided in An ex A

5 Polarization index

The p larization in ex is the q otient of the in ulation resistan es me s red at two dif ferent

times t = 1 min an t

0

= 10 min af ter a pl cation of direct voltage:

1i

10i

RR

Variants of the p larization in ex definition with a q otient of in ulation resistan e of other

me s rin times may b u ed in sp cial a pl cation an ne d to b in icated (se An ex E)

More me s rement p ints d rin the 10 min interval may yield ad itional inf ormation

The p larization in ex des rib s the variation of the IR b twe n two sp cif i p ints in time

an therefore, b t er than with a sin le in ulation resistan e value, it may in icate

contamination an /or moisture de osition on the win in , or a sorb d moisture in the

win in However, it may not in icate internal void cau ed by impro er impreg ation or

thermal deterioration

The p larization in ex can b u ed to estimate the s ita i ty of the win in f or a plcation of

a voltage with tan test or f or o eration It may provide inf ormation f or as es in the

con ition of the in ulation s stem

Owin to the negl gible p larization c r ents in the time interval f rom 1 min to 10 min, the

determination of the p larization in ex may not a ply for smal mac ines with ran om-wou d

win in s, f or the field win in s in generator rotors, for non-in ulated f ield an s uir el-cage

The p larization in ex de en s on the typ of the in ulation s stem, esp cial y on the nature

of the in ulation materials an proced res u ed for win in man facture (for s nthetic resin

b sed or s elac- an asphalt b sed, se 7.1) Furthermore it dep n s on the kin of stres

control co tin (see Clau e C.3) an the mag itu e of the test voltage (se 6.4.1) The

influen e of the temp rature on the p larization in ex is not sig ificant u der the con ition

that the win in temp rature is con tant b twe n the 1 min an 10 min re din s of the

in ulation resistan e (se 6.1.2)

Before a win in is recommen ed for a voltage with tan test or f or o eration, the

p larization in ex s ould have a minimum value (se recommen ation in 8.3)

6.1 Inf lue c s on the me s reme t of th ins lation re ista c

6.1.1 Ge eral

The resistan e me s rement res lt de en s on en ironmental f actors, mainly on the win in

temp rature an on the h midity content of the air The win in temp rature in uen e can b

o tained f rom empirical data or an exp rimental me s rement an u ed for the cor ection of

me s rement res lts taken at dif ferent temp ratures (se 6.1.2)

The air relative h midity af fects the s r ace le kage c r ent an can u ual y not b estimated,

as its ef fect further de en s on the air temp rature, s rf ace pro erties of the in ulation an

the nature of an s rf ace contamination F r this re son it is general y recommen ed to

p rorm in ulation resistan e me s rements at win in temp ratures a ove the dew p int

6.1.2 Win ing temperature cor e tion

The variation of temp rature aff ects al of the identified c r ent comp nents, ex e t the

ca acitive c r ent I

C, b cau e an in re se in temp rature s p l es thermal energ , whic

f e s ad itional c arge car iers an so red ces resistivity Therefore the in ulation resistan e

value of a win in de en s on the win in temp rature

To alow a comp rison of in ulation resistan e values o tained at diff erent temp ratures it is

recommen ed that al IR values me s red be cor ected to a common b se temp rature of

for a s nthetic resin b sed in ulation s stem is me s red at a temp rature les than

4 °C, then no cor ection is ne ded [4] Otherwise the cor ection factor is calc lated u in

Formula (2):

XT

K

−

=4

T5

Where

4 is the b se temp rature (°C);

T is the win in temp rature (°C);

X is the slo e p rameter for an in ulation s stem (K)

Formula (2) is b sed on Formula (A.3), takin into ac ou t al relevant c r ent comp nents

NOT 1 This f ormula e pre s s th t th IR is re u e b h lf , if th win in temp rature T in re s s b X Kelvin

Th s me empiric l relatio c n b e u ly e pre s d b e p n ntial fu ctio s with oth r b s s, lk e Th slo e

p rameter c n b dire tly tra sf orme , in c s of a b sis e b dividin X with -ln(0,5)

i

at the b se temp rature is o tained by multiplyin the resistan e value me s red at a

win in temp rature T with the cor ection factor K

T(F rmula (3) :

T

Ti

ci

KR

is the temp rature cor ection factor

The slo e p rameter X in Formula (2) c aracterizes the degre of in ulation resistan e

temp rature de en en y of an in ivid al in ulation s stem Prefera ly, this p rameter is

estimated exp rimental y The recommen ed method

is by p rf ormin

IR me s rements at

sev eral win in temp ratures in the exp cted ran e where me s rements may be made,

in lu in 4 °C, al a ove the dew p int, an plot in the res lts on a semi-logarithmic s ale

From the res lt of an exp nential a proximation the slo e p rameter Xcan b derived An

example for the proced re is given in An ex B If exp rimental data are not avai a le for an

in ulation s stem, the values for X in Ta le 1 can b u ed, Ta le 1 is b sed on empirical

data, an there is no a p rent re son for the dis ontin ity at 4 °C

The temp rature cor ection with an exp nential a proximation by eq ation 2 an 3 can

cau e sig ificant er ors with an in re sin diff eren e b twe n win in temp rature an b se

temp rature It is recommen ed to a ply this method only for a win in temp rature ran e as

given in Ta le 1, whic is derived f rom exp rimental me s rements

NOT 3 If dif fere t in ulatio s stems are u e in th slot a d th e d win in re io s, th n it is th in ulatio

s stem in th slot re io th t is rele a t f or temp rature c re tio

Table 1 – Value of the parameter X f or the temperature cor e tion

T pe of insulatio system Slo e parameter X

These values are b sed on exp riments an are con idered to b a con ervative a pro c ,

i.e minimum values Typical y the temp rature de en en y (F rmula 2) is smal er, i.e the

slo e p rameter is hig er

For the estimation of the p larization index PI, the temp rature cor ection is not req ired as

the diff eren e in win in temp rature d rin the me s rement of R

re din s b low 5 0 0 MΩ, a digital in trument s ould have at le st the fol owin

c aracteristic :

• Display: 3 digits

• Ac urac : ± 5 % of re din , ± 5 digits

If no in ulation resistan e meter is avaia le, the in ulation resistan e can b o tained f rom a

a sta i zed DC voltage source, a voltmeter an a micro ammeter can b u ed The voltage

f l ctuation of a re l DC voltage source wi introd ce a variation of i

c(t) = C

0

dU

0/d Sin e the

ca acitan e C

0

of most hig voltage mac ines is large, a minimum sta i ty an noise is

req ired for the DC s p ly to neglect this eff ect The in ulation resistan e is calc lated fom

the volt- an ammeter re din s u in Formula (4)

t

ti

IU

is the in ulation resistan e (MΩ) at time t;

U is the me s red voltage (voltmeter re din ) of the DC voltage source (V);

I

t

is the me s red c r ent (ammeter re din ) (µA) at time t

For the me s rement of hig IR values a meter with g ard o tion is recommen ed, to avoid

le kage an ca acitive influen es f rom the me s rin ca le

The in trumentation s al take no more than 5 s to re c the test voltage

6.3 Te t obje t a d me s ring circ it

De en in on the aim of the test an the desig of the test o ject, diff erent me s rin circ its

a ply For c ec in the recommen ed minimum IR the test s al b p rormed on the entire

win in In order to c ec f or in ulation pro lems on e c phase win in an b twe n phase

win in s, me s rements s al b p r ormed phase by phase if e c phase win in can b

e si y dis on ected fom one another For tren in purp ses, the same con ection s al

alway b a pl ed

If p s ible, external elements s c as ca les, switc es, ca acitors, c r ent tran f ormers, etc

s al b dis on ected f om the win in Items sti con ected to the win in ne d to b

recorded

To o tain in ulation resistan e me s rements on directly water co led win in s, the water

s ould b removed an the internal circ it thorou hly dried In some water co led win in s

the man f acturer may have provided a me n of me s rin the in ulation resistan e without

the ne d f or the co lant to be drained In general, if the water is not removed then the

con u tivity of the water s ould be les than what is recommen ed by the mac ine

man facturer In this case, the water cond ctivity wi largely dominate the in ulation

resistan e; an th s PI = 1 an R

i10

= 1 MΩ may b exp cted

In any case the win in elements that are not u der test s al b con ected with s ort le d

to mac ine e rth to avoid any u desira le ef fects, s c as eq al zin c r ents or AC c r ent

in u ed to test circ it

6.3.2 Thre -pha e stator windings

6.3.2.1 Conne tion f or me s reme ts of the e tire winding to e rth

Figure 2 – Conne tion for te ting of the e tire winding

6.3.2.2 Conne tion f or pha e- o-e rth me s reme ts

Testin e c phase win in se arately (se Fig re 3) is the pref er ed method f or tren in

purp ses an to ac ieve comp rative values f or the in ivid al phases The same circ it is

a pl ca le f or testin more than one phase win in to e rth, by con ectin the desired phase

win in s to b tested together an the others to e rth

In case the me s rin in trument is eq ip ed with a g ard o tion, in te d of con ectin the

remainin phase win in s to e rth they can b con ected to the g ard in order to el minate

the ef fects f rom c r ents b twe n phase win in s, l ke le kage an stres control co tin

c r ents Sin e this is a deviation f rom the stan ard proced re, it has to b noted in the test

re ort

Figure 3 – Conne tion for pha e- o-e rth me s reme t

6.3.2.3 Conne tion f or pha e- o-pha e me s reme ts

This test is not a stan ard me s rement proced re, however it may provide ad itional

inf ormation of a diagnostic nature Se An ex D

6.3.3 Oth r win ing

Other win in s, lke field win in s or a hig voltage rotor win in s al b con ected simi arly

to the phase-to-e rth me s rement of the stator win in (se 6.3.2.2)

Al elements that are not p rt of the me s rin circ it, s c as bru h rig in fom static

ex itation, ne d to b dis on ected f rom the test o ject e.g by l f tin or removin the

bru hes Rotatin diodes s al b brid ed, to avoid pro lems in the case of win in damage

d rin testin The same has to b done with p rmanently in tal ed monitorin eq ipment

IEC

IE C

6.4 Me s rin volta e

6.4.1 Type a d ma nitude

The me s rement of the in ulation resistan e req ires the a pl cation of a DC voltage The

voltage mag itu e s al b restricted to a value a pro riate for the voltage ratin of the

win in an the b sic in ulation con ition This is p rtic larly imp rtant in the case of low

voltage mac ines or wet win in s If the voltage mag itu e is to hig , it may overstres the

in ulation an le d to an in ulation fai ure Guidel nes f or voltage mag itu es are presented

in Ta le 2

Table 2 – Guidel ne for DC volta e ma nitude to be appl e

during the in ulation re ista c me s reme t

In ulation resistan e me s rements are u ualy con u ted at con tant DC voltage havin

negative p la ty Negative

p la ty is pref ered to ac ommodate the phenomenon of el

ectro-en osmosis in case of h midity in ide the in ulation s stem

6.5 Me s ring time

Re din s of the IR are taken after the test voltage has b en a pl ed for the sp cified time,

e.g after one min for in ulation resistan e R

i1

an after 10 min for in ulation resistan e R

i10

6.6 Saf ety

Due to the p larization eff ect, the win in in ulation wi maintain a c arge for a lon time

af ter the voltage s p ly has b en dis on ected It is imp rtant after the test to red ce the test

voltage to zero an to e rth the test o ject b fore the test in trument is dis on ected The

e rth s al remain a pl ed to the terminals of the phase or win in for at le st 4 times the

d ration of voltage a pl cation

6.7 Me s reme t proc dure

6.7.1 Sta dard proc dure

The test voltage s ould b a pl ed in tantane u ly an held con tant throu hout the test The

test voltage s al b re c ed in no more than 5 s The cou tin of the me s rin time starts

af ter re c in the sp cified test voltage level The re din s are taken af ter the sp cified

times There are dif ferent proced res for testin al win in phases at the same time an

me s rin one phase at a time in multiphase win in s

When me s rin al phases at the same time in a multiphase win in , or the win in in a

sin le phase win in , af ter test completion, the test voltage is red ced to zero an for safety

re son , the test o ject s ould b immediately s ort-circ ited, e rthed an dis harged for at

In multiphase win in s where e c phase is tested one at a time (with the other phases

e rthed), af ter the test on the first phase is completed, the e rth s al b a pl ed an remain

a pl ed f or at le st 4 times lon er than the d ration of voltage a plcation As lon as the first

phase remain e rthed, the secon an other phases can b tested in turn by con ectin the

test in trument to it with the other phases e rthed As an alternative, con ection can b

c an ed when the dis harge c r ent with the terminal e rthed is s ff i iently low, e.g.10 nA

When in ulation me s rement is an imp rtant asp ct of maintenan e, it is recommen ed to

c ec the o eration of the in trument u in hig voltage resistan es in the ran e of 10 MΩ

The c ec s ould b p r ormed b f ore an after the tests, an is esp cial y imp rtant f or

b tery o erated in truments

a) Estimation of the s ita i ty of a mac ine f or the a pl cation of voltage with tan test, or

for o eration, may b b sed on o tainin the recommen ed minimum values f or the

in ulation resistan e an /or the p larization in ex (se 7.2)

b) The in ulation resistan e an p larization in ex history of a given mac ine, me s red at

u iform con ition so f ar as the control a le varia les are con erned, is recog ized as a

u eful way of tren in some asp cts of the in ulation con ition over ye rs (se 7.3)

c) The in ulation resistan e an p larization in ex values of in ivid al phase win in s or of

diff erent mac ines of the same design, me s red at u iform con ition so f ar as the

control a le varia les are con erned, can b u ed to comp re the in ulation con ition

For these sp cif i purp ses a), b) an c) the me s rement influen es s al b con idered

(se 7.4)

7.2 Suitabi ity for te ting a d o eration

In ulation resistan e an p larization in ex may b u ed to estimate the s ita i ty of a

mac ine for the a plcation of a voltage with tan test or for o eration Mac ines s ould have

b th the in ulation resistan e an the p larization in ex a ove the recommen ed minimum

values (se Ta les 3 an 4) If the me s red values are b low the recommen ed minimum

values, a voltage with tan test an o eration of the mac ine are not recommen ed, u les

there is a desig re son or there is a p st history for s c low re din s

In ulation resistan e testin may also b u ed to determine if a win in has f ai ed a voltage

with tan test If the me s red values taken after a voltage with tan test are sig if i antly

lower than the values b fore the voltage with tan a pl cation, in ulation damage may have

oc ur ed d rin the voltage with tan testin , even if the test voltage did not col a se d e to

hig fault c r ents

If the me s red values are b low the recommen ed minimum values d e to dirt or ex es ive

moisture an an in ulation defect can b ex lu ed, cle nin an dryin the win in may

improve the me s red res lts The in ulation resistan e re din s can b u ed to monitor the

If the in ulation resistan e R

i1

at 4 °C is gre ter than 5 0 0 MΩ, the p larization in ex may

b ambig ou an can b disregarded

For s el ac an asphalt b sed in ulation s stems, a very hig p larization in ex (for example

gre ter than 8) may in icate that the in ulation has b en thermal y aged, an may have a ris

of f aiure If ph sical in p ction ( a pin on the in ulation, for in tan e) confirms that the

in ulation is dry an brit le, it is b st not to at empt cle nin or voltage with tan testin the

win in Fai ure may oc ur at an time if the mac ine is returned to o eration

7.3 Tre ding of ins lation condition

If the in ulation resistan e (R

i1

history of a mac ine is avaia le, comp rison of the present

test res lt with previou tests wi help to as es the in ulation con ition It is imp rtant,

however, to comp re tests u der simi ar con ition , that is, win in temp rature, voltage

mag itu e, test d ration an relative h midity (se 6.1) an , if p s ible, the u e of identical

test eq ipment For comp rison of tests con u ted at dif ferent win in temp ratures, the

res lts s ould b cor ected to the same b se temp rature (se 6.1.2)

A s arp decl ne in the R

i1

or PI f om the previou re din may in icate s race contamination

or moisture When a low PI oc urs at an elevated temp rature (a ove 60 °C), a secon

me s rement p rf ormed b low 4 °C, but a ove the dew p int, is recommen ed as a c ec

b twe n identical mac ines, may in icate s r ace contamination, moisture or in ulation

damage In s c a case, the re son for the diff eren e s ould b in estigated

7.5 Ef fe ts at v ry hig v lue of ins lation re ista c

De en in on the test voltage level an the con ition of the in ulation, the me s red c r ent

may not ex e d values in the s b-micro mp re- an e Any noise fom external sources, s c

as voltage flu tuation generated by the test in trument, but also temp rature flu tuation and

even electroc emical eff ects cau ed by moisture, may generate c r ents that ex e d the

c r ents throu h the in ulation False re din s are a con eq en e of s c

Esp cial y d rin lon er me s rements ses ion , the con ition can vary so that the res lts

may le d to misinterpretation

As general ad ice, the p larization in ex is not in icative of in ulation con ition when the

in ulation resistan e re c es values in the ran e of 5 0 0 MΩ an a ove

It is ad isa le to con ult the man al of the test in trument Al man facturers s ould publ s

the ac urac of the test in trument This may also help ju gin the res lts

7.6 Limitations of the ins lation re ista c te t

In ulation resistan e test data is u ef ul in evaluatin the presen e of some in ulation

pro lems s c as contamination, a sorb d moisture, or severe crac in ; however, some

l mitation are as fol ows:

a) In ulation resistan e of a win in is not directly related to its dielectric stren th an if the

in ulation s stem of a win ing wi fai in service The in ulation resistan e me s rements

give no in ication of the distribution of we k p ints within the in ulation, e.g large void

cau ed by impro er impreg ation an thermal deterioration

b) Win in s havin an extremely large en win in s r ace are , large or slow-sp ed

mac ines, rou d rotor field win in s or mac ines with commutators may have in ulation

resistan e values that are les than the recommen ed value In these cases, historical

tren in of in ulation resistan e is u eful in evaluatin in ulation con ition

c) Becau e in ulation resistan e tests are con u ted whi e a mac ine is at stan sti , these

tests wi not detect pro lems d e to rotation, s c as lo se coi s, or vibration le din to

en win in movement

8 Recommended l mits of insulation resistance a d polarization index

The fol owin in ulation resistan e an p larization in ex values are the lowest values at

whic the win in is recommen ed f or a voltage with tan test or f or o eration In some

cases, sp cial in ulatin materials or desig s may provide lower values (e.g at sp cial

desig s of stres control co tin s, se Clau e C.3) Minimum values f or s c mac ines

s ould b b sed on comp rison with historic data

The recommen ed values are not val d for complete win in s whic have not yet b en

cor ected to the b se temp rature of 4 °C

Table 3 – Re omme de minimum ins lation re ista c v lue

Hig v lta e s ela -a d a p alt b s d in ulatio s stems a d al f i ld win in s rate v lta e (kV) +1

Hig v lta e s nth tic re in b s d in ulatio s stems (f orm-wo n win in s) 10

If the minimum value of in ulation resistan e for the complete win in is not met, refer to

section 6.3.2.2 for ad itional me s rements that may b made For more inf ormation, ref er to

An ex D

8.3 Polarization in e

Ta le 4 provides recommen ed minimum p larization in ex values of an entire win in

These values are only val d for in ulation s stems of hig voltage mac ines, in whic a

c aracteristic p larization c r ent I

Pcan le d to a marked c an e in the re din s of the

in ulation resistan e at 1 min an at 10 min

If the win in temp rature do s c an e a precia ly b twe n the 1 min an 10 min re din s

of the in ulation resistan e, the values cor ected to the b se temp rature of 4 °C are to b

u ed for the calc lation of the p larization in ex

Table 4 – Re omme de minimum v lu s of polarization inde

f or high volta e ins lation s stems

T pe of hig v ltage insulatio system Minimum p lariz tio in ex P I

These recommen ed minimum values are not a plca le for:

• Smal mac ines with ran om-wou d win in s, mac ines with non-in ulated f ield win in s,

s uir el-cage rotor win in s an DC mac ine armatures (se Clau e 5)

• Mac ines with stres control co tin s havin galvanic contact with the win in

The test re ort s ould, as a minimum, provide the data, the test decision and, if u ed f or

tren in an comp rison of in ulation con ition , a statement on the interpretation of the

me s rement res lts The test re ort s ould contain the fol owin items:

• Identif i ation inf ormation

– Cal bration d e date

• Test circ it, in trument set in s an test con ition

– Test voltage an p larity

– Duration, if eq ired

• In ulation c aracteristic at me s rement con ition , an cor ected to the b se

temp rature:

– 1 min in ulation resistan e

– 10 min in ulation resistan e, if a pl ca le

– Polarization in ex, if a pl ca le

– Temp rature cor ection, if a plca le

• Gra hical display of me s rement c rves, if a pl ca le

– Me s red c r ent v time, pref era ly in a log-log diagram

– Me s red in ulation resistan e v time, prefera ly in a log-log diagram

• Name of test p rson el

• Doc ment sp cific information

• Comment section e.g for deviation f om stan ard test proced re, o servation , etc

It is ad isa le to store the me s rement res lts in the most original data-ormat for f uture

referen e

De en in on the typ of doc ment, stan alone test certificate or p rt of a con ition

as es ment re ort, the content of the me s rement rep rt may b s p lemented by

ad itional inf ormation, e.g op rational mode, mac ine age, etc

9.2 New windin s

The in ulation resistan e me s rement for new win in s may b a s p lementary p rt of the

hig voltage with tan test, with a 1 min in ulation resistan e test b fore an af ter the hig

voltage with tan test

Con eq ently, most of the inf ormation req ired f or complete re ortin is p rt of the with tan

test protocol A selection of the req ired information defined in 9.1 may b s ff i ient

The total c r ent is the s m of al c r ents cau ed by the a pl cation of the DC test voltage in

the test o ject It is avai a le for me s rement in the le d b twe n voltage source an test

o ject (Fig re 1)

The general time c aracteristic of the total c r ent an the contribution of the comp nents

are s own in Fig re A.1 Examples for sp cif i in ulation con ition are given in An ex C

Fig re A.1 – Relations ips betwe n dif fere t c r e ts a d time

A 2 Capacitive c r ent I

The ca acitive c r ent is cau ed by c argin or dis hargin the ca acitan e of the win in

via the circ it resistan e, con istin of the internal resistan e of the voltage source an the

le d resistan es The ca acitive c r ent i

c( ) de en s on time ( ) in secon s ac ordin to

Formula (A.1)

0m

e)

(

m0

c

CRt

RU

ti

When a voltage ste of ∆U

fu ction is the prod ct of the resistan e R

m

an the ca acitan e C

0 Even for large win in s

with a ca acitan e in the ran e of microf arad , the time con tant would not ex e d one

secon or so with typical me s rement in truments havin a relatively low R

m Therefore the

ca acitive c r ent has to s ort a d ration to af fect the me s rement of the in ulation

resistan e an the p larization in ex

A 3 Conduction cur ent I

G

The con u tion c r ent is cau ed by directed movement of electron an ion in the electric

f ield At low field mag itu es, con u tion is predominantly cau ed by ion At hig f ield

mag itu es, when a pro c in the dielectric bre k own stren th, electron in re sin ly

contribute to con u tion

Given that R

m

< R

V, the con u tion c r ent I

G

of an in ulation volume element b twe n

p ral el electrodes is given by Formula A.2):

VG0

RU

R

⋅

=

VV

κ(Ω);

The con u tion c r ent throu h the in ulation bulk de en s on the dielectric material, the

electric field mag itu e, the temp rature an the moisture content of the in ulation The

sp cific volume con u tivity is not very de en ent on the electric field mag itu e, as lon as

the stres d rin me s rement is wel b low the bre k own stren th The temp rature may

not have a sig ificant ef fect if the activation energ of the in ulation material is low, in whic

case the activation energ de en s on the mo i ty of ion an electron in the in ulation

material with temp rature ac ordin to:

TkW

is the activation energ (eV);

k is the Boltzman con tant (eV/K);

T is the a solute temp rature (K)

The volume con u tivity, an theref ore the con u tion c r ent, exp nential y de en s on the

activation energ an the temp rature (se Formula A.2 an A.3)

The presen e of h midity within the in ulation stru ture in re ses the con u tion c r ent d e

to the con u tivity of the water itself an by an in re sin n mb r of ion f rom the

in re sin h midity content the sp cif i volume con u tivity can in re se by several orders of

the f ield, cal ed relaxation, req ires time, whic de en s on the inertia in erent to the

mec anism In the eq ivalent circ it the diff erent mec anisms are re resented by R-C series

combination (Fig re 1, in ices 1,2…n) The relaxation time con tants t

vary in a wide ran e, f om extremely fast proces es l ke the electron p larization up to

relatively lon d ration for slow proces es l ke the inter acial p larization, with time

con tants ran in f rom mi isecon s to hours

Given that the internal an le d resistan es are of negl gible influen e (R < R

j), the

p larization c r ent I

P(t) can b generaly expres ed as a f un tion of time (t) by

Formula (A.4)

jt

n

jjRU

ti

10P

)(

R ⋅

=t

is the c aracteristic resistan e an ca acitan e of a p larization mec anism j (Ω), (F)

Becau e the c r ents of the fast proces es decay toward zero in the s b-mi isecon s ran e,

f or the in ulation resistan e me s rement only slow p larization proces es are of relevan e

Therefore, only the interacial p larization ef fect remain It is cau ed by the slow drift of ion

an a minor amou t of f e electron , whic ac umulate at the internal interaces an at the

electrodes As there are dif ferent kin s of micros o ic an macros o ic interf aces present in

in ulation with comp site dielectric , the p larization c r ent is a s p rp sition of c r ent

comp nents with dif ferent mag itu es an time con tants An empirical expres ion f or the

p larization c r ent s m is given by Formula (A.5) Some of the time con tants may b many

min tes or hours

n

tCUt

∆β)

is the ge metric ca acitan e (F);

n is the exp nent re resentin the in ulation an its condition

As a hig er temp rature in re ses ion mo i ty an electron avai a i ty, the p larization

c r ent raises with in re sin temp rature

Also the presen e of h midity an contamination wi in re se the avai a i ty of c arge

For dry an cle n in ulation the p larization c r ent is dominant Sin e the p larization

c r ent is a straig t l ne in a log-log diagram, the prefer ed way to display the c r ent

gra hical y is a log(i) vers s log( ) diagram

A 5 Surf ace leak ge cur ent I

L

The s r ace le kage c r ent is cau ed by con u tion in foreig layers, in case of in erently

con u tin sol d de osits mainly by electron or, in oi fi ms an wet layers fom water,

predominantly by ion Wet s r ace layers can b formed either by direct wet in or by

ac umulation of water molec les fom the air d e to ad orption, h gros o icity of f oreig

layers an con en ation

The s rf ace le kage c r ent de en s on the s r ace resistan e ac ordin to Formula (A.6):

L0

L

RU

R

⋅

=

LL

κ(Ω);

The s race le kage resistan e can dro with in re sin relative h midity of air in the ran e

b twe n 3 % an 9 % by several orders of mag itu e an fol ows c an es in the relative

h midity ne rly in tantly

When con en ation con ition arise (s r ace temp rature < dew p int temp rature), the

s rf ace resistan e wi f urther decre se, as the amou t of water on the s race in re ses

The s rf ace le kage resistan e of wet layers de en s on the temp rature by an empirical

exp nential relation hip

Dry de osition of layers f rom intrin ical y con u tin materials, lke co p r or carb n d st,

may red ce the s race le kage resistan e sig if i antly The pro erties an de en en ies

are dominated by electron con u tion mec anisms As metal d st oxidizes with air to metal

oxides, its s r ace le kage resistan e is hig er than that of carb n d st layers

The contribution of s rf ace le kage c r ent de en s on the in ulation con ition of the en

win in ; in the case of a me s rement p r ormed on e c phase win in with the others

con ected to e rth, this contribution is mainly d e to the phase bre k p rtion of the en

win in

A 6 Stress control coating c r ent I

S

If a contin ou stres control co tin covers the complete en win in of a stator, an the

co tin is in galvanic contact with the win in con ection by desig , a stres control co tin

c r ent oc urs Sin e this sp cific stres control desig is a u iq e desig , the tec nical

b c grou d an the con eq en es for the me s rement an the interpretation of in ulation

Annex B

(inf or mative)

Graphical estimation of the slope parameter Xfor temperature

cor ection from measurement data

The f olowin is an example of the estimation of X f rom an in ulation resistan e me s rement

of a complete win in The values in Ta le B.1 are o tained at dif ferent temp ratures

Table B.1 – Ex mple d ta from ins lation re ista c

me s reme ts at dif fere t winding temperature

Win in temperature T

For the estimation of the slo e p rameter X of Formula 2, the sin le data p ints are ploted in

a semi-logarithmic diagram The data p ints may b fited with either one or two straig t l nes

For two lnes, the fitin proced re s ould b se arated into two temp rature region : more

than 4 °C an les than 4 °C The l ne fit in can b ac ieved mathematical y by regres ion

with an exp nential fu ction of an b sis With the f un tional a proximation l ne in the

diagram, the p rameter X can b directly o tained by c o sin an in ulation resistan e value

R’ an a secon value R’, with R’ = 2 x R’, an re din the cor esp n in temp rature values

of the fit in l ne The dif feren e b twe n the temp rature values is eq al to the slo e

p rameter X (se Fig re B.1) For the example data in Ta le B.1, the slo e p rameter X is

Fig re B.1 – Graphic l e timation of th slope parameter X

IEC

The diagram in Fig re C.1 s ows the DC c r ent comp nents an the total c r ent vers s

time of voltage a pl cation for a win in with dry an cle n s r aces

Fig re C.1 – Total c r e t v rs s time on a cle n a d dry ins lation

The s ale are logarithmic

From the total c r ent vers s time the in ulation resistan e vers s time an the p larization

IE C

Figure C.2 – Ins lation re ista c v rs s time

on a cle n a d dry ins lation

C.2 Ma hine with a wet a d contaminate surfac

The diagram in Fig re C.3 s ows the DC c r ent comp nents an the total c r ent vers s

time of voltage a pl cation for a win in with wet an contaminated s r aces

Figure C.3 – Total c r e t v rs s time on a wet

Ca a it iv e c re t a d p lariz tio c re ts of fa t pro e s s

Polariz at io c re t s of slow pro e s s

10–

From the total c r ent vers s time the in ulation resistan e vers s time an the p larization

in ex is o tained (see Fig re C.4) Due to the s r ace le kage c r ent contribution, the

p larization in ex may dro sig if i antly in comp rison to the cle n an dry con ition (se

Clau e C.1)

Fig re C.4 – Ins lation re ista c vers s time on a wet

a d contaminate ins lation

C.3 Ma hine with continuous stress control layers in galva ic conta t with

high voltage conductors

C.3.1 Stre s control coating c r e t I

If a stres control co tin covers the complete en win in of a stator, a stres control

co tin c r ent oc urs The c r ent f lows throu h contacts b twe n the f il er p rticles

emb d ed in the resin b sed co tin , whic is a pl ed on to of the in ulation as a varnis or

a ta e The fi er, mostly do ed si con carbide or iron oxide, b haves as semi-con u tor an

ex ibits a strong de en en y on the a pled electrical field, whic can b a proximated by a

f ormula with an exp nential de en en y fom the a pled voltage (Formula C.1):

S0

S

RU

R

⋅

=

SS

κ(Ω);

κ

S

is the sp cif i s r ace con u tivity

lU

e0

0S

⋅

⋅

=β

l is the len th of the s race layer b twe n electrodes (m);

b is the layer width (m);

IEC

The p sitive exp nent of the sp cific s r ace con u tivity in icates that the stres control

co tin resistan e decre ses, when the sp cific voltage stres U

0

l in re ses

The stres control co ting resistan e decre ses with in re sin temp rature

The contribution of the stres control co tin c r ent to the total c r ent b comes sig ificant

only if the layer is in galvanic contact with the win in con u tors by desig or by a low series

resistan e, e.g f rom h midity or con u tive de osits at the in ulatin s rf ace b twe n stres

control co tin en an the win in con ection or in ulation crac s in the en win in

C.3.2 Ef fe ts on ins lation re ista c a d polarization inde

If the en win in is covered with a stres control co tin that has galvanic contact to win in

con ection at hig voltage p tential, the stres control co tin c r ent may b come

dominant The in ulation resistan e values may fal b low the recommen ed l mits, esp cial y

if the en win in s are s ort The same eff ect influen es the p larization in ex The c rve of

time de en en e of the in ulation resistan e b comes flat, so that con eq ently the

p larization in ex may b close to 1 (se Fig re C.6) Due to these ef fects, the recommen ed

minimum values in Ta le 3 an 4 do not a ply

In order to identify s c a stres control co tin desig , the a pled test voltage s ould b

varied Due to the exp nential de en en y of the stres control co tin c r ent f rom the

a pl ed voltage, the in ulation resistan e would ex ibit a decl ne with in re sin test voltage

level

C.3.3 Ex mple of test re ults

The diagram in Fig re C.5 s ows the DC c r ent comp nents an the total c r ent vers s

time of voltage a plcation f or a win in with a contin ou stres control co tin whic is

galvanical y con ected to the phase win in con ection The s rf ace is dry an cle n

Figure C.5 – Total c r e t v rs s time on a dry a d cle n s rf ac

IE C

From the total c r ent vers s time the in ulation resistan e vers s time an the p larization

in ex is o tained (se Fig re C.6) Becau e of the c r ent throu h the stres control co tin ,

the p larization in ex may dro sig ificantly in comp rison to a win in without contin ou

stres control co tin (se Clau e C.1)

Figure C.6 – Ins lation re ista c v rs s time on a dry a d

IEC

Annex D

(inf or mative)

Measurement of leakage cur ent to assess

interphase insulation resistance

Durin a phase-to-e rth test of a sin le phase win in , c r ent may flow fom the energized

phase win in to the other e rthed phase win in s d e to le kage in the en win in Typical

sources of interphase le kage c r ents could b crac ed in ulation an /or p rosity of the

in ulation, mec anical erosion of the in ulation d e to vibration, contaminated mec anical

s p ort bloc in b twe n phase win in s or at the terminals, etc A hig phase-to-phase

le kage c r ent in o eration may le d to a phase-to-phase s ort circ it, that u ual y res lts in

severe win in damage

Testin of the IR b twe n phases is a non-stan ard proced re For testin the IR b twe n

phase win in s, the desired phase win in s are con ected b twe n the test le d of the

me s rin in trument an the remainin phase win in (s) ne d to b e rthed (Fig re D.1)

Extreme care is neces ary af ter e c test, sin e the ful test voltage may sti b present on

the win in s, even if the test in trument has an automatic s ort circ it a pled at the

con lu ion of the test Both phase win in s have to b dis harged to e rth by a pro riate

me n

Figure D.1 – Conn ction f or pha e- o-pha e me s reme t

The te t instrume t s al be f loating with re pe t to e rth

Other pha e to ph s combinations are permit e

A variation of the phase-to-e rth me s rement test circ it in Fig re D.1 can b u ed to

as es the interphase in ulation resistan e (Fig re D.2) For this phase-to-phase test, the

resistan e b twe n phase win in s is ac ou ted for An HV s p ly with an ac es ible ( o the

u er) g ard p int is req ired, but it may b diff i ult to f i d a commercial in trument

Alternatively some commercial megohmeter test in truments may have a g ard circ it

When phase 1 in Fig re D.2 is energized, the c r ent is the s mmation of the phase 1 to e rth

(

1

an the phase 1 to 2 le kage (

12), an phase 1 to 3 le kage (

13) Phase 2 an 3 are

e rthed to the g ard of the voltage s p ly via an ammeter The voltage dro of the ammeter

is very low comp red to the f ul a pl ed voltage Therefore, phase 2 to e rth an phase 3 to

e rth c r ent may b neglected

The total c r ent del vered by the voltage source is in F rmula (D.1):

IEC

By con ectin the return c r ent of the e rthed phase win in s ammeter to the g ard of the

hig voltage s p ly, the contribution of e c phase win in is se arated

An alternative con ection to as es the interphase le kage when there is no g ard avai a le

on the me s rement in trument is s own in Fig re D.3

A phase-to-phase le kage c r ent hig er than 0,1 times the phase-to-e rth c r ent may

in icate an a normal con ition b twe n the phase win in s, if there is no desig re son for

this b haviour

Figure D.2 – Me s reme t of interpha e le k g c r e t with

a me s reme t instrume t e uip e with a g ard con e tion

Figure D.3 – Me s reme t of interpha e le k g c r e t with

IE C

IEC

Annex E

(inf or mative)

Other DC tests

The in ulation resistan e me s rement an determination of the p larization in ex, des rib d

in this stan ard, are not the only method whic are u ed in practice to gain a b t er

u derstan in of the con ition of a win in by me n of direct voltage an c r ent In ad ition

to the determination of the in ulation resistan e an p larization in ex, modern in ulation

testers provide other o tion

The target of this an ex is to provide an overview of these ad itional method to gain a b t er

u derstan in of the con ition of a win in of a rotatin mac ine, by me n of c an in the

me s rin time, direct test voltage or en ironmental con ition whic are u ed nowaday

E.2 Diele tric absorption ratio (DAR)

The p larization in ex (PI) is traditional y defined as the ratio of the 10 min in ulation

resistan e (R

i10

) to the 1 min in ulation resistan e (R

i1 In older in ulation s stems, s c as

asphaltic-mica, the p larization c r ents of ten takes 10 min or more to decay to ne rly zero

However in ran om-wou d low voltage mac ines an in f ield win in s, the p larization

c r ent may decay to ne rly zero in les than a min te Th s some u ers calc late variants of

the con entional PI The variants in lu e, but are not lmited to, the one s own in Formula

is the in ulation resistan e re din after the a plcation of voltage f or 3 s

Fig re E.1 s ows an example of the in ulation resistan e me s re in the f irst min te fom a

ran om wou d stator win in , where the R

i1

of 4 0 3 MΏ an R

i3

of 3 7 5 MΏ, yieldin a

DAR of 1,0 Some in ulation resistan e testers have an automatic test setin to p rorm the

DAR me s rement The distin uis in f eature is the s orter time the direct voltage is a pl ed

an th s the s orter time that the win in s ould b e rthed (se 6.6) Sin e with in ulation

s stems that do not contain mica the p larization c r ent may b es ential y zero after a

min te, by u in s orter times for the ratio, the test time can b con idera ly s ortened with

l t le los of information a out the degre of contamination or moisture p larization present

When me s rin R

i3, the test o ject s al re c the test voltage in les than 1 s

There are l mitation in a plyin other ratios:

a) There is no stan ard for what time intervals the R

ivalues are to b recorded Diff erent

organization u e diff erent ratios (se 3.3)

b) There is no agre d p s - ai criterion for the DAR, as have b en esta ls ed for the

Fig re E.1 – Me s reme t of c r e t a d ins lation

re ista c that re ults in a DAR of 1,0

E.3 Monitoring charge and discharge cur e ts

As stated in Clau e A.4, the p larization c r ent is primari y determined by the interf acial

p larization within the in ulation Both are temp rature de en ent

After the a pl ed direct voltage is removed, the dis harge c r ent can b monitored as a

f un tion of time u in a s ita le dis harge circ it The dis harge c r ent manifests itself in

two comp nents: a ca acitive dis harge c r ent comp nent, whic decay ne rly

in tantane u ly, de en in up n the dis harge resistan e; an the p larization dis harge

c r ent, whic wi decay f rom a hig initial value to ne rly zero with c aracteristic simi ar to

the initial c argin c r ent, but with the o p site p larity Normal y, neither the s race

le kage nor the con u tion c r ent aff ects the dis harge c r ent Fig re E.2 b low s ows the

c arge an dis harge c r ents f or the thre -phase win in s of a 5 MVA h dro-generator in

l ne r s ale (a) an in logarithmic s ale (b) The se aration b twe n the c arge an

IE C Time

a)

b)

Figure E.2 – Charge a d dis harge c r e ts af er a step volta e of 2,5 kV

f or th thre -pha e win ing of a 5 MVA hy ro-ge erator:

a) l ne r s ale; b) logarithmic s ale with the dis harge time re et

to zero a d u ing a positiv v lue f or the dis harg c r e t

Sin e the dis harge c r ent is the res lt of the de olarization c r ents, an a normal y hig

dif feren e b twe n the c arge to dis harge c r ent may in icate an internal in ulation

pro lem s c as lac of c rin , thermal agin , mec anical damage or moisture p larization

within the bulk in ulation wal

As umin that the p larization c r ent comp nent I

P

is identical with the dis harge c r ent

with the reversed p larity, the empirical p rameters of Formula A.5 can b o tained fom the

–0,5∙10

–

–10–

0,1

10–

10–

10–

10–

10–

E.4 High voltage DC tests

An in ulation resistan e me s rement is traditional y p rormed at a con tant voltage level

that is wel b low the p ak voltage to whic the win in wi b s bjected to in service

However some in ulation testers provide the p s ibi ty to determine the c r ent with an

in re sin DC voltage level to voltages gre ter than the p ak voltage in service Thre

tec niq es have b en u ed f or direct-voltage testin at hig er levels: u if orm-time ste ,

graded-time ste , an ramp d voltage test These tests are des rib d in more detai in [2]

• DC voltage can s ort-circ it stres gradin co tin s an le d to local electric field

en an ements in in ulation s stem, with p s ible ir eversible eff ects

• Per ormin a hig voltage DC test on site on a stator win in in u k own con ition, could

ris f las over or in ulation bre k own

E.4.2 Uniform- ime volta e step te t

This method in olves a pl cation of the high direct voltage in a series of u iform voltage ste s

at reg lar time intervals Cur ent re din s are taken at the en of e c interval and the

c r ent vers s voltage c rve is plot ed Durin an after testin , the c rve is examined for

s d en in re ses or other variation in con u tion c r ent vers s a pl ed voltage resp n e,

that may in icate in ulation we k es

Dielectric p larization may dominate the c r ent me s rements an mas sig ificant

variation in con u tion c r ent To minimize this ef fect, the test voltage may b held at e c

level lon enou h to al ow the p larization c r ent to decay to a neglgible value For older

asphalt-mica an s el ac mica-ol um win in s, a time interval of 1 min is the minimum; but up

to 6 min is employed by some u ers of this method For modern e ox -mica in ulation

s stems, the time interval is u ual y 1 min

E.4.3 Gra e - ime volta e step te t

It is u ual y not practical to hold e c voltage ste lon enou h to make the p larization

c r ent negl gible To avoid introd cin er or f rom in omplete p larization c r ent decay an

to s orten the time req ired to o tain c r ent vers s voltage c rves, complex volt-time

s hed les were develo ed The b sic ide of these test s hed les is to adju t the voltage, in

ste s, ac ordin to a diminis in time s hed le so the p larization comp nent of the

me s red c r ent is l ne rized, i.e pro ortional to the a pl ed voltage By folowin a

predetermined test s hed le, relative c an es in con u tion c r ent b come more re di y

dis ernible Althou h the graded-time ste test is more diff i ult to p r orm than the u iform

-time ste test, it do s red ce the total test time an res lts in b t er as es ment of the

con u tion c r ent The time d ration at e c voltage ste is varia le an is determined by

the c aracteristic of the in ulation

E.4.4 Rampe - ime volta e step te t

The ramp d voltage test method can b con idered a ste voltage test in whic the voltage

ste s an time intervals are made very smal As the size of the voltage an time in rements

a pro c es zero, a voltage ramp is formed A programma le direct-voltage test set is u ed to

automatical y ramp the voltage at a preselected rate In ulation c r ent vers s a pl ed voltage

is recorded, providin contin ou o servation an analy is of the c r ent resp n e as the test

progres es The a pl cation of a ramp d voltage, in te d of dis rete voltage ste s,

automatical y l ne rizes the ge metric ca acitive an p larization comp nents of the c r ent

so that me nin ful variation in the me s red c r ent are more e si y o served

_ _ _ _ _ _

The prin ip l advantages of the ramp d voltage test over the con entional ste p d voltage

method are that it gives b ter control an improved warnin of imp n in fai ure to avoid

damage to the in ulation El mination of the h man varia le f om the time, voltage, an

c r ent p rameters yield overal test res lts whic are mu h more re e ta le The rate of

in re se in voltage is typical y 1 kV/min or 2 kV/min

E.5 Wet insulation resistance measurement

In stator win in s that may b s bjected to severe contamination, it is more an more

common to detect if the in ulation s stem is se led again t contamination by wet in the

in ulated s r aces an s bseq ently me s rin the in ulation resistan e an p larization

in ex (se [3]) Thre diff erent method are u ed:

• The in ulation resistan e is determined d rin local y wetin a cle n an dry in ulation

s stem with a del cate mist of water containin a non-ionic wet in agent by sprayin

• The in ulation resistan e is determined after completely wet in the win ing of a cle n

and dry in ulation s stem with water containin a non-ionic wet in agent by sprayin

• The in ulation resistan e is determined after completely s bmergin the win in s of a

cle n an dry in ulation s stem in water containin a non-ionic wetin agent

The wetin agent is ad ed to the water in a pro ortion s f ficient to red ce the s race

ten ion of water to a value of 3 mN/m or les at 2 °C The in ulation resistan e

me s rement is p rormed with a direct voltage of 5 0 V