Testing Power Transformers

ABB Stromberg Power

TESTING POWER TRANSFORMERS

Tes t procedures and equipment used f o r t h e t e s t i n g o f l a r g e power transformers a t Stromberg's Vaasa Works a r e d e a l t with i n t h e fol lowing sec t ions . The t e s t i n g o f d i s t r i b u t i o n t ransformers is no t included. Due t o t h e i r l a rge manufacturing numbers d i s t r i b u z i o n t ransformers a r e rou t ine t e s t e d by means of computerized (automatic) t e s t equipment. The measuring equipment d i f f e r s from those explained here in . The p r i n c i p l e s of rou t ine , type and s p e c i a l t e s t s a r e however s imi l a r and thus t h i s bookle t is app l i cab le f o r t e s t i n g o f d i s t r i b u t i o n t ransformers too .

The e l e c t r i c a l c h a r a c t e r i s t i c s and d i e l e c t r i c s t r e n g t h of t h e t ransformers a r e checked by means o f measurements and t e s t s def ined by s tandards .

The t e s t s a r e c a r r i e d o u t i n accordance with I E C Standard 76, Power Transformers, un less o therwise s p e c i f i e d i n t h e c o n t r a c t documents.

CONTENTS Pages

1. Summary of d i e l e c t r i c t e s t s 1- 1

Routine t e s t s

2. Measurement of vo l t age r a t i o and check of connection symbol

3 . Measurement of winding r e s i s t a n c e 3-1. . .3-2

4. Measurement of impedance vol tage and load l o s s

5. Measurement of no-load l o s s and cu r ren t 5-1. . .S-3 6. Induced overvol tage withstand t e s t 6-1. . .6-2

7 I . Separate-source voi tage wirhstand t e s t

8. Operation t e s t s on on-load tap-cnanger 8-1

Type t e s t s and s p e c i a l t e s t s

9. Measurement of zero-sequence i2pedance 9-1

10. Capacitance measurement 10-1...10-2

11. Insu la t ion r e s i s t a n c e measurement 11-1

13. Measurement of t h e e l e c t r i c s t rength of the i n s u l a t i n g o i l

CONTENTS Pages

14. Temperature-rise test X)

15. Lightning impulse test X)

16. Test with lightning impulse, chopped on the tail X)

17. Switching impulse test X)

18. Partial discharge measurement X)

19. Measurement of acoustic sound level X) X ) on request

List of equipment

1. SUMMARY OF DIELECTXC TESTS

According t o t he Standard IEC 76 -3 t h e d i e l e c t r i c t e s t requirements f o r a t ransformer winding depend on the h ighes t vo l t age f o r equipment Um app l i cap l e t o the winding and on whether t h e winding i n s u l a t i o n i s uniform o r non-uniform.

Category of rwlnalng Tes t s Sec t lon U- C 300 kV -Separate-source withstand t e s t 7 ~!iform i n s u l a t i o n

-Induced overvol tage wi ths tand 6 t e s t wi th symmetrical th ree- phase vo l t ages ( r o u t i n e t e s t )

-Lightning impulse t e s t 15 -for l i n e te rmina ls ( t y p e t e s t ) - for n e u t r a l t e rmina l ( s p e c i a l t e s t

U- c300 kV -Separate-source wi ths tand t e s t 7 m

Non-uniform i n s u l a t i o n

corresponding t o i n s u l a t i o n l e v e l o f n e u t r a l ( r o u t i n e t e s t )

-Induced l ine- to-ear th over- 6 vo l tage withstand t e s t ; t h r e e single-phase t e s t s ( r o u t i n e t e s t )

-Lightning inpulse t e s t 1) 15 - for l i n e te rmina ls ( t y p e t e s t ) - for n e u t r a l t e rmina l ( s p e c i a l t e s t

U-% 300 kV -Separate-source wi ths tand t e s t 7 ~ % i - u n i f o m i n s u l a t i o n

corresponding t o i n s u l a t i o n l e v e l o f n e u t r a l ( r o u t i n e t e s t )

-Induced l ine- to-ear th overvol tage 6 withstand t e s t ; t h r e e single-phase t e s t s ( r o u t i n e t e s t )

Tes t ing according -Lightning x p u l s e t e s t l ) 15 t o method 1 , s e e - for l i n e te rmina ls ( r o u t i n e IEC 76-3 - for n e u t r a l t e rmina l ( s p e c i a l

t e s t ) U , 3 300 kV -Separate-source wi ths tand t e s t 7 Non-unif orm corresponding t o i n s u l a t i o n i n s u l a t i o n l e v e l o f neucral ( r o u t i n e t e s t )

-Switching impulse t e s t f o r l i n e 17 te rmina ls ( r o u t i n e t e s t )

-Lightning ixpulse t e s t l ) 15 Tes t ing according -for l i n e te rmina ls ( r o u t i n e t e s t ) L - - L , - . 7 - !,G t ! ! r l - r w c r L . S Y P -fcr n e u t r a l termlnal ( s p e c i a l IEC 76-3 t e s t J

- P a r t i a l d i scharge measurement 18 ( r o u t i n e t e s t j 2 )

"chopped-wave Lightning i e p u l s e :ast is a s p e c i a l t e s t , s e e Sec t ion 15.

' ) ~ a r o t h e r ca rego r i s s o f windings :he p a r t i a l Cischarge measuremenr ii a s p e c i a l t e s t .

2. MEASUREMENT OF VOLTAGE RATIO AND CHECK OF CONNECTICN SYMBOL

Purpose o f t he measurement

The vo l t age r a t i o of t h e t ransformer is t h e r a t i o of vo l tages ( i n three-phase t ransformers l ine- to- l ine v o l t a g e s ) a t no-load, e .g . , 110000 V/10500 V.

The purpose of t h e measurement i s t o check t h a t t he devia t ion of t h e vol tage r a t i o from t h e s p e c i f i e d value does n o t exceed t h e l i m i t g iven i n t h e r e l e v a n t t ransformer s tandard ( g e n e r a l l y 0.5 % ) .

The connection symbol of t h e transformer is checked a t t h e same time.

Performance and r e s u l t s o f t h e measurement

The vo l t age r a t i o measurements a r e c a r r i e d o u t by means o f a vo l tage r a t i o measuring br idge ; t h e e r r o r of t h e b r idge i s l e s s than +-0.1 %. The supply vol tage is 220 V a . c . The f u n c t i o n o f t he br idge is shown i n Fig. 2-1. The vol tages o f t h e t ransformer t o be checked are compared t o t h e corresponding vo l t ages o f t he r e g u l a t i n g t ransformer, which is provided with a decade d i s p l a y u n i t and l o c a t e d i n t he br idge cas ing . When t h e br idge i s balanced, t he vol tage r a t i o of the decade transformer is equal t o t h a t of t h e t ransformer under t e s t . The r e s u l t can be seen d i r e c t l y from the numeral d i sp l ay o f t h e br idge.

Fig. 2-1.

Bridge measurement (o f the vol tage r a t i o ) .

T t ransformer t o be measured, 1

T r e g u l a t i n g t ransformer equipped 2 ~ i t h a decade d i s p l a y , P zero-

1 sequence vol tmeter , U, supply voltage o f t h e bridge; U secondary

2 voltage o f t h e t ransformer.

S ince t h e measuring device i s a single-phase br idge, the vol tage r a t i o o f a p a i r o f windings mounted on the same l e g is measured a t a t ime. I t is t o be observed t h a t t h e r a t i o i nd ica t ed by t h e bridge does no t alway: correspond t o the r a t i o of t h e l ine- to- l ine vol tages. The r e s u l t depend^ on t h e connect ion symbol of t h e t ransformer. For each winding connected t o t h e br idge l t 1s important t o observe whether the number of t u r n s r e l a t e s t o the l ine- to- l ine o r l ine- to-neut ra l voltage. For example, the vol tage r a r i o of a 120/21 kV Yd-connected t ransformer is 120000:

\/3/21000 V = 3.299. The reading o ~ t a i n e a from t h e bridge i s t o be compared t o t h i s va lue .

The connect ion sy rn~o l of t h e transformer is checked i n conjunct ion w i t k t h e vol tage r a t i o neasurement. When t h e measuring leads from t h e

transformer a r e connected t o t h e b r idge according t o t h e re levant vec to r diagram i n Table 2-2, the bridge can be balanced only if the t ransformer connection i s c o r r e c t .

The vol tage r a t i o s a r e measured f o r each tapping connection of t h e transformer. In t h e r epor t t h e spec i f i ed tapping vol tage r a t i o s a r e s t a t e d , a s well a s the measured r a t i o s and t h e i r dev ia t ions from the spec i f i ed r a t i o s .

Table 2-2.

Determination of t h e connection symbol.

Clock hour f i g u r e ( l e f t ) , connection symbol (middle) and vec tor diagram ( r i g h t ) .

CHECKING OF THE VECTOR GROUPS

Phase U on HV-side and phase u on LV-side are connected together. The transformer isenergised by a symmetric 3-phase 400 V. Voltages of the terminals are measured andvector group symbol is determinated by following chart.

HV-sideMain voltage

LV-sideMain voltage

Voltage betweenHV and LV terminals

Terminals U/V Terminals U/V Terminals U/VU-V u-v V-vV-W v-w V-wW-U w-u W-v

W-w

Vector groupsymbol

Voltage relationshipbetween terminals

0 Ww<Vw=Wv>Ww<UV1 Ww<Vw>Wv=Ww<UV2 Ww<Vw>Wv<Ww<UV3 Ww<Vw>Wv<Ww≥UV4 Ww<Vw>Wv<Ww>UV5 Ww=Vw>Wv<Ww>UV6 Ww>Vw=Wv<Ww>UV7 Ww>Vw<Wv=Ww>UV8 Ww>Vw<Wv>Ww>UV9 Ww>Vw<Wv>Ww≥UV10 Ww>Vw<Wv>Ww<UV11 Ww=Vw<Wv>Ww<UV

The vector group symbol is _____________________________________

3 . NEASUREMENT OF W I N D I N G R E S I S T A N C E

Purpose o f t h e measurement

The r e s i s t a n c e s between a l l p a i r s of phase t e rmina l s o f each transformer winding a r e measured us ing d i r e c t current . The measurement is performed f o r each connection of connectable windings and f o r each tapping connection. Furthermore t h e corresponding winding temperature is measured.

The measured r e s i s t a n c e s a r e needed i n connect ion wi th the load l o s s measurement when t h e load l o s s e s a r e co r r ec t ed t o correspond t o t he re ference temperature. The r e s i s t a n c e measurement w i l l a l s o show whether t h e winding j o i n t s a r e i n o rde r and the windings c o r r e c t l y connected.

Apparatus and measuring c i r c u i t

The measurement is u s u a l l y performed by means of a Thornson-Wheatstone r e s i s t a n c e br idge.

Fig. 3-1

Resistance measurement using Thomson-bridge.

R dec R V

The p r i n c i p l e of measurement is a s follows: The vol tage drops caused o y t he d i r e c t cu r r en t I a c r o s s the r e s i s t ances R and R a r e compared by

X means of t he br idge (Fig. 3-1) . Here R is t h e r e s i s r a n c e t o be measured and R a s tandard r e s i s t a n c e . The accGacy i s b e t t e r than +-0.1 % and N - 7 t h e measuring range 100...10 (with Thomson-connection).

The temperature is measured by means of thermometers with an accuracy o f +-O.l°C. Di rec t c u r r e n t is obtained from a b a t t e r y .

Performance o f t h e measurement

Before t he measurement s t a r t s t h e transformer is s tanding f o r a t l e a s t ? hours f i l l e d with o i l and without exc i t a t i on . During t h i s per iod t h e temperature d i f f e r e n c e s o f t he transformer w i l l equa l i ze and the w i n l l n g temperature w i l l become equal t o the o i l temperature. The average winding temperature is obta ined by determining t h e average o i l temperature. The average o i l temperature is obta ined by measuring t h e top o i l temperature i n an o i l - f i l l e d thermometer pocket s i t u a t e d I n

cover, and t h e bottom o i l temperature i n t he d r a i n va lve , and tak ing the average o f t h e s e two.

When switching on the supply vol tage E t o t h e measuring c i r c u i t t h e winding inductance L tends t o r e s i s t t h e i nc rease of t h e cur ren t . The r a t e o f i nc rease depends on the time cons tan t o f t h e c i r c u i t :

t = t ime from switching on L/R = t ime cons tan t of t h e c i r c u i t R = t o t a l r e s i s t a n c e o f t h e c i r c u i t

To shor ten t h e t ime f o r t h e c u r r e n t t o become s t e a d y s o high a measuring cur ren t is used t h a t t h e co re w i l l be s a t u r a t e d and t h e inductance w i l l be low. The measuring c u r r e n t is usua l ly 5...10 times t h e no-load cur ren t o f t h e winding. However, t he cu r r en t should be l e s s than 10 % o f t he r a t e d c u r r e n t o f t h e winding, otherwise t h e temperature r i s e of t h e winding caused by t h e measuring c u r r e n t w i l l g i v e r i s e t o measuring e r r o r s . Furthermore the t ime cons tan t can be reduced by using as h igh a supply vol tage a s poss ib l e enabl ing an increased s e r i e s r e s i s t a n c e i n the c i r c u i t . When us ing a b a t t e r y , t h e supply v o l t a g e is approximately constant and t h e c u r r e n t is ad jus ted by means of t h e s e r i e s r e s i s t a n c e

Re g

When swi tch ing on and a d j u s t i n g t h e measuring c u r r e n t t h e br idge i s n o t connected with t h e te rmina ls of R , s o the re is no r i s k t h a t t h e induced vol tage w i l l damage the br idge. ~ f i e n t h e ammeter i n d i c a t e s t h a t t h e cu r r en t is almost s teady t h e br idge is connected and balanced by means of t he zero- indica tor . The br idge r e s i s t a n c e r ead ings a r e noted down.

Test r e s u l t

The r e s i s t a n c e v a i ~ e s and the average temperature a r e ca lcu la ted . In t h e r epo r t t h e termina-S, between which t h e r e s i s t a n c e s a r e measured, t he connection, t h e tapping pos i t i on and t h e average temperature of t he windings dur ing t h e measurement a r e s t a t e d .

L i t e r a t u r e

(3-1) Kiiskinen,E.: Determining t h e temperature r i s e i n a t ransformer winding using t h e r e s i s t a n c e method. S W o - E l e c t r i c i t y i n Finland 47 (19741, No 1.

Measurement of w i n d i n a r e s i s t a n c e

The o h m i c r e s i s t a n c e o f e a c h w i n d i n g is measu red b e t w e e n t h e a p p r o p r i a t e b u s h i n g s o f e a c h p h a s e and a t a l l t a p p i n g s o f t h e t a p p i n g r a n g e ; b e f o r e t h e r e s i s t a n c e measu remen t , t h e appropriate w i n d i n g t e m p e r a t u r e is m e a s u r e d .

T e s t c i r c u i t

The r e s i s t a n c e i s de t e r r r i i ned by means. o f t h e M c u r r e n t - v o l t a g e me thod" . Wi th t h i s m e t h o d , t h e DC test c u r r e n t a n d t h e v o l t a g e d r o p a c r o s s t h e w i n d i n g t o be m e a s u r e d a r e d e t e r m i n e d by means + o f t e s t i n s t r u m e n t s of a c c u r a c y c l a s s - 0 . 2 % (see F i g u r e ) .

R e s i s t a n c e measurement

1 S t a b i l i z e d power s u p p l y o r b a t t e r y 2 Ammeter 3 V o l t m e t e r

4 T r a n s f o r m e r u n d e r t e s t

The q u o t i e n t o f t h e v o l t a g e d r o p a c r o s s t h e w i n d i n g and t h e DC t e s t c u r r e n t f l o w i n g t h r o u g h t h e w i n d i n g g i v e s t h e ohmic w ind ing r e s i s t a n c e . The DC t e s t c u r r e n t i s o b t a i n e d f rom e i t h e r a s t a b i - l i z e d power s u p p l y o r a t e s t b a t t e r y .

T h e t e m p e r a t u r e of the w i n d i n g o r o f t h e i n s u l a t i n g o i l s u r r o u n d i n g i t i s m e a s u r e d by means o f me rcu ry t h e r m o m e t e r s + w i t h a n a c c u r a c y o f - 0 . 1 " C .

T h e t e s t r e s u l t s a r e documented i n a t e s t c e r t i f i c a t e .

4. MEASUREMENT OF IMPEDANCE VOLTAGE AND LOAD LOSS

Purpose o f the measurement

The measurement is c a r r i e d o u t t o determine t h e load-losses of t h e t ransformer and t h e impedance vol tage a t r a t e d frequency and r a t e d cu r r en t . The measurements a r e made s e p a r a t e l y f o r each winding p a i r (e .g. , t he p a i r s 1-2, 1-3 and 2-3 f o r a three-winding t r ans fo rmer i , and furthermore on t h e p r i n c i p a l and extreme tappings.

Apparatus and measuring c i r c u i t

Fig. 4-1 C i r c u i t f o r t h e impedance and load-loss measurement.

G supply gene ra to r , T step-up t ransformer, T transformer t o be 1 1 2 t e s t e d , T cu r r en t t ransformers , T vol tage t ransformers , P wattgecers ,

3 4 1 P ammeters (r.rn.s. v a l u e ) , P voltmeters (r.m.s. va lue ) , C capac i tor 2 bank. 3 1

The supply and measuring f a c i l i t i e s a r e descr ibed i n a sepa ra t e measuring apparatus l ist (Sec t ion 20).

Current is gene ra l ly suppl ied t o the h.v. winding and the 1.v. winding is shor t - c i r cu i t ed .


I f t he r e a c t i - ~ e power suppl ied by the genera tor G i s no t s u f f i c i e n t 1 when measuring l a r g e t ransformers , a c a p a s i t o r Sank C is used t o

1 compensate p a r t of t h e induc t ive r eac t ive power taken by the t ransformer T2..

The vol tage of t h e supply genera tor is r a i s e d u n t i l t h e cu r r en t has a t t a ined t h e requi red value (25...100 % of t h e r a t e d c u r r e n t according t o the s tandard 4 .1 ) . I n o r d e r t o i nc rease t h e accuracy of readings w i l l be taken a t s e v e r a l c u r r e n t values near the r e q u i r e d l e v e l . If a winding i n t he p a i r t o be measured is equipped with an o f f - c i r c u i t o r on-load tap-changer, the measurements a r e c a r r i e d ou t on t h e p r i n c i p a l and extreme tappings. The readings have t o be taken a s quick ly as poss ib l e , because t h e windings tend t o warm up due t o t h e c u r r e n t and the l o s s values obtained i n t he measurement a r e accordingly t o o high.

If the t ransformer has more than two windings a l l winding p a i r s a r e measured sepa ra t e ly .

Results

Correct ions caused by t h e instrument transformers a r e made t o t h e measured cu r ren t , vo l tage and power values. The power va lue co r r ec t ion caused by t h e phase displacement is ca l cu la t ed as fol lows:

P = co r rec t ed power P' = power read from t h e meters

e bU = phase displacement o f the vol tage t ransformer i n minutes

6 = phase displacement o f t h e c u r r e n t t ransformer i i n minutes

cp = phase angle between cu r ren t and vo l t age i n t h e measurement P i s pos i r ive a t i nduc t ive load 1

K = c o r r e c t i o n

The co r rec t ion K obta ined from equat ion (4.1) is shown a s a s e t of curves i n Fig. 4-2.

The co r rec t ions caused by t h e instrument t ransformers a r e made sepa ra t e ly f o r each phase, because d i f f e r e n t phases may have d i f f e r e n t power f a c t o r s and t h e phase displacements of t h e instrument t ransformers a r e gene ra l ly d i f f e r e n t .

If the measuring c u r r e n t I d e v i a t e s from t h e r a t e d c u r r e n t I the N'

power P and t h e vol tage Ukm a t r a t e d cu r r en t are obta ined by applying km corrections t o t h e values P and U r e l a t i n g t o t h e measuring cu r ren t .

C C The co r rec t ions a r e made a s fol lows:

20 Fig. 4-2 181 Q02

The correction caused by 10 Q03 the phase displacement of 8 Q04 instrument transformers. 6

0.06 K correction in percent,

4 0.08 6 - 6. phase displacement U 1

0,lO in minutes, cos power factor of the measurement. The sign

2 OJ5

of K is the same as that of 0.2 0

SU - hi. 1 Q8 0.30 0.6

0,L 0 0.50

=OrY

a1

) min I

Mean values are calculated of the values corrected to the rated current and the mean values are used in the following. According to the standards the measured value of the losses shall be corrected to a winding temperature of 75OC (80°C, if the oil circulation is forced and directed). The transformer is at ambient temperature when the measurements are carried out, and the loss values are corrected to the reference temperature 75OC according to the standards as follows.

The d.c. losses P at the measuring temperature are calculated using m

the resistance va?!es R and R obtained in the resistance measurement 1 2m

(for windings 1 and 2 be?ween llne terminals) :

The additional losses P at the measuring temperature are am

Here P is the measured power, to which the corrections caused by the km instrument transformer have been made, and which is corrected to the rated current according to Equation (4 .2 ) .

The short-circuit impedance Z and resistance R at the measuring km km temperature are

(4.6) U

'km = 100 km % and -

'n

'km is the measured short-circuit voltage corrected according to

Equation (4.3); U is the rated voltage and S the rated power. The short circuit reactance X does not depend on t!!e losses and X is the k k same at the measuring temperature (* ) and the reference temperature

m (75OC), hence

1

When the losses are corrected to 75OC, it is assumed that d.c. losses vary directly with resistance and the additional losses inversely with resistance. The losses corrected

Now the short

235O for Copper 225O for Aluminium

circuit resistance at the reference temperature can

to 75OC are obtained as follows:

Rkc and the short circuit impedance Z

be determined: k c

Results

The report indicates for each winding pair the power S and the N following values corrected to 75OC and relating to the principal and extreme tappings.

- d.c. losses P Oc ( PDC ) - additional losses P

a c (PA) - load losses Pkc (PK - short circuit resistance R

k c (RK) - short circuit reactance X k c (XK) - short circuit impedance Z kc (ZK)

Literature

5. MEASUREMENT OF NO-LOAD LOSS AND CURRENT

Pur~ose of the measurement

In the no-load measurement the no-load losses P O and the no-load current I of the transformer are determined at rated voltage and rated 0 frequency. The test is usually carried out at several voltages below and above the rated voltage U N , and the results are interpolated to correspond to the voltage values from 90 to 115 % of UN at 5 % intervals.

The asymmetric voltage at the neutral terminal is also measured in certain cases. The harmonics on the no-load current are also measured on request . Apparatus and measuring circuit

Fig. 5-1 Circuit for the no-load measurement.

G, supply generator, T, step-up transformer, T, transformer to be t$sted, T current trahsformers , T voltage trhsformers, P wattmeters,

3 1 P ammeters, P voltmeters (r.m.~.~value), P 4 voltmeters (mean value x 2 1.11). 3

The available supply and measuring facilities are described in a separate measuring instrument list (Section 20).

Performance

The following losses occur at no-load

- iron losses in the transformer core and other constructional parts - dielectric losses in the insulations

- load l o s s e s caused by t h e no-load c u r r e n t

While t h e two l a s t mentioned l o s s e s a r e small, they a r e genera l ly ignored.

The fol lowing formula is v a l i d f o r t h e i r o n l o s s e s

PO = measured i r o n l o s s e s kl = c o e f f i c i e n t r e l a t i n g t o h y s t e r e s i s l o s s e s

P = c o e f f i c i e n t r e l a t i n g t o eddy-current l o s ses = frequency

U' = mean value of vo l tage X 1.11 ( read ing o f a r e c t i f i e r voltmeter s ca l ed t o read t h e r . m . s . value of a s i n u s o i d a l v01 tage )

U = r.m.s. value o f t h e vol tage

When ca r ry ing o u t t h e no-load measurement, t h e vol tage wave shape may somewhat d i f f e r from t h e s i n u s o i d a l form. This is caused by t h e harmonics i n t h e magnetizing c u r r e n t which cause add i t i ona l vo l tage drops i n t h e impedances of t he supply. The readings o f t h e mean va lue meter and r.m.s. meter w i l l be d i f f e r e n t .

Because t h e l o s s e s a r e t o be determined under s tandard condi t ions , it is necessary t o apply a wave shape co r rec t ion whereby t h e lo s ses a r e co r r ec t ed t o correspond t o t e s t condi t ions where t h e supply vol tage is s i n u s o i d a l .

I n t h e t e s t t h e vol tage i s ad jus t ed s o t h a t t h e mean value vol tmeter i n d i c a t e s t h e requi red vol tage value. Then t h e h y s t e r e s i s l o s s e s correspond t o s tandard condi t ions , but t he eddy-current l o s s e s must be co r r ec t ed . From (5.1).

'on = l o s s e s a t s inuso ida l vo l tage under s tandard condi t ions

p1 = r a t i o , expressed a s a percentage, o f h y s t e r e s i s l o s s e s t o t o t a l i r o n l o s s e s

P 2 = r a t i o , expressed a s a percentage, o f eddy-current l o s s e s

t o t o t a l i r o n l o s s e s

The l o s s value corresponding t o s tandard condi t ions i s obtained from the measured value P a s fol lows:

0

I t is assumed t h a t f o r o r i en t ed s h e e t s p = p2 = 50 %. 1

The c u r r e n t and power readings o f d i f f e r e n t phases a r e usua l ly d i f f e r e n t ( t h e power can even be negat ive i n some phase) . This is due t o t he

asymmetric construction of the 3-phase transformer; the mutual inductances between different phases are not equal.

The report shows the corrected readings at each voltage value, as well as the mean values of the currents of all three phases.

A regression analysis is carried out on the corrected readings. From the no-load curve thus obtained no-load losses and no-load apparent power corresponding to voltage values from 90 to 115 % of U at 5 % intervals N are determined and stated. Furthermore the no-load current in percentage on the rated current is stated.

6. INDUCED OVERVOLTAGE WITHSTAND TEST

Purpose of the test

The object of the test is to secure that the insulation between the phase windings, turns, coils, tapping leads and terminals, for non- uniformly insulated windings also the insulation between these parts and earth, withstand the temporary overvoltages and switching overvoltages to which the transformer may be subjected during its lifetime.

Performance

The excitation voltage is applied to the terminals of the low-voltage winding. The other windings are left open-circuited. The machines and the equipment are described in the test equipment list (Section 20).

The tapping of the off-circuit or on-load tap-changer is chosen so that in all windings the voltage during the test is as near as possible the rated test voltage.

The test frequency is either 165 Hz or 250 Hz. The duration of the test is

rated frequency . 120 seconds. test frequency

The test is successful if no collapse of the test voltage occurs.

a. Uniformly insulated windings

The test voltage connection is essentially the same as in service. A three-phase winding is tested with symmetrical three-phase voltages induced in the phase windings. If the winding has a neutral terminal, it is earthed during the test.

The test voltage is twice the rated voltage. However, the voltage developed between line terminals of any winding shall not exceed the rated short duration power-frequency withstand voltage.

The voltage is measured from terminals to earth or between terminals of the low voltage winding using voltage transformers. Alternatively the capasitive taps of the bushings on the high voltage side are used for voltage measurement. The voltage is so adjusted, that the average of the voltage values measured from terminals to earth or between terminals 1s equal to the required test voltage value.

Procedure in accordance with IEC 60076-3 (2000) as per pages 6-1-A and 6-1 -B below.

a.1 Transformers with Um < 72,5 kV

The phase-to-phase test voltage shall not exceed the rated induced AC withstand voltages intables 2 or 3 of IEC 60076-3 (2000). As a rule, the test voltage across an untapped winding ofthe transformer shall be as close as possible to twice the rated voltage. Normally, no partialdischarge measurements are performed during this test.

The test shall be commenced at a voltage not greater than one-third of the test value and thevoltage shall be increased to the test value as rapidly as is consistent with measurement. At theend of the test, the voltage shall be reduced rapidly to less than one-third of the test value beforeswitching off.

The test is successful if no collapse of the test voltage occurs.

a.2 Transformers with Um > 72,5 kV

These transformers shall all, if not otherwise agreed, be tested with partial dischargemeasurement. The phase-to-phase test voltages shall not exceed the rated AC withstandvoltages of tables 2, 3 or 4 of IEC 60076-3. As a rule, the test voltage across an untappedwinding of the transformer shall be as close as possible to twice the rated voltage.

The partial discharge performance shall be controlled according to the time sequence for theapplication of the voltage as shown in figure 6.0 below.

In order not to exceed the rated withstand voltage between phases according to tables 2, 3 and4, the partial discharge evaluation level U2 shall be:

1,3 Um / √3 phase-to-earth and

1,3 Um phase-to-phase

The voltage with respect to earth shall be:

– switched on at a level not higher than one-third of U2;

– raised to 1,1 Um / √3 and held there for a duration of 5 min;

– raised to U2 and held there for a duration of 5 min;

– raised to U1, held there for the test time as stated in 12.1;

– immediately after the test time, reduced without interruption to U2 and held there for aduration of at least 5 min to measure partial discharges;

– reduced to 1,1 Um / √3 and held there for a duration of 5 min;

– reduced to a value below one-third of U2 before switching off.

6-1-A TESTING OF POWER TRANSFORMERS

Figure 6.0 – Time sequence for the application of test voltage with respect to earth

During the raising of the voltage up to a level and reduction from U2 down again, possible partialdischarge inception and partial discharge extinction voltages shall be noted.

The background noise level shall not exceed 100 pC.

NOTE: It is recommended that the background noise level should be considerably lower than 100 pC in order toensure that any inception and extinction of partial discharge can be detected and recorded. The above-mentionedvalue of 100 pC at 1,1 Um / √3 is a compromise for the acceptance of the test.

The test is successful if

– no collapse of the test voltage occurs;

– the continuous level of ‘apparent charge’ at U2 during the second 5 min does not exceed300 pC on all measuring terminals;

– the partial discharge behaviour does not show a continuing rising tendency;

– the continuous level of apparent charges does not exceed 100 pC at 1,1 Um / √3.

A failure to meet the partial discharge criteria shall lead to consultation between purchaser andsupplier about further investigations (annex A of IEC 60076-3). In such cases, a long-durationinduced AC voltage test (see clause 18 hereinafter) may be performed. If the transformer meetsthe requirements of 12.4 of IEC 60076-3, the test shall be considered successful.

6-1-B TESTING OF POWER TRANSFORMERS

b. Non-uniformly in su la t ed windings

r-lN

Fig . 6-1. Tes t c i r c u i t f o r induced overvol tage withstand t e s t on non-uniformly in su la t ed winding o f three-phase transformer

G supply genera tor , T step-up t ransformer, T t ransformer unde r t e s t , 1 T cu r ren t transformer: T vo l tage transformer: L compensating r e a c t o r , 3 4 E vol tage d i v i d e r , P ammeter, P vol tmeter , P vo l tmeter (r.m.s.

1 value ) , P vol tmeter ( p e a t value?. 3 4

The t e s t connection shown i n Fig. 6-1 is app l i cab le t o three-phase t ransformers i f t h e i n s u l a t i o n l e v e l o f the n e u t r a l t e rmina l is a t l e a s t one t h i r d of t h e i n s u l a t i o n l e v e l o f t he terminals . The t e s t vo l tage is appl ied t o t h e ind iv idua l phases i n succession. During each app l i ca t ion t h e t e s t vo l tage from te rmina l t o e a r t h is equal t o t h e r a t e d withstand voltage.

The vol tage is measured with a capac i t i ve vol tage d i v i d e r i n conjunction with vol tmeters responsive t o peak andr .m . s . va lues . The peak vol tmeter i nd ica t e s t h e peak value d iv ided by b 2: The t e s t vo l t age is ad jus ted according t o t h i s vo l tmeter .

Test r epo r t

The t e s t vo l tage , f requency, t e s t dura t ion and tapping a r e s t a t e d i n the r epo r t .

Procedure in accordance with IEC 60076-3 (2000) as per pages 6-2-A and 6-2-B below.

b.1 Short-duration AC withstand voltage test (ACSD) for transformers with non-uniformlyinsulated high-voltage windings (Transformers with Um > 72,5 kV)

For three-phase transformers, two sets of tests are required, namely:

a) A phase-to-earth test with rated withstand voltages between phase and earth according totables 2, 3 or 4 of IEC 60076-3 with partial discharge measurement.

b) A phase-to-phase test with earthed neutral and with rated withstand voltages betweenphases according to tables 2, 3 or 4 with partial discharge measurement. The test shall becarried out in accordance with 12.2.2 of IEC 60076-3.

On single-phase transformers, only a phase-to-earth test is required. This test is normally carriedout with the neutral terminal earthed. If the ratio between the windings is variable by tappings,this should be used to satisfy test voltage conditions on the different windings simultaneously asfar as possible. In exceptional cases, see clause 6 of IEC 60076-3, the voltage on the neutralterminal may be raised by connection to an auxiliary booster transformer. In such cases, theneutral should be insulated accordingly.

The test sequence for a three-phase transformer consists of three single-phase applications oftest voltage with different points of the winding connected to earth at each time. Recommendedtest connections which avoid excessive over-voltage between line terminals are shown in figure6.2. There are also other possible methods.

Other separate windings shall generally be earthed at the neutral if they are star-connected, andat one of the terminals if they are delta-connected.

The voltage per turn during the test reaches different values depending on the test connection.The choice of a suitable test connection is determined by the characteristics of the transformerwith respect to operating conditions or test plant limitations. The test time and the time sequencefor the application of test voltage shall be as described in 12.1 and 12.2.2 of IEC 60076-3.

For the partial discharge performance evaluation, during the phase-to-phase test,measurements should be taken at U2 = 1,3 Um.

NOTE The value U2 = 1,3 Um is valid up to Um = 550 kV with AC test values greater than 510 kV. For Um = 420 kVand 550 kV with AC test values of 460 kV or 510 kV, the partial discharge evaluation level should be reduced toU2 = 1,2 Um in order not to exceed the AC withstand voltages of table 4 of IEC 60076-3.

For the three single-phase tests for the phase-to-earth insulation, U1 is the test voltageaccording to tables 2, 3 or 4 and U2 = 1,5 Um / √3.NOTE 1 In the case of transformers with complicated winding arrangements, it is recommended that the completeconnection of all windings during the test be reviewed between supplier and purchaser at the contract stage, in orderthat the test represents a realistic service stress combination as far as possible.

NOTE 2 An additional induced AC withstand test with symmetrical three-phase voltages produces higher stressesbetween phases. If this test is specified, the clearances between phases should be adjusted accordingly and specifiedat the contract stage.

The test is successful if no collapse of the test voltage occurs and if partial dischargemeasurements fulfil the requirements as stated in 12.2.2 of IEC 60076-3 with the followingalteration:


The continuous level of ‘apparent charge’ at U2 during the second 5 min does not exceed500 pC on all measuring terminals for single-phase tests at U2 = 1,5 Um / √3 line-to-earth, or300 pC for phase-to-phase tests at U2 = 1,3 Um or as may be required at extremely low a.c.co-ordination values at 1,2 Um.

Figure 6.2 – Connections for single-phase induced AC withstand voltage tests (ACSD) ontransformers with non-uniform insulation

Connection a) may be used when the neutral is designed to withstand at least one-third of thevoltage U. Three different generator connections to the low-voltage winding are shown. Only a1)is possible if the transformer has unwound magnetic return paths (shell form or five-limb coreform).

Connection b) is possible and recommended for three-phase transformers having unwoundmagnetic return paths for the flux in the tested limb. If there is a delta-connected winding, it hasto be open during the test.

Connection c) shows an auxiliary booster transformer, which gives a bias voltage Ut at theneutral terminal of an auto-transformer under test. Rated voltages of the two auto-connectedwindings are Ur1, Ur2, and the corresponding test voltages U, Ux. This connection may also beused for a three-phase transformer without unwound magnetic return paths having the neutralinsulation designed for less than one-third of the voltage U.

6-2-B TESTING OF POWER TRANSFORMERS

7 . S E P A R A T E - S O U R C E VOLTAGE W I T H S T A N D TEST

Purpose o f t h e t e s t

The o b j e c t of the t e s t i s t o secure t h a t t h e in su la t ion between the windings and the i n s u l a t i o n between windings and ear thed p a r t s , withstand t h e temporary overvol tages and swi tch ing overvoltages which may occur i n s e rv i ce .

Test c i r c u i t

Fig. 7-1. Tes t c i r c u i t f o r separate-source vol tage withstand t e s t

G supply genera tor , T t e s t t ransformer , T transformer under t e s t , 1 1 c u r r e n t t ransformer, L compensating reactor: E vol tage d i v i d e r , 3

ammeter, P voltmeter ( r . m . s . v a lue ) , P vol tmeter (peak va lue ) . P1 2 3

The vol tage i s measured us ing a c a p a c i t i v e vol tage d iv ide r i n conjunct ion with vol tmeters responsive t o r . m . s . and peak values. The peak-voltmeter i nd ica t e s t he peak value d iv ided by \'2. The t e s t vol tage is ad jus t ed according t o t h i s meter.

The gene ra to r s and t h e equipment a r e descr ibed i n t he t e s t equipment list (Sec t ion 20 ) .

Performance

The t e s t is made with single-phase vol tage o f r a t e d frequency. The t e s t vo l tage is appl ied f o r 60 seconds between t h e winding under t e s t and a l l t e rmina ls o f the remaining windings, co re and tank of t he t ransformer, connected toge ther t o e a r t h ( F i g . 7-1).

The t e s t is successfu l i f no co l l apse of t h e t e s t vol tage occurs .

The l i n e te rmina ls of non-uniformly i n s u l a t e d windings a r e t e s t e d by induced t e s t according t o Sec t ion 6.5.

Tes t r e p o r t

The t e s t vo l tage , frequency and t e s t du ra t ion a r e s t a t e d i n t he r e p o r t .

8 . OPERATION TESTS ON ON-LOAD TAP-CHANGER

After t h e tap-changer is f u l l y assembled on t h e t ransformer, t he fol lowing t e s t s a r e performed a t (wi th except ion of b ) 130 % o f the r a t e d a u x i l i a r y supply vol tage:

a 8 complete opera t ing cyc les wi th t h e transformer no t energized

b 1 complete opera t ing cycle with t h e t ransformer not energized, with 85 % o f t h e r a t e d a u x i l i a r y supply vol tage

C 1 complete ope ra t ing cycle with t h e transformer energized and r a t e d vol tage and frequency a t no load

d ) 10 tap-change opera t ions with +- 2 s t e p s on e i t h e r s i d e o f t h e p r i n c i p a l tapping with a s f a r a s poss ib l e t h e r a t e d c u r r e n t of t h e t ransformer, with one winding shor t -c i rcu i ted .

I n p r a c t i c e t h e ope ra t ing t e s t with t h e r a t e d cu r r en t i s usua l ly performed by one complete opera t ing cyc le from one extreme tapping t o another . The c u r r e n t is as f a r a s poss ib l e t h e r a t e d c u r r e n t of each tapping.

9. MEASUREMENT OF ZERO-SEQUENCE IMPEDANCE

Purpose of t h e measurement

The zero-sequence impedance is usua l ly measured f o r a l l star-connected windings o f t h e t ransformer. The measurement is c a r r i e d o u t by supplying a cu r r en t o f r a t e d frequency between the p a r a l l e l connected phase terminals and the n e u t r a l t e rmina l . The zero-sequence impedance per phase is t h r e e times t h e impedance measured i n t h i s way. The zero- sequence impedance i s needed f o r ear th- fau l t p r o t e c t i o n and ea r th - f au l t cu r r en t ca l cu la t ions .

Measuring c i r c u i t and performance o f measurement

Fig. 9-1. C i r x i t f o r zero-sequence impedance measurement

G supply genera tor , T t ransformer t o be t e s t e d , T vol tage 1 transformer, T currenk t ransformer, P vo l tmeter , 6 ammeter, I t e s t

cu r r en t . 3 2 3

The zero-sequence impedance i s dependent on t h e c u r r e n t flowing througn the winding. Usually t h e va lue corresponding t o r a t e d cu r r en t I i s

N s t a t e d . This impl ies t h a t t he measurement is c a r r i e d out with a t e s t cu r r en t of 3 . I . However, t h i s i s not always poss ib l e i n p r a c t i c e s i n c e t h e curren! must be l imi t ed t o avoid excess ive temperature o f m e t a l l i c cons t ruc t iona l p a r t s . The zero-sequence impedance i s measured a s func t ion o f t e s t c u r r e n t , and when necessary t h e f i n a l r e s u l t i s obtained by ex t r apo la t ion .

Resul t

The zero-sequence impedance i s usua l ly given a s a percentage of the r a t e d phase impedance. When t h e transformer has a three-limb core and no delta-connected windings, t he zero-sequence impedance is about 30. . . C ' . % . When t h e t ransformer has a delta-connected winding, t he zero-sequencn impedance i s 0 .8 . . . 1 .0 times t h e corresponding s h o r t - c i r c u i t impedanc->.

In t h e t e s t r e p o r t t he zero-sequence impedance va lues a t t he p r inc l c I .

and extreme tappings a r e s t a t e d .

10. CAPACITANCE MEASUREMENT

Purpose of the measurement

Fig. 10-1

The purpose of the measurement is to determine the capacitances between the windings and the earthed parts and between the different windings of the transformer.

The capacitance values are needed when planning transformer overvoltage protection and calculating the overvoltages affecting the transformer. In addition, the results are used by the manufacturer for design purposes.

Performance of the measurement

All line terminals of each winding are connected together durlng the measurement. The winding capacitances of two- and three-winding transformers are shown on Fig. 10-1.

Transformer capacitances.

a two-wlnding transformer b three-winding transformer

Because the partial capacitances (C) in Fig. 10-1 cannot be measured separately, the values of resulting capacitances (K), obtained by combining the partial capacitances, are measured and the required partial capacitance values are calculated from the measured values. The measurement is carried out by means of a capacitance bridge.

A two-winding transformer is measured as follows:

- The capacitance K between earth and winding No. 1 is 1 measured, when win%ng No. 2 is earthed.

- The capacitance K between earth and winding No. 2 is measured. when wiz& No. 1 is earthed.

- The capacitance K from the interconnected windings No. 1 and 12 2 to earth is measured.

The p a r t i a l capacitances C . C and CZ0 are determined by solving the s e t of equations ( 10. l 1. . .+PO. 33. For transformers with three o r more windings a s imi lar method is used. The number nk of p a r t i a l capacitances (and measurement combinations) is

n = the number o f windings

Test report

The p a r t i a l capacitances a r e given per phase, thus three-phase capacitance values obtained i n the measurement a r e divided by 3 .

Litera ture

(10.1) Bertula, T . , Palva , V . : Transformer capacitances, S a k o - Elec t r i c i ty in Finland 39 (1966) No. 10, p 289...293.

11. INSULATION RESISTANCE MEASUREMENT

W p o s e o f t h e measurement

The purpose o f the measurement is t o determine the leakage current r e s i s t a n c e o f t h e insu la t ion . This is a funct ion of the moisture and impurity contents o f the i n s u l a t i o n and o f i ts temperature such t h a t when these parameters a r e increased the insu la t ion res i s t ance , a s measured a t a cons tant voltage d i f ference across the insu la t ion , depend on t h e s t r e n g t h o f the e l e c t r i c f i e l d during the measurement and thus on the s i z e and const ruct ion of the transformer. This measurement g ives information about t h e condit ion o f the i n s u l a t i o n and secures t h a t the leakage cur ren t is adequately small.


The i n s u l a t i o n r e s i s t a n c e is measured by means o f an insu la t ion r e s i s t a n c e meter a t a voltage o f 5000 V d.c . Each winding is measured separa te ly by connecting the vol tage between t h e winding t o be t e s t ed and e a r t h , while the o the r windings a r e ear thed. The r e s i s t a n c e readings

and R a r e taken 15 S and 60 S a f t e r connecting the voltage. The 60 a so rb t ion r a t i o R 6 0 : ~ 1 5 i s normally 1 , 2 ... 3 i n d r i ed tranformers. The type of meter used, the measuring voltage, temperature, RI5, R60 and R60/R15 a r e s t a t e d i n t h e r epor t .

The readings should be taken after 1 5s1 60s1 180s and 600s! The absorbtion ratio ~ 6 0 \ : R1 5 is not covered by any recognised standard. The polarisation index R600 : R60 (in IEEE called R1 0 : R I ) shall be higher than 1 .l !

\

Readings shall be refered to 20 "C by multiplying the reading at ambient temperature T(hbieno by correction factor given in the table below.

Guide for information only! Not covered by any recognised standard.

Textfeld

Fig. 12-1

12. LOSS FACTOR MEASUREMENT

D i f f e r e n t e l e c t r i c a l measurements can be c a r r i e d out t o check t h e condi t ion of i n s u l a t i o n s between t ransformer windings and between windings and ear thed p a r t s . The r e s u l t ( l eakage cu r ren t r e s i s t a n c e ) obtained i n t h e i n s u l a t i o n r e s i s t a n c e measurement (compare item 11) desc r ibes i n the f i r s t hand t h e behavior of i n s u l a t i o n d i s t a n c e s a t d i r e c t cu r r en t vol tage. The leakage c u r r e n t r e s i s t a n c e depends on t h e measuring vol tage. The l o s s f a c t o r is p r imar i ly a c h a r a c t e r i s t i c quan t i t y f o r the i n s u l a t i o n i t s e l f , and t h e r e f o r e r e s u l t s ob ta ined f o r t ransformers of d i f f e r e n t s i z e s cannot d i r e c t l y be compared t o each o t h e r .

The l o s s f a c t o r measurement w i l l be c a r r i e d o u t by means of a s p e c i a l measuring br idge and a s tandard capac i to r . The measuring vo l t age i s usua l ly 5 kV o r 10 kV. The capac i tance CS o f each winding end t h a t o i pair-wise connected windings and the l o s s f a c t o r tan4 a g a i n s t e a r t h (connec t ions a s i n i tem 10) a r e gene ra l ly def ined i n t he measurement.

Paral le l-connect ion o r ser ies-connect ion (F ig . 12-11 can be used a s equiva len t c i r c u i t o f t he i n s u l a t i o n d i s t a n c e 1 t o 2 t o be measured.

Val id f o r t h e ser ies-connect ion:

Obtained f o r the p a r a l l e l connection

t a n s = 1 . R = l

" 2 = a ! , + t a n 2 6 , c = S

P S tan- ? L + tan' 6

The l o s s f a c t o r tan6 i s proportional, t o t h e e f f e c t i v e r e s i s t a n c e 3- of t h e i n s u l a t i o n di ,s tance a t t h e AC vo l tage . The r e s i s t ance depends zn t he

dielectric losses of the insulation as well as on the leakage current component caused by the AC voltage. The loss factor tan6 can be used as a standard to be noted that the loss factor is the function of the insulation temperature and humidity content. The following equation distance to be measured.

The losses caused by the polarization generated in the insulation of the electrical field are proportional to the square of the voltage. Tan6 corresponding to these losses is independent of the voltage. If tan6 increases when the voltage is raised the reason for it may be that the leakage current resistance decreases (humidity, etc.) or discharges take place.

If a rounding electrode made of half-conducting material has been installed on top of the core, the electrode has an important effect on the size of the displacement angle. In such cases the tan -value does not correctly describe the condition of the insulation in this insulation distance.

Results

The insulation distances measured, the measuring voltages , the t a d , capacitance and temperature of the insulation are stated in the report.

The dielectric dissipation factor (tan delta) shall not exceed 0.5% at an oil temperature of 20°C for new transformers (IEEE Std 62 - 1995). For temperature corrections refer to page 12-2-A below.

Temperature correction factors for tan δ values as per IEEE Std C57.12.90 - 1999

Temperature correction factors for the insulation power factor depend upon the insulatingmaterials and their structure, moisture content, etc. Values of correction factor K listed in theTable below are typical and are satisfactory for practical purposes for use in the equation below.

where

Fp20 is the power factor corrected to 20 °C,

Fpt is the power factor measured at T,

T is the test temperature (°C),

K is the correction factor.

Insulation temperature may be considered to be that of the average liquid temperature. Wheninsulation power factor is measured at a relatively high temperature and the corrected values areunusually high, the transformer should be allowed to cool; and the measurements should berepeated at or near 20 °C.

Table — Temperature correction factors for insulation power factors

Test temperature T(°C)

Correction factor K

10 0.8

15 0.9

20 1.00

25 1.12

30 1.25

35 1.40

40 1.55

45 1.75

50 1.95

55 2.18

60 2.42

65 2.70

70 3.00

NOTE — The correction factors listed above are based oninsulating systems using mineral oil as an insulating liquid.Other insulation liquids may have different correction factors.


CRPACI TQNCE & POWER FkCTOR

Measuring Bridge : Teitex - 5chering

l. kasurmeni o f the result ing caoacitances kasuring voltace 10 hV , 50 H: at 25°C

1.2 Hioh-voltage t o W-Winding earthed HV- k S7Wintiiry

CX(,?)= C2+C3tC6= 31675 pF

tan = 0.268 %

1.3 High-voltage t o m l d i n d i n g earthed W- L Lv-Uindhg

CX(3)= Cl+C2tCS= 12190 DF

tan = 0.2775:

1.4 i i i + d t a g e t o W- L. LV-~indinq part hed . p l j i n d i n q

tan = 0.253%

It ransforber-ran~ earthed)

CRPRCI TANCE R POWER FRCTDR

1.5 Hi@rvoltaoe t o g w - II LV-Windirpc! earthed HV-Windity

tan = 0 . 3 2 t :

tan = 6.314s

2. calculation o f the m r t i a i coacitances

Cl= K X G ) +EX (6)-CX (4) ) /2= 130G pF i= 636 trFiPhase)

G= (CX (2)KX (3)-C): (5) )/2= 22936 of (= 7645 $/Phase)

C;:= (EX (:)+CX (2) -CX (4) )E= 753,' d (= 2511 &/Phase)

C4= (CX(l)+CT(6)<XI5))/2= 6310 DF (= 2IW OFIPhase)

E= (CX (4)+CX (5)-CX (6)-EX (2) 1 /l= 212 pr' (= 71 $;Phase)

C&= (CX(4)+CX (5)-CX (1)-CX13)) /L- 1237 d (= 402 $/Phase)

13. MEASUREMENT OF THE ELECTRIC STRENGTH OF THE INSULATING O I L

The e l e c t r i c s t r e n g t h o f o i l is given by t h e breakdown vol tage , measured using an e l ec t rode system i n accordance with IEC 156 (13.1). The e l ec t rodes a r e s p h e r i c a l sur faced with 25 mm r a d i u s and a r e 2.5 mm apa r t . The measurement is c a r r i e d out a t 50 Hz, t h e r a t e of increase of t he vol tage being 2 kV/s. The e l e c t r i c s t r e n g t h is the average of s i x breakdown vol tage values.

The e l e c t r i c s t r e n g t h o f new t r e a t e d o i l should be a t l e a s t 60 kV. O i l which does no t withstand t h i s vol tage may con ta in a i r bubbles, dus t o r moisture.

In p r a c t i c e t h e breakdown vol tage is about 70 kV.

L i t e r a tu re

(13.1) I E C 156 (1963) Method f o r t h e determinat ion of t h e e l e c t r i c s t r e n g t h of i n s u l a t i n g o i l s .

(13.2) I E C 296 (19821, Spec i f i ca t ion forunusedmineralinsulating o i l s f o r t ransformers and switchgear.

14. TEMPERATURE-RISE TEST

Purpose o f t h e measurement

The purpose is t o check t h a t t h e temperature rises of the o i l and windings do n o t exceed t h e limits agreed on o r s p e c i f i e d by t h e s tandards . Apparatus

The supply and measuring f a c i l i t i e s a s w e l l a s t h e measuring c i r c u i t a r e t he same a s i n load l o s s measurement (Sec t ion 4 ) and i n t h e r e s i s t a n c e measurement (Sec t ion 3 ) . In add i t i on thermometers a r e used f o r t h e measurement of t he temperature o f the o i l , coo l ing medium and t h e ambient temperature and f u r t h e r a temperature r eco rde r and Pt-100 r e s i s t i v e senso r s are used f o r t he measurement o f c e r t a i n temperatures and f o r equi l ibr ium c o n t r o l .

Performance o f t he measurement

The t e s t is performed by us ing t h e s h o r t - c i r c u i t method. The temperatur rise o f t h e windings is determined by t h e r e s i s t a n c e method. The t e s t i, performed a s follows:

Cold r e s i s t a n c e measurement

The r e s i s t a n c e and t h e corresponding o i l temperature a r e measured. Resis tances a r e measured between l i n e t e rmina l s e .g . , A-B and 2A-2B. The winding temperature i s t h e same a s the o i l temperature.

Determination of t h e temperature r i s e o f o i l --------p---------

The power t o be suppl ied t o t h e transformer is t h e sum of t h e no-load l o s s e s and load l o s s e s on the tapping on which t h e temperature-rise t e s t is t o be performed ( g e n e r a l l y t h e maximum l o s s tapping) . With t h i s power the t ransformer is warmed up t o thermal equi l ibr ium. The supply values and t h e temperatures o f d i f f e r e n t po in ts a r e recorded a t s u i t a b l e -1-e i n t e r v a l s . The o i l temperature r i s e above t h e coo l ing medium temperarur can be ca l cu la t ed from :he equilibrium tempera tures .

Determination of t h e temperature r i s e o f windings ---------------- - - Without i n t e r r u p t i n g t h e supply the cu r r en t is reduced t o r a t e d c u r e ? ? f o r 1 h. The supply va lues and t h e temperatures a r e recorded a s above. When t h e c u r r e n t has been c u t o f f the ho t - r e s i s t ance measurement is performed. The t e s t connect ion is changed f o r ca r ry ing ou t t h e r e s i s t a n c e measurement and a f t e r t he induc t ive e f f e c t s have disappeared the resistance-time-curves a r e measured f o r a s u i t a b l e per iod of tr-a ( z e r o time is the i n s t a n t o f switching o f f t h e s u o p l v ) . The r e s i s t a ~ r - is measured between t h e same l i n e t e r a i n a i s a s i n t h e cold resistance measurement.

The r e s i s t a n c e of t h e windings a t shut-down a r e obtained by ex t r apo la t ing the res i s tance- t ime -curves t o t h e i n s t a n t o f swi teh l r ; o f f . The temperature r r s e s of the windings above t h e o i l temperature :re

calculated on the basis of the "hot'' and "cold" res i s t ance values and the o i l temperature. The temperature r i s e s o f the windings above the cooling medium temperature a r e found by adding the temperature r i s e of o i l above the cooling medium temperature to the before mentioned winding temperature r i ses .

For multi-winding transformers the l a t t e r p a r t of the temperature r i s e t e s t is generally carried ou t several times i n order t o determine the individual winding temperature r i s e s a t the speci f ied loading combination.

For air-cooled transformers with na tu ra l a i r c i r cu l a t i on the temperature of the cooling medium is t he same a s the ambient temperature. The ambient temperature is measured by means of a t l e a s t th ree thermometers, which a r e placed a t d i f f e r en t points around the transformer a t a distance defined by the standards approximately half-way up the transformer. For forced-air cooled transformers the temperature of the ingoing a i r is measured. If water is used as cooling medium, the water temperature a t the intake o f the cooler is the reference temperature.

The top o i l temperature is measured by a thermometer placed i n an o i l - f i l l e d thermometer pocket on the cover o r i n t he tube leading t o - t h e coolers. If transformer has separate cooler, the top o i l temperature is measured from the tube leading t o the cooler near the transformer. Furthermore the temperatures of the o i l coming from o r going t o the transformer are measured and a l so some other temperatures which may be in teres t rng.

The readings of the thermometers mounted on the transformer a r e checked i n connection with the temperature r i s e t e s t , and the power taken by the o i l pump and fan motors is measured.

Results

The temperature r i s e s a re ca lcula ted as follows:

O i l temperature r i s e - - The temperature r i s e of t o p o i l a is

t o

ra ted losses P 75OC + PO k power supplied during the t e s t exponent according t o the standard top o i l temperature cooling medium temperature

ii2 = temperature of o i l going i n t o t h e cooler 63 = temperature of o i l coming from the cooler

Temperature r i s e o f windings

The average temperature of o i l ' before the hot-resistance measurement 0

i S

The average temperature of winding+_ is

23S°C f o r Copper 22S°C foe Aluminium cold r e s i s t a n c e hot r e s i s t a n c e the average temperature of o i l during cold res is tance measurement

The average temperature r i s e 8 of the winding above the oil r 0 temperature i s

'N = ra ted cur ren t o f the winding It = t e s t current y = exponent according t o the standard

The average temperature r i s e a of the winding above the ambient r temperature i s

The temperature r i s e 8 of the hot spot o f the winding above the ambient temperature i s h S

The winding temperature ind ica to r , i f any, w i l l be adjusted on the bas is o f the temperature r i s e 8

hs '

Results

- cold res i s t ance values and the corresponding o i l temperature

- temperatures o f o i l and cooling medium i n thermal equilibrium and the corresponding losses

- hot resistances at shut-down and the corresponding cur-ents

- temperature rises calculated from the measuring results

In addition information on the winding combination or combinations involved in the test, the tapping position, the cooling method and the time of delay is given.

Literature

(14.1) Kiiskinen, E.: Determining the temperature rise in a transformer winding using the resistance method. Sahko-Electricity in Finland 47 (19741, No. 1.

15. LIGHTNING IMPULSE TEST

Purnose of the test

The purpose of the impulse voltage test is to secure that the transformer insulations withstand the lightning overvoltages which may occur in service.

Testing equipment

Impulse generator

Fig. 15-1

Basic circuit diagram of the impulse generator.

C impulse capacitor 1 R charging resistor C R series resistor S R low-ohmic discharging a resistor for switching impulse,

R high-ohmic discharging resistor for switching impulse ... F main spark-gaps, . . . auxiliary spark-gaps . Fal an

Test is type test up to Um=72.5kV, and is routine test for transformers with windings of Um=l OOkV and above for all power windings.

The impulse generator design is based on the Marx circuit. The basic circuit diagram is shown on Fig. 15-1. The impulse capacitors C (12 capacitors of 750 nF) are charged in parallel through the charg?ng resistors R (45 kG) (highest permissible charging voltage 200 k V ) . Nhen

C - . . the chai-gii-~g voltage k~aa rrilc~sri tile ~.eqillr'ed v ~ ~ l u e . Lr.r&J~lwri n L I:!Y

spark-gap F is initiated by an external triggering pulse. When F 4 breaks downf the potential of the following stage (points B and C.

rises. Because the series resistor R is of low ohmic value compared with the discharging resistor R (4.5 kQ) and the charging resistor R

b and since the low-ohmic discharging resistor R is separated from the C ' circuit by the auxiliary spark-gap F

a1 ' the pgtential difference across the spark-gap F rises considerably and the breakdown of F is 2 2

initiated. Thus the spark-gaps are caused to break down in sequence. Concequently the capacitors are discharged in series-connection. The high-ohmic discharge resistors R are dimensioned for switching impulses

b and the low-ohmic resistors R for lightning impulses. The resistors R are connected in parallel witg the resistors R when the auxiliary a

spark-gaps break down, with a time delay of a P;w hundred nanoseconds. This arrangement is necessary in order to secure the functioning of the generator.

The required voltage is obtained by selecting a suitable number of series-connected stages and by adjusting the charging voltage. In order to obtain the necessary discharge energy parallel or series-parallel connections of the generator can be used. In these cases some of the capacitors are connected in parallel during the discharge.

Max. test voltage amplitudes: 2.1 MV lightning impulse, 1.6 MV switching impulse.

Test circuit

Fig. 15-2 Equivalent diagram of the impulse test circuit.

C resulting impulse capacitance, Rsr r resulting series resistance, R resulting discharge resistance, L L stray inductances, er input capacitance of transformer, 'lp

l transformer inhctance, C capacitance of voltage dlvider, F 1 - - - - l . ---- - 0 : - - . . l -- ---,.--&A- '. r r p a -. .,,tp...== =ca,C.. .". , Fa calibratf i i i ; p k t ~ ~ ~ g ~ p , n'

2 protective resistor.

The required impulse shape is obtained by selecting the series and discharge resistors of the generator suitably.

The front time can be calculated approximately from the equation:

and the time to half value from the equation:

The factor k depends on the quantities R sr* Rar' L. 1 and Cr.

In practice the testing circuit is dimensioned according to experience.

Voltage measuring circuit ........................ The impulse shape and the peak value of the impulse voltage are measured by means of an oscilloscope and a peak voltmeter which are connected to the voltage divider (Fig. 15-3). The measuring range can be changed by short-circuiting part of the high voltage capacitors or changing the low voltage capacitor of the divider.

Fig. 15-3

The impulse voltage measuring circuit.

E damped capacitive voltage divider, W measuring cable (=wave impedance = R ) , P1 oscilloscope, 'P peak

2 voltmeter, R terminal resistance OF the measuring cable, R damping resistor oh voltage divider, C high 1 voltage capacitor of voltage divider, C2 low voltage capacitor of divider.

The measuring circuit is checked in accordance with the standards (15-2) and (15.3). If necessary the sphere-gap calibration of the measuring circuit can be performed in connection with the testing according to the s t ~ ~ c i a r d (15.G!.

Transformer testing and fault detection connections

The lightning impulse test is normally applied to all windings. The impulse test-sequency is applied successively to each of the line

terminals of the tested winding. The other line terminals and the neutral terminal are earthed (single-terminal test, Fig. 15-4a and b) . When testing low voltage windings of high power the time to half-value obtained is often too short (Fig. 15-51. However, the time to half-value can be increased by connecting suitable resistors (R in Fig. 15-4b) between the adjacent terminals and earth. According to the standard IEC 76-3 the resistances of the resistors must be selected so that the voltages at the adjacent terminals do not exceed 75 % of the test voltage and the resistance does not exceed 500(1 . A delta-connected winding (and star-connected winding, unless the neutral is available) is also tested with an impulse test-sequence applied to the line terminals of the tested winding connected together, while the other windings are earthed (three-terminal test, Fig. 15-4c).

For delta-connected windings the single and three-terminal testings can be combined by applying the impulse to two line terminal6 at a time, while the other line terminals are earthed (two-terminal te&ing, Fig. 15-4d). In this case two phases are simultaneously tested in a single- terminal connection and one phase in a test connection corresponding to three-terminal testing.

The two- and three-terminal testings are not included in the standard (15.5), but they can be done if it is so agreed.

Fig. 15-4 Transformer impulse testing and fault detection connections.

a and b 1- terminal testing, c 3- terminal testing, d 2- terminal testing, e test with transferred voltages, f neutral terminal testing.

When the low voltage winding cannot in service be subjected to lightning overvoltages from the low voltage system (e.g. step-up transformers, tertiary windings) the low voltage winding may (by agreement between customer and manufacturer) be impulse tested simultaneously with the impulse tests on the high voltage winding with surges transferred from the high voltage winding to the low voltage winding (Fig. 15-4e, test with transferred voltages). According to IEC 76-3 the line terminals of the low voltage winding are connected to earth through resistances of such value (resistances Ra in Fig. 15-4s) that the amplitude of transferred impulse voltage between line terminal and earth or between different line terminals or across a phase winding will be as high as possible but not exceeding the rated impulse withstand voltage. The resistance shall not exceed 5000q . The neutral terminal is normally tested indirectly by connecting a high-ohmic resistor between the neutral and earth (voltage divider Ra, R ) and by applying the impulse (Fig. 15-4d) to the line terminals U connected together. The impulse test of a neutral terminal is performed only if requested by the customer.

For fault detection in single-terminal and two-terminal tests the neutral of star-connected windings are earthed via a low-ohmic resistor (R 1. The current flowing through the detection resistor during the test isUrecorded by means of an oscilloscope. Evidence of insulation failure arising from the test would be given by significant discrepancies between the calibration impulse application and the full voltage applications in recorded current wave-shapes. Certain types of faults give rise to discrepancies in the recorded voltage wave-shapes as well.

For fault detection in three-terminal tests and tests on the neutral terminal the adjacent winding is earthed thro~gh a low-ohmic resistor. The fault detection is then based on recording the capacitive current which is transferred to the adjacent winding.

Performance of the impulse test

The test is performed with standard lightning impulses of negative polarity. The front time (T ) and the time to half-value (T ) are

2 defined in accordance with &he standard (15.4) (Fig. 15-51,

Fig. 15-5 Standard l i g h t n i n g impulse

Front time T1 = 1.2 "S +- 30 %. Time t o half-value T2 = 50 bs +- 20 %.

I n p r a c t i c e t h e impulse shape may devia te from t h e s tandard impulse when t e s t i n g low-voltage windings o f h igh r a t e d power and windings o f high input capac i tance .

The vo l t age measurement is based on the reading o f t h e peak vol tmeter . If r equ i r ed t h e vol tage measuring system, inc luding t h e peak vol tmeter , is c a l i b r a t e d by means o f sphere-gap, i n connect ion with t h e t e s t i n g o f t h e f i r s t l i n e te rmina l . I n t e s t i n g the o t h e r t e rmina l s t h e vol tage . measurement i s based on t h e reading of t h e c a l i b r a t e d peak vol tmeter . The vol tage c a l i b r a t i o n i s performed a t 60 % o f t h e vol tage t e s t l e v e l .

Osc i l lographic records a r e made o f the app l i ed vo l t age and t h e vol tage ac ros s t h e f a u l t d e t e c t i o n r e s i s t o r RU during c a l i b r a t i o n a t 62.5 % of t h e vol tage t e s t l e v e l and dur ing t h e 100 % vol tage app l i ca t ions .

A t f u l l t e s t vo l tage each l i n e te rmina l i s t e s t e d with a s many impulses a s is r equ i r ed by the s t anda rd . I n order t o f a c i l i t a t e t he d e t e c t i o n o f nnccible r--.-- d i c c r e ~ a n c i e ~ in the c s c i l l o g r ~ z f i i c r $ ~ c r 5 ~ ~ the ~ = ~ i 1 1 ~ ~ ~ ~ ~ ~ - a t t e n u a t i o n is adjus ted such t h a t t he curves recorded during t h e f u l l wave a p p l i c a t i o n s can be brought t o co inc ide wi th those obta ined during the c a l i b r a t i o n .

Unless agreed otherwise d i f f e r e n t tappings a r e s e l ec t ed f o r t h e impulse t e s t s on t h e th ree phases o f a three-phase transformer, u sua l ly t h e two extreme tappings and the p r i n c i p a l tapping.

Tes t r e p o r t

The summary of t e s t r e s u l t s is given on a form termed "Report o f impulse vol tage withstand t e s t on transformer". The osc i l l og raph ic r eco rds and measurement records a r e s t o r e d i n t h e a r ch ives , where they a r e a v a i l a b l e when requi red .

L i t e r a t u r e

(15.1) IEC Publ. 60-3 (1976): High-voltage t e s t techniques. P a r t 3: Measuring devices .

(15.2) I E C Publ. 60-4 (1977): High-voltage t e s t techniques. P a r t 4: Applicat ion guide f o r measuring devices .

(15.3) IEC Publ . 52 ( 1960) : Recommendations f o r vo l tage measurement by means of sphere gaps.

(15 .4) I E C Publ. 60-2 (1973): High-voltage t e s t techniques. P a r t 2 : Tes t procedures.

(15.5) I E C Publ. 76-3 (1980) : Power t ransformers . P a r t 3: I n s u l a t i o n l e v e l s and d i e l e c t r i c t e s t s .

16. TEST WITH LIGHTNING IMPULSE CHOPPED ON THE TAIL

Purpose o f t h e t e s t

The purpose o f t h e chopped l i g h t n i n g impulse t e s t is t o secure t h a t t h e t ransformer i n s u l a t i o n s withstand t h e vo l t age s t r e s s e s caused by chopped l i g h t n i n g impulse, which may occur i n s e r v i c e .

Tes t ing equipment

For t h e l i g h t n i n g impulse t e s t t h e same t e s t i n g and measuring equipment and t h e same t e s t i n g and f a u l t de t ec t i on connec t ions a r e used a s f o r t h e s tandard l i g h t n i n g impulse t e s t . The impulse is chopped by means o f a t r iggered-type chopping gap connected t o t h e t e rmina l t o which t h e impulse is app l i ed . The de lay o f t h e chopping-gap i g n i t i o n impulse i n r e l a t i o n t o t h e i g n i t i o n o f t h e impulse g e n e r a t o r is ad jus t ab l e , thus t h e time T from t h e s ta r t o f t h e impulse t o t h e chopping can be ad jus ted ( f ig . 16-11.

Performance o f t h e t e s t

The t e s t is performed wi th impulses o f nega t ive p o l a r i t y . The dura t ion T from t h e beginning o f t h e impulse t o t h e chopping can vary wi th in t h e

C range o f 2 . . .6 PS ( F i g . 16-1). According t o t h e s t anda rd (16.1) t h e amount o f overswing t o oppos i te p o l a r i t y s h a l l be l im i t ed t o n o t more than 30 % o f t h e ampli tude o f t h e chopped impulse ( F i g . 16-1). If necessary t h e overswing amplitude w i l l be l i m i t e d t o t h e value mentioned by means o f a damping r e s i s t o r i n s e r t e d i n t h e chopping c i r c u i t .

The peak value of the chopped impulse 'is 1 .l times the amplitude of full impulse.

Fig . 16-1 Chopped l i g h t n i n g impulse. T = 1 . 2 k s +- 3 0 % ( *2 = 50 +- 20 % ) Tc = 2 . . .6 a s

The voltage measurement is based on the peak voltmeter indicat ion. I f necessary t h e voltage measuring c i r c u i t can be c a l i b r a t e d with t h e a i d of a sphere-gap.

The t e s t with chopped l ightning impulse is combined with the t e s t ca r r i ed ou t with standard impulse.

The following order of pulse appl ica t ions is recommended by the standard (16.1)

- one 62.5 % f u l l impulse - one 100 % f u l l impulse - one o r more 62.5 % chopped impulses - two 100 % chopped impulses - two 100 % f u l l impulses

The f a u l t de tec t ion is a l s o f o r chopped impulses primari ly based on the comparison of voltages and winding currents obtained a t 62.5 % c a l i b r a t i o n voltages and 100 % t e s t voltages. In order t o make the comparison of f a u l t de tec t ion oscillograms obtained a t 100 % voltage a r e of one same s i z e a s c a l i b r a t i o n oscillograms obtained a t 62.5 % voltage. A t chopped impulse the f a u l t de tec t ion i s add i t iona l ly secured s ince the t e s t sequence includes the appl ica t ion of two standard impulses a f t e r t h e applicaton o f t h e chopped impulses. A t high t e s t voltages (> 750 kV) t he re is a small delay i n the i g n i t i o n s of the chopping-gap, which causes d i f ferences i n t h e f a u l t de tec t ion and c a l i b r a t i o n oscillograms of voltages and winding currents . In t h i s case the f a u l t de tec t ion must be based primari ly on the recordings obtained a t the app l i ca t ion of f u l l impulses.

When carry ing out the chopped-impulse t e s t , unless otherwise agreed, d i f f e ren t tappings a r e se lec ted f o r the t e s t s on the th ree phases o f a three-phase transformer, usually the two extreme tappings and the p r inc ipa l tapping.

Test r e o o r t

The t e s t voltage values, impulse shapes, tappings and the number of impulses a t d i f f e r e n t voltage l e v e l s a r e s t a t e d i n the repor t .

The osc i l lographic records and measurement records a r e s tored i n the archives , where they a re ava i l ab le when required.

L i t e ra tu re

(16.1) I E C Publ. 76-3 (1980): Power transformers, Pa r t 3: Insula t ion l e v e l s and d i e l e c t r i c t e s t s .

17. SWITCHING IMPULSE TEST

Purpose o f the t e s t

The purpose of the switching impulse t e s t is t o secure t h a t the insula t ions between windings, between windings and ea r th , between l i n e terminals and e a r t h and between d i f f e r e n t terminals withstand the switching overvoltages, which may occur i n se rv ice .

Performance of the t e s t

The same t e s t i n g and measuring equipment a s f o r t h e l ightning impulse t e s t a r e used here.

According t o the s tandard (17.1) the switching impulse t e s t is ca r r i ed out on each l i n e terminal of a three-phase winding i n sequence. A single-phase no-load t e s t connection is used i n accordance with Fig. 17-1. The voltage developed between l i n e te rminals during the t e s t is approximately 1.5 times t h e t e s t voltage between l i n e and neu t ra l terminals .

The f l u dens i ty i n the magnetic c i r c u i t inc reases considerably during the t e s t . When the co re reaches sa tu ra t ion t h e winding impedance i s d r a s t i c a l l y reduced and a chopping of the appl ied voltage takes place (F ig . 17-2). The time t o sa tu ra t ion determines t h e durat ion of the switching impulse. Because the remanent f l u x can amount t o even 70 t o 80 % of the s a t u r a t i o n f l u x , the i n i t i a l remanence of the core has a g rea t influence on the vol tage durat ion. By introducing remanent f lux of opposite p o l a r i t y i n r e l a t i o n t o the f l u x caused by the switching impulse, the maximum poss ib le switching impulse durat ion can be increased. The remanence of opposite p o l a r i t y is introduced i n the core by applying low vol tage impulses of opposi te p o l a r i t y t o the transformer before each f u l l voltage t e s t impulse.

Fig. 17-1 Transformer switching impulse t e s t ing and f a u l t de tec t ion connections.

Test is not applicable up to Um=170kV (IEC 60076-3, 2000), and is routine test for windings with Um=245kV and above.

The test is performed with impulses of negative polarity. The requirements on the switching impulse shape given in the standard IEC 76-3 are summarized in Fig. 17-2.

The voltage measurement is based on the peak voltmeter indication. The voltage measuring circuit can be calibrated with the aid of a sphere-gap when required.

Fig. 17-2 Switching impulse

Front time T1 > ~ O O Ys Time above 90 % Td > 200 ks Time to the first zero passage T > 500 PS z

Calibration oscillograms of voltages and winding currents are recorded at 62.5 % voltage level for comparison with the fault detection oscillograms recorded at 100 % voltage.

At full test voltage each phase will be tested with the number of impulses required by the relevant standard. In order to facilitate the comparison of oscillograms the oscilloscope will be attenuated so that the fault detection oscillograms are of the same size as the calibration oscillograms,

When comparing the fault detection and calibration oscillograms it is to be noticed that the magnetic saturation causes drastical reduction of voltage and increase in winding current and the time to saturation is dependent on the amplitude of the applied voltage. Thus voltage and current oscillograms obtained at full test voltage and at 62.5 % voltage level will deviate from each other in this respect. In additon

dis turbances caused by corona d ischarges i n t h e t e s t c i r c u i t may be found on the cu r r en t osc i l lograms recorded a t t e s t vol tage.

The f a u l t de t ec t ion is mainly based on t h e vol tage osci l lograms. The t e s t is successfu l i f no sudden co l l apse o f vo l tage caused by f lashover o r breakdown is ind ica t ed on t h e vol tage osc i l lograms and no abnormal sound e f f e c t s a r e observed. ' h e n t h e core reaches s a t u r a t i o n a s l i g h t no ise caused by magnetos t r ic t ion can be heard from t h e t ransformer.

Tes t r epo r t

The t e s t vol tage va lues , impulse shapes, and number o f impulses a t d i f f e r e n t vo l tage l e v e l s a r e s t a t e d i n t h e r e p o r t . The osc i l l og raph ic records a r e s to red i n t h e a r ch ives , where they a r e a v a i l a b l e when requi red .

L i t e r a t u r e

(17.1) IEC Publ. 76-3 (1980): Power t ras formers . P a r t 3: I n s u l a t i o n l e v e l s and d i e l e c t r i c t e s t s .

18. PARTIAL DISCHARGE MEASUREMENT

Scope and object

A partial discharge in an insulating medium is a localized electrical discharge, which does not bridge the electrodes of the insulation structure. The field strength of a weak part of the dielectric may exceed the dielectric stregth, which causes a breakdown. It is, however, to be observed that the weak parts mentioned may form a small portion of the insulation structure only. The remaining whole insulating gap can, therefore, withstand voltage stresses corresponding even to the test voltage, and the breakdown remains partial. The ionic discharge following the test voltage, and the breakdown is called a partial discharge for the above mentioned reasons.

Resulting from a partial breakdown the voltage difference across the weak part of the dielectric decreases so much that the discharge currenc is interrupted. Due to the sinusoidal variation of the applied voltage the electrical field strength increases again after the discharge has been extinguished. When the field strength reaches its critical value, a new discharge occurs. Thus discharges take place repeatedly. (Fig. 18- l) .*

The situation is enlightened by the simple analogue circuit of a c 3 ' ; : - 7 (Fig. 18-21. C is the capacitance of the whole insulating gap, t h e spark-gap and ?he capacitance C represent the cavity and the capacitance C represenrs the dfelectric in series with Cc. b

When t h e vol tage U across C has increased enough, t h e spark-gap C i g n i t e s . The c a p a c ~ t a n c e C s i s cha rges and t h e vol tage d i f f e r ence ac ros s

C t he c a v i t y vanishes wi th in 1. . .1000 ns. The d ischarge magnitude o r apparent charge q and t h e vol tage Uc a r e r e l a t e d by t h e following equation:

The discharge g ives r i s e t o a c u r r e n t pu lse , which causes a f a s t vo l tage change a t t he te rmina ls o f t h e t ransformer; t h i s change can be measured by means of a capac i t i ve vol tage d iv ide r and a pulse t ransformer.

Fig. 18-2

Analogue c i r c u i t o f a gas - f i l l ed cavi ty .

The p a r t i a l discharges do no t l e a d t o an immediate breakdown. They have, however, o the r e f f e c t s on t h e i n s u l a t i n g medium:

- t he su r f ace of t h e d i e l e c t r i c is bombarded by iones , which cause temperature-r ise and may r e s u l t i n degrading and chemical changes i n t h e i n s u l a t i n g ma te r i a l

Chemical changes may g i v e r i s e t o mater ia l components, which speed up ageing. On t h e o the r hand the p a r t i a l discharges may a l s o be ext inguished by t h e inf luence of some o the r degradation products

- discharges cause high l o c a l f i e l d s t r e n g t h s near t h e discharge s i t e .

These phenomena r e s u l t i n degrada t ion of the d i e l e c t r i c p rope r t i e s of t he i n s u l a t i n g medium, and inc rease of l o s ses .

The o b j e c t of t he p a r t i a l d i scha rges measurement i s t o revea l t h e above mentioned weak p a r t s of t h e d i e l e c t r i c , ,which may cause des t ruc t ion o f J-he t-..--*--.l-r ; - - - - - - . 4 -- -A." I* -.."I "l a l l b . A... a b L Y A k b .

Measurement c i r c u i t

I

Fig. 18-4 Measurement o f p a r t i a l - discharges.

G feeding genera tor 1

T t ransformer t o be t e s t e d 1

T pulse t ransformer 2

T s t e p up t ransformer 3

L compensating r e a c t o r s 1

P ammeters 1

Z low-pass f i l t e r s 1

P volt-meter (peak va lue) 2

Z terminal r e s i s t o r s o f measuring cable P osc i l loscope 2

W measuring cables 3

1 P volt-meter

E c a p a c i t i v e vol tage d iv ide r 4

Z reactance 3

The feeding and measuring instruments used a r e described on a sepa ra t e measuring instrument list (Sect ion 20) .

Performance of t he measurement

The measurement i s based on observing and evalua t ing the apparent charge i n a c c ~ r d z n c c x i t n tne standard ( : Y . t j j LEC 76-3. The neasuring system is b a s i c a l l y a wide-band system, but a narrow-band instrument can be connected t o the system i f necessary.

S t a b i l i t y t e s t

F i e . 18-3

Cal ibra t ion .

Due t o i n t e r n a l capaci tances, t h e vol tage on t h e high vol tage s i d e of t he t ransformer under t e s t may r i s e t o an unacceptably high value when connecting the generator t o t h e feeding c i r c u i t . For t h i s reason t h e s t a b i l i t y of t h e generator vo l tage con t ro l must be t e s t e d .

The s t a b i l i t y is t e s t e d a t a vo l t age equal t o h a l f t h e measurement vol tage. Therefore, spark-gaps a r e connected between t h e high vol tage te rmina ls and e a r t h . The spark-gaps a r e s e t according t o t h e maximum permissable vol tage o f t h e t ransformers .

In t h e c a l i b r a t i o n measurement (F ig . 18-31 a n apparent charge q is i n j e c t e d between each high vol tage te rmina l and e a r t h . The vol tgge pulse caused by t h e i n j e c t e d charge is measured by means o f an o s c i l l o p e with the a i d o f pu lse t ransformers connected t o t h e t e s t t a p of t h e bushings. The reading on t h e osc i l loscope corresponds t o t h e charge q . The high- vol tage s i d e o f t he step-up t ransformer is ear thed during tRis measurement.

C c a l i b r a t i o n gene ra to r , wfiich produces charge pulses of magnitude q .

0

P a r t i a l d i scharge measureaent

The vol tage i s increased s tepwise , f i r s t up t o t h e measuring vol tage U when t h e occurance of d i scharges i s checked. The t e s t vol tage is

2 '

increased t o t h e pre-s t ress vol tage l e v e l U and he ld there f o r a 1

dura t ion of 5 seconds. The pre-stress vol tage is applied i n order t o i g n i t e t he discharges. Thereaf te r , the vol tage is rapid ly reduced t o U2 and maintained a t t h i s value f o r t he agreed dura t ion o f t ime t (Fig . 18-51. During t h i s per iod t h e occurence of d ischarges is beingm%ecked a t t h e terminals o f t h e transformer. I f d i scharges occur, t h e r e s u l t s a r e recorded i n o rde r t o determine the d ischarge magnitudes. I f there a r e discharges a t t h e vol tage l eve l U t h e vol tage is decreased stepwise a f t e r t he dura t ion o f time Tmes i n orger t o determine t h e extension voltage. The vol tage measurement is c a r r i e d ou t a t t he high vol tage s ide of t h e transformer t o be t e s t e d (Fig. 18-4).

Old IEC-Cycle! For procedure jn accordance with IEC 60076-3 (2000) refer to next page.

F i g . 18-5 Test vol tage

U pre - s t r e s s vol tage 1

U measuring vol tage 2

U . p a r t i a l discharge incept ion vol tage l

U p a r t i a l d ischarge ex t inc t ion vol tage e

According :G tha standard (18.6) I F Z 76-3 tile ~ e s i is car r ie r i cur , ..:?g t h e following values of t e s t vol tages between l i n e and n e u t r a l ter-:nais and t e s t period dura t ions : -

U1 = um

- U2

= e l t h e r 1.3 urn/V3 with q < 300 pc o r l. 5 urnf13 with q ? - 500 pc

- t, = 5 min +L = 30 min mes

ACLD sequence as per IEC 60076-3, clause 12.4

The voltage shall be

– switched on at a level not higher than one-third of U2;

– raised to 1,1 Um / √3 and held there for a duration of 5 min;

– raised to U2 and held there for a duration of 5 min;

– raised to U1, held there for the test time as stated in 12.1 of IEC 60076-3;

– immediately after the test time, reduced without interruption to U2 and held there for aduration of at least 60 min when Um ≥ 300 kV or 30 min for Um < 300 kV to measure partialdischarges;

– reduced to 1,1 Um / √3 and held there for a duration of 5 min;

– reduced to a value below one-third of U2 before switching off.

The duration of the test, except for the enhancement level U1, shall be independent of thetest frequency.

A = 5 minB = 5 minC = test timeD = 60 min for Um ≥ 300 kV or 30 min for Um < 300 kVE = 5 min

Figure 8.5.a – Time sequence for the application of test voltage for induced AC long-duration tests(ACLD) as per 12.4 of IEC 60076–3

During the whole application of the test voltage, partial discharges shall be monitored.

The voltages to earth shall be:

U1 = 1,7 Um / √3U2 = 1,5 Um / √3

NOTE For network conditions where transformers are severely exposed to over-voltages, values for U1 and U2 canbe 1,8 Um / √3 and 1,6 Um / √3 respectively. This requirement shall be clearly stated in the enquiry.

The background noise level shall not exceed 100 pC.

As long as no breakdown occurs, and unless very high partial discharges are sustained for along time, the test is regarded as non-destructive. A failure to meet the partial dischargeacceptance criteria shall therefore not warrant immediate rejection, but lead to consultationbetween purchaser and supplier about further investigations as described in IEC 60076-3.


When the t e s t is ca r r i ed out a s a spec ia l t e s t , the t e s t procedure can

Test r epor t ltest for windings with Um=245kV and above.

be separately agreed upon.

A summary of t e s t r e s u l t s i s put down on a form made f o r t h i s purpose. The form is s to red i n the archives, and is then ava i l ab le when requested.

Test is special test up to Um=170kV, and is routine

Litera ture

ELECTRA No. 19, November, 1971. ELECTRA No. 11, December, 1969. Brown, R . D . , Corona measurement on high voltage apparatus using t h e bushing capacitance tap . IEEE Trans. Power Apparatus and Systems 84 (1965) , pp 667-671. Q, Harrold, R . T . and Dakin, T.W., The r a l a t i o n s h i p between the picocoulomb and microvolt f o r corona measurements on h.v. transformers and o ther apparatus. IEEE Paper T 72086-2, 1972. IEC Publicat ion 270, 1968. P a r t i a l discharge measurements. I E C Publ. 76-3 (1980): Power Transformers. Pa r t 3: Insula t ion l eve l s and d i e l e c t r i c t e s t s .

Auswertung der Me8ergebnisse bei sdrmalbandigw M m g mit dem RIV

Berucks~chtigung der Freauem der PrGfsoannung und der Frequent d e s fi-m-rt @wmrWn9sbJf'J~ nacb CISPR) I

Wertetabelle Mr qo = 500 PC

19. MEASUREMENT OF ACOUSTIC SOUND LEVEL

Puraose o f t h e measurement

The purpose of t he sound l e v e l measurement is t o check t h a t t h e sound l e v e l of t h e t ransformer meets t h e s p e c i f i c a t i o n requirements, i . e . requirements given i n r e l e v a n t s tandards , e.g. (19.1) o r (19 .2 ) , o r guarantee values given by t h e t ransformer manufacturer. A sound spectrum ana lys i s is c a r r i e d o u t f o r t h e transformer a t t h e customer 's r eques t . The sound spectrum i n d i c a t e s t h e magnitude of sound components (measured a t a given band-width) a s a func t ion of frequency.

Measurine e a u i ~ m e n t

A p rec i s ion sound l e v e l meter complying wi th s tandards (19.11, (19.2) and (19.3) is used i n t h e sound l e v e l measurements. The measilrements a r e performed us ing the weightning curve A . The sound spectrum a n a l y s i s of t he t ransformer is c a r r i e d o u t by recording the sound band l e v e l s automatical ly a s a func t ion o f frequency. This is done with t h e a i d of an ana lyse r , which i s both mechanically and e l e c t r i c a l l y connected t o t he recorder o r with t h e a i d o f an octave f i l t e r s e t joined t o t h e sound l e v e l meter.

The measuring equipment is descr ibed i n a s epa ra t e l ist of equipment (Sec t ion 20).


The measurement is c a r r i e d o u t a t measuring p o s i t i o n s loca t ed around the t ransformer a s d e t a i l e d i n t h e s tandards (19 .1 ) , (19.2) and (19 .3 ) . According t o t h e s t anda rds (19 .1) and (19 .3) t h e microphone p o s i t i o n i n t he v e r t i c a l d i r e c t i o n s h a l l be on ho r i zon ta l planes a t one t h i r d and two t h i r d s o f one t ransformer tank he ight , when t h e height o f t h e tank is equal t o o r g r e a t e r than 2 .5 m . When t h e tank he ight is l e s s than 2.5 m , and when t h e measurement i s c a r r i e d o u t i n accordance wi th t h e s tandard (19.21, the measuring plane is loca t ed a t ha l f t he tank he ight . The microphone is d i r e c t e d perpendicular ly a g a i n s t the su r f ace of t h e t ransformer ( t h e p r i n c i p a l r a d i a t i n g s u r f a c e ) .

Before and a f t e r the t ransformer sound l e v e l measurement t h e background no i se l e v e l is measured. Preferab ly the background l e v e l should be a t l e a s t 9 dB(A) below t h e measured combined sound l e v e l . I f t h e d i f f e r e n c e is l e s s than 9 dB(A) bu t no t l e s s than 3 dB(A), a co r r ec t ion f o r background l e v e l w i l l be app l i ed according t o s tandards (19.11, (19 .2) and (19 .3 ) .

The t ransformer w i l l be l oca t ed a t the t e s t s i t e s o t h a t t h e f r e e d i s t ance from the t ransformer t o r e f l e c t i n g o b j e c t s is s u f f i c i e n t l y l a rge .

The measurement i s c a r r i e d ou t a t r a t ed vol tage and frequency.

Test r e p o r t

The mean value w i l l be c a l c u l a t e d from t h e measurement r e s u l t s . Correct ions f o r background Level and environmental co r r ec t ion a r e made t o t h e mean value.

Literature

(19.1) NEMA Standards Publication No. TR 1-1980. Transformers, regulators and reactors.

(19.2) VDE 0532, Teil 1/03.82. Bestimmungen fur Transformatoren und Drosselspulen.

(19.3) IEC Publication 551, 1976. Measurement of transformer and reactor sound levels.

Measurement o f a c o u s t i c sound l e v e l

A c o u s t i c sound l e v e l measu remen t s , i .e . t h e d e t e r m i n a t i o n o f t h e A-weighted sound p r e s s u r e l e v e l o r A-weighted sound power l e v e l a t t r a n s f o r m e r s a n d r e a c t o r s s h o u l d be c o n d u c t e d i n a c c o r d a n c e w i t h

I E C 551 .

T h i s I E C ~ e g u l a t i o n c o r r e s p o n d s . t o D I N 4 5 635 , P a r t 30 and VDE 0532 P a r t l , P a r a g r a p h 8 -1 - 3 .

The most i m p o r t a n t c o n d i t i o n s f o r sound l e v e l measurements a r e a s f o l l o w s :

1 . The t es t o b j e c t must be e x c i t e d w i t h r a t e d v o l t a g e and r a t e d f r e q u e n c y , i . e . f o r t r a n s f o r m e r s : no- load e x c i t a t i o n .

2 . The f a n s and pumps ( i f f i t t e d ) o f t h e c o o l i n g sys t em must b e o p e r a t e d a t r a t e d v o l t a g e and r a t e d f r e q u e n c y .

3 . If t h e t es t object h a s b e e n . e x c i t e d i n a c c o r d a n c e w i t h l . , and t h e t r a n s f o r m e r c o o l i n g d e v i c e s described i n 2 . ' a r e n o t i n o p e r a t i o n , t h e measu r ing d i s t a n c e ( t h e d i s t a n c e between t h e r e f e r e n c e s u r f a c e and t h e microphone) i s 0 .3 m .

4 . I f t h e t es t o b j e c t is e x c i t e d i n a c c o r d a n c e w i t h l . , and t h e t r a n s f o r m e r c o o l i n g d e v i c e s d e s c r i b e d i n 2 . a r e i n o p e r a t i o n , t h e measu r ing d i s t a n c e i s 2 m .

Microphone positions

5. The h e i g h t of t h e microphone H is f o r t a n k h e i g h t s h h and f o r t a n k h e i g h t s h 2 2.5 m: H = and $ h h ~ 2 . 5 m: H = -Z,

6 . The sound l e v e l meter meets t h e r e q u i r e m e n t s of I E C 651 i n accordance w i t h D I N 45 6 3 3 .

Attenuator I Attenuator I1

Block diagram of t h e sound p r e s s u r e meter

Manufacturer: B r u e l & K j a e r , Copenhagen, Denmark

7 . Environment c o n d i t i o n s f o r - 3 r e e - f i e l d o r i n d o o r measurements

7.1 The ambient A-weighted sound l e v e l s h o u l d be a t l e a s t 1 0 dB below t h e sound l e v e l of t h e t e s t o b j e c t .

7.2 R e f l e c t i n g s u r f a c e s , a p a r t from t h e f l o o r , must b e a t a d i s t a n c e o f more t h a n 3 m from t h e s u r f a c e of t h e t es t o b j e c t .

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Testing Power Transformers