Testing Power Transformers

ABB Stromberg

Power

TESTING POWER TRANSFORMERS

T e s 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 f o l l o w i n g s e c t i o n s . The t e s t i n g o f d i s t r i b u t i o n t r a n s f o r m e r s is n o t i n c l u d e d . Due t o t h e i r l a r g e manufacturing numbers d i s t r i b u z i o n t r a n s f o r m e r s a r e r o u t i n e t e s t e d by means of computerized ( a u t o m a t i c ) t e s t equipment. The measuring equipment d i f f e r s from t h o s e e x p l a i n e d h e r e i n . The p r i n c i p l e s o f r o u t i n e , type and s p e c i a l t e s t s a r e however s i m i l a r and t h u s t h i s b o o k l e t i s a p p l i c a b l e 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 r a n s f o r m e r s t o o .

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 r a n s f o r m e r s a r e checked by means o f measurements and t e s t s d e f i n e d by s t a n d a r d s .

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, u n l e s s o t h e r w i s e 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 o f v o l t a g e r a t i o and check of connection symbol

3 . Measurement o f winding r e s i s t a n c e 3-1.

.

.3-2 4. Measurement o f impedance v o l t a g e and

load l o s s

5. Measurement of no-load l o s s and c u r r e n t 5-1.

.

.S-3

6 . Induced o v e r v o l t a g e w i t h s t a n d t e s t 6-1.

.

.6-2 7

I . Separate-source v o i t a g e 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 o f zero-sequence i2pedance 9-1

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

11. I n s u l a t i o n 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 r e n g t h o f t h e 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 o f acoustic sound level X ) X ) on request

1. SUMMARY OF DIELECTXC TESTS

According t o t h e Standard

IEC

7 6 - 3 t h e d i e l e c t r i c t e s t r e q u i r e m e n t s f o r a t r a n s f o r m e r winding depend o n t h e h i g h e s t v o l t a g e f o r equipment Um a p p l i c a p l e t o t h e 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 o f rwlnalng T e s t s S e c t l o n U- C 300 kV -Separate-source w i t h s t a n d t e s t 7 ~!iform i n s u l a t i o n -Induced o v e r v o l t a g e w i t h s t a n d 6 t e s t w i t h symmetrical t h r e e - phase v o l t a g e s ( r o u t i n e t e s t ) - L i g h t n i n g impulse t e s t 1 5 - f o r l i n e t e r m i n a l s ( t y p e t e s t ) - f o r n e u t r a l t e r m i n a l ( s p e c i a l t e s t U- c 3 0 0 kV -Separate-source w i t h s t a n d t e s t 7 m Non-uniform i n s u l a t i o n c o r r e s p o n d i n g 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 i n e - t o - e a r t h over- 6 v o l t a g e w i t h s t a n d t e s t ; t h r e e s i n g l e - p h a s e t e s t s ( r o u t i n e t e s t ) - L i g h t n i n g i n p u l s e t e s t 1) 1 5 - f o r l i n e t e r m i n a l s ( t y p e t e s t ) - f o r n e u t r a l t e r m i n a l ( s p e c i a l t e s t U-% 300 kV -Separate-source w i t h s t a n d t e s t 7 ~ % i - u n i f o m i n s u l a t i o n c o r r e s p o n d i n g 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 i n e - t o - e a r t h o v e r v o l t a g e 6 w i t h s t a n d t e s t ; t h r e e s i n g l e - p h a s e t e s t s ( r o u t i n e t e s t ) T e s t i n g a c c o r d i n g - L i g h t n i n g x p u l s e t e s t l ) 15 t o method 1 , s e e - f o r l i n e t e r m i n a l s ( r o u t i n e IEC 76-3 - f o r n e u t r a l t e r m i n a l ( s p e c i a l t e s t ) U , 3 300 kV -Separate-source w i t h s t a n d t e s t 7 Non-unif orm c o r r e s p o n d i n g 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 n e u c r a l ( r o u t i n e t e s t ) -Switching impulse t e s t f o r l i n e 1 7 t e r m i n a l s ( r o u t i n e t e s t ) - L i g h t n i n g i x p u l s e t e s t l ) 1 5 T e s t i n g a c c o r d i n g - f o r l i n e t e r m i n a l s ( 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 t e r m l n a l ( s p e c i a l IEC 76-3 t e s t J - P a r t i a l d i s c h a r g e measurement 18 ( r o u t i n e t e s t j 2 ) "chopped-wave L i g h t n i n g i e p u l s e :ast i s a s p e c i a l t e s t , s e e S e c t i o n 15. ' ) ~ a r o t h e r c a r e g o r i s s o f windings :he p a r t i a l C i s c h a r g e measuremenr ii

2. MEASUREMENT OF VOLTAGE RATIO AND CHECK OF CONNECTICN SYMBOL Purpose o f t h e measurement

The v o l t a g e r a t i o o f t h e t r a n s f o r m e r i s t h e r a t i o of v o l t a g e s ( i n t h r e e - p h a s e t r a n s f o r m e r s l i n e - t o - l i n e v o l t a g e s ) a t no-load, e . g . , 110000 V/10500 V.

The purpose o f t h e measurement i s t o check t h a t t h e d e v i a t i o n o f t h e v o l t a g e r a t i o from t h e s p e c i f i e d v a l u e d o e s n o t exceed t h e l i m i t g i v e n i n t h e r e l e v a n t t r a n s f o r m e r s t a n d a r d ( g e n e r a l l y 0 . 5 % ) .

The c o n n e c t i o n symbol of t h e t r a n s f o r m e r i s checked a t t h e same t i m e . Performance and r e s u l t s o f t h e measurement

The v o l t a g e r a t i o measurements a r e c a r r i e d o u t by means o f a v o l t a g e r a t i o measuring b r i d g e ; t h e e r r o r o f t h e b r i d g e i s l e s s t h a n +-0.1 %.

The s u p p l y v o l t a g e i s 220 V a . c . The f u n c t i o n o f t h e b r i d g e i s shown i n F i g . 2-1. The v o l t a g e s o f t h e t r a n s f o r m e r t o be checked are compared t o t h e c o r r e s p o n d i n g v o l t a g e s o f t h e r e g u l a t i n g t r a n s f o r m e r , which i s p r o v i d e d w i t h a decade d i s p l a y u n i t and l o c a t e d i n t h e b r i d g e c a s i n g . When t h e b r i d g e i s b a l a n c e d , t h e v o l t a g e r a t i o o f t h e decade t r a n s f o r m e r i s e q u a l t o t h a t o f t h e t r a n s f o r m e r under t e s t . The r e s u l t c a n be s e e n d i r e c t l y from t h e numeral d i s p l a y o f t h e b r i d g e . Fig. 2-1. Bridge measurement ( o f t h e v o l t a g e r a t i o ) . T t r a n s f o r m e r t o be measured, 1 T r e g u l a t i n g t r a n s f o r m e r equipped 2 ~ i t h a decade d i s p l a y , P zero- 1 sequence v o l t m e t e r , U, s u p p l y v o l t a g e o f t h e bridge; U secondary 2 v o l t a g e o f t h e t r a n s f o r m e r . S i n c e t h e measuring d e v i c e i s a s i n g l e - p h a s e b r i d g e , t h e v o l t a g e r a t i o o f a p a i r o f windings mounted on t h e same l e g i s measured a t a t i m e . I t i s t o be observed t h a t t h e r a t i o i n d i c a t e d by t h e b r i d g e does n o t alway: c o r r e s p o n d t o t h e r a t i o o f t h e l i n e - t o - l i n e v o l t a g e s . The r e s u l t depend^ on t h e c o n n e c t i o n symbol of t h e t r a n s f o r m e r . For each winding connected t o t h e b r i d g e l t 1s i m p o r t a n t t o o b s e r v e whether t h e number of t u r n s r e l a t e s t o t h e l i n e - t o - l i n e o r l i n e - t o - n e u t r a l v o l t a g e . For example, the v o l t a g e r a r i o o f a 120/21 k V Yd-connected t r a n s f o r m e r i s 120000:

\/3/21000 V = 3.299. The r e a d i n g o ~ t a i n e a from t h e bridge i s t o be compared t o t h i s v a l u e .

The c o n n e c t i o n s y r n ~ o l of t h e t r a n s f o r m e r i s checked i n c o n j u n c t i o n w i t k t h e v o l t a g e r a t i o neasurement. When t h e measuring l e a d s from t h e

transformer a r e connected t o t h e b r i d g e a c c o r d i n g t o t h e r e l e v a n t v e c t o r diagram i n Table 2-2, t h e b r i d g e c a n be balanced only if t h e t r a n s f o r m e r connection i s c o r r e c t .

The v o l t a g e r a t i o s a r e measured f o r each t a p p i n g connection of t h e transformer. I n t h e r e p o r t t h e s p e c i f i e d t a p p i n g v o l t a g e r a t i o s a r e s t a t e d , a s well a s t h e measured r a t i o s and t h e i r d e v i a t i o n s from t h e s p e c i f i e d r a t i o s . Table 2-2. Determination o f t h e connection symbol. Clock hour f i g u r e ( l e f t ) , connection symbol (middle) and v e c t o r 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 is

energised by a symmetric 3-phase 400 V. Voltages of the terminals are measured and

vector group symbol is determinated by following chart.

HV-side

Main voltage

Main voltage

LV-side

HV and LV terminals

Voltage between

Terminals

U/V

Terminals

U/V

Terminals

U/V

U-V

u-v

V-v

V-W

v-w

V-w

W-U

w-u

W-v

W-w

Vector group

symbol

Voltage relationship

between terminals

0

Ww<Vw=Wv>Ww<UV

1

Ww<Vw>Wv=Ww<UV

2

Ww<Vw>Wv<Ww<UV

3

Ww<Vw>Wv<Ww≥UV

4

Ww<Vw>Wv<Ww>UV

5

Ww=Vw>Wv<Ww>UV

6

Ww>Vw=Wv<Ww>UV

7

Ww>Vw<Wv=Ww>UV

8

Ww>Vw<Wv>Ww>UV

9

Ww>Vw<Wv>Ww≥UV

10

Ww>Vw<Wv>Ww<UV

11

Ww=Vw<Wv>Ww<UV

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 r m i n a l s o f each transformer winding a r e measured u s i n g d i r e c t c u r r e n t . The measurement i s performed f o r each connection o f c o n n e c t a b l e windings and f o r each t a p p i n g

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 c o n n e c t i o n w i t h t h e load l o s s measurement when t h e l o a d l o s s e s a r e c o r r e c t e d t o correspond t o t h e r e f e r e n c e 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 r d e r and t h e 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 b r i d g e .

Fig. 3-1

Resistance measurement using Thomson-bridge.

R

dec R V

The p r i n c i p l e of measurement i s a s follows: The v o l t a g e drops caused o y

t h e d i r e c t c u r r e n t I a c r o s s t h e r e s i s t a n c e s R and R a r e compared by

X

means o f t h e b r i d g e (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 t a n d a r d r e s i s t a n c e . The a c c G a c y 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 i s measured by means o f thermometers with an accuracy o f +-O.l°C. D i r e c t c u r r e n t i s o b t a i n e d from a b a t t e r y .

Performance o f t h e measurement

Before t h e measurement s t a r t s t h e transformer i s s t a n d i n g f o r a t l e a s t ?

hours f i l l e d with o i l and without e x c i t a t i o n . During t h i s p e r i o d t h e temperature d i f f e r e n c e s o f t h e transformer w i l l e q u a l i z e and t h e w i n l l n g temperature w i l l become e q u a l t o t h e o i l t e m p e r a t u r e . The average

winding temperature i s o b t a i n e d by determining t h e average o i l temperature. The average o i l temperature i s o b t a i n e d by measuring

cover, and t h e bottom o i l temperature i n t h e d r a i n v a l v e , and t a k i n g t h e average o f t h e s e two.

When s w i t c h i n g on t h e supply v o l t a g e E t o t h e measuring c i r c u i t t h e winding i n d u c t a n c e L t e n d s t o r e s i s t t h e i n c r e a s e of t h e c u r r e n t . The r a t e o f i n c r e a s e depends on t h e time c o n s t a n t o f t h e c i r c u i t :

t = time from s w i t c h i n g on

L/R = time c o n s t a n 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 s h o r t e n t h e t i m e 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 c u r r e n t is used t h a t t h e c o r e 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

u s u a l l y 5...10 times t h e no-load

c u r r e n t o f t h e winding. However, t h e c u r r e n t s h o u l d be l e s s t h a n 10 % o f t h e r a t e d c u r r e n t o f t h e winding, otherwise t h e t e m p e r a t u r e r i s e o f 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 t h e time c o n s t a n t can be r e d u c e d by u s i n g as h i g h a supply v o l t a g e a s p o s s i b l e e n a b l i n g a n i n c r e a s e d s e r i e s r e s i s t a n c e i n t h e c i r c u i t . When u s i n g a b a t t e r y , t h e supply v o l t a g e is approximately c o n s t a n t and t h e c u r r e n t i s a d j u s t e d 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 s w i t c h i n g 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 b r i d g e i s n o t connected w i t h t h e t e r m i n a l s o f R

,

s o t h e r e i s no r i s k t h a t t h e induced v o l t a g e w i l l damage t h e b r i d g e . ~ f i e n t h e ammeter i n d i c a t e s t h a t t h e c u r r e n t i s a l m o s t s t e a d y t h e b r i d g e i s connected and balanced by means o f t h e z e r o - i n d i c a t o r . The b r i d g e r e s i s t a n c e r e a d i n g s a r e noted down. T e s t r e s u l t

The r e s i s t a n c e v a i ~ e s and t h e average temperature a r e c a l c u l a t e d . I n t h e r e p o 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 h e connection, t h e t a p p i n g p o s i t i o n and t h e a v e r a g e temperature of t h e windings d u r i n g 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 t e m p e r a t u r e r i s e i n a t r a n s f o r m e r winding u s i n g 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 F i n l a n d 47 (19741, No 1.

M e a s u r e m e n t of w i n d i n a r e s i s t a n c e T h e 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 i s m e a s u r e d 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 a n d 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 m e a s u r e m e n t , t h e appropriate w i n d i n g t e m p e r a t u r e i s m e a s u r e d . T e s t c i r c u i t T h e r e s i s t a n c e i s d e t e r r r i i n e d by means. o f t h e M c u r r e n t - v o l t a g e m e t h o d " . W i t h t h i s m e t h o d , t h e DC t e s t 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 % ( s e e F i g u r e ) . R e s i s t a n c e m e a s u r e m e n t 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 T h e 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 a n d 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 o h m i c w i n d i n g r e s i s t a n c e . T h e DC t e s t c u r r e n t i s o b t a i n e d f r o m e i t h e r a s t a b i - l i z e d p o w e r 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 t h e 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 m e a n s o f m e r c u r y 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 d o c u m e n t e d 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 t h e measurement

The measurement

i s

c a r r i e d o u t t o d e t e r m i n e t h e l o a d - l o s s e s of t h e t r a n s f o r m e r and t h e impedance v o l t a g e a t r a t e d frequency and r a t e d c u r r e n 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 h e p a i r s 1-2, 1-3 and 2-3 f o r a three-winding t r a n s f o r m e r i , and f u r t h e r m o r e on t h e p r i n c i p a l and extreme t a p p i n g s .

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 l o a d - l o s s measurement.

G s u p p l y g e n e r a t o r ,

T

step-up t r a n s f o r m e r , T t r a n s f o r m e r t o be 1 1 2 t e s t e d , T c u r r e n t t r a n s f o r m e r s ,

T

v o l t a g e t r a n s f o r m e r s , P w a t t g e c e r s , 3 4 1 P ammeters (r.rn.s. v a l u e ) , P v o l t m e t e r s (r.m.s. v a l u e ) , C c a p a c i t o r 2 bank. 3 1

The s u p p l y and measuring f a c i l i t i e s a r e d e s c r i b e d i n a s e p a r a t e measuring a p p a r a t u s l i s t ( S e c t i o n 2 0 ) .

C u r r e n t i s g e n e r a l l y s u p p l i e d t o t h e h . v . winding and t h e 1.v. winding is s h o r t - c i r c u i t e d .

Performance o f t h e measurement

I f t h e r e a c t i - ~ e power s u p p l i e d b y t h e g e n e r a t o r G i s n o t s u f f i c i e n t 1

when measuring l a r g e t r a n s f o r m e r s , a c a p a s i t o r Sank C i s used t o 1

compensate p a r t of t h e i n d u c t i v e r e a c t i v e power taken by t h e t r a n s f o r m e r T2..

The v o l t a g e of t h e supply g e n e r a t o r is r a i s e d u n t i l t h e c u r r e n t has a t t a i n e d t h e r e q u i r e d v a l u e (25...100 % of t h e r a t e d c u r r e n t according t o t h e s t a n d a r d 4 . 1 ) . I n o r d e r t o i n c r e a s e t h e a c c u r a c y of r e a d i n g s w i l l be taken a t s e v e r a l c u r r e n t v a l u e s n e a r t h e r e q u i r e d l e v e l . If a winding i n t h e p a i r t o be measured is equipped with a n o f f - c i r c u i t o r on-load tap-changer, t h e measurements a r e c a r r i e d o u t on t h e p r i n c i p a l and extreme tappings. The r e a d i n g s have t o be t a k e n a s q u i c k l y a s p o s s i b l e , because t h e windings t e n d t o warm up due t o t h e c u r r e n t and t h e l o s s values o b t a i n e d i n t h e measurement a r e a c c o r d i n g l y t o o high.

If t h e t r a n s f o r m e r has more t h a n two windings a l l winding p a i r s a r e measured s e p a r a t e l y .

Results

C o r r e c t i o n s caused by t h e i n s t r u m e n t transformers a r e made t o t h e measured c u r r e n t , v o l t a g e and power values. The power v a l u e c o r r e c t i o n caused by t h e phase displacement is c a l c u l a t e d as f o l l o w s :

P = c o r r e c t e d power

P

'

= power r e a d from t h e meters e bU = phase displacement o f t h e v o l t a g e t r a n s f o r m e r i n minutes 6 = phase displacement o f t h e c u r r e n t t r a n s f o r m e r i i n minutes

cp = phase a n g l e between c u r r e n t and v o l t a g e i n t h e measurement P i s p o s i r i v e a t i n d u c t i v e l o a d 1

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

The c o r r e c t i o n K o b t a i n e d from e q u a t i o n (4.1) i s shown a s a s e t o f curves i n Fig. 4-2.

The c o r r e c t i o n s caused by t h e instrument t r a n s f o r m e r s a r e made

s e p a r a t e l y f o r each phase, because d i f f e r e n t p h a s e s 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 o f t h e i n s t r u m e n t t r a n s f o r m e r s a r e g e n e r a l l y d i f f e r e n t .

If t h e 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 t h e N'

power P _km and t h e v o l t a g e

Ukm

a t r a t e d c u r r e n t are o b t a i n e d by applying corrections t o t h e v a l u e s P and U r e l a t i n g t o t h e measuring c u r r e n t .

C C

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

S U

-

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

(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)

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

-

l o a d 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, t h e y a r e g e n e r a l l y i g n o r e d . The f o l l o w i n g formula i s 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 s e s = frequency U ' = mean v a l u e of v o l t a g e X 1.11 ( r e a d i n g o f a r e c t i f i e r v o l t m e t e r s c a l e d t o r e a d t h e r . m . s . v a l u e o f a s i n u s o i d a l v01 t a g e ) U = r.m.s. v a l u e o f t h e v o l t a g e

When c a r r y i n g o u t t h e no-load measurement, t h e v o l t a g e wave s h a p e may somewhat d i f f e r from t h e s i n u s o i d a l form. T h i s i s caused by t h e harmonics i n t h e magnetizing c u r r e n t which c a u s e a d d i t i o n a l v o l t a g e drops i n t h e impedances of t h e s u p p l y . The r e a d i n g s o f t h e mean v a l u e 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 t a n d a r d c o n d i t i o n s , i t i s n e c e s s a r y t o a p p l y a wave shape c o r r e c t i o n whereby t h e l o s s e s a r e

c o r r e c t e d t o correspond t o t e s t c o n d i t i o n s where t h e supply v o l t a g e i s s i n u s o i d a l . I n t h e t e s t t h e v o l t a g e i s a d j u s t e d s o t h a t t h e mean v a l u e v o l t m e t e r i n d i c a t e s t h e r e q u i r e d v o l t a g e v a l u e . Then t h e h y s t e r e s i s l o s s e s c o r r e s p o n d t o s t a n d a r d c o n d i t i o n s , b u t t h e eddy-current l o s s e s must be c o r r e c t e d . From (5.1). 'on = l o s s e s a t s i n u s o i d a l v o l t a g e under s t a n d a r d c o n d i t i o n s p1 = r a t i o , e x p r e s s e d a s a p e r c e n t a g e , 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 , e x p r e s s e d a s a p e r c e n t a g e , 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 v a l u e c o r r e s p o n d i n g t o s t a n d a r d c o n d i t i o n s i s o b t a i n e d from t h e measured v a l u e P a s f o l l o w s : 0 I t i s assumed t h a t f o r o r i e n t e d s h e e t s p = p2 = 50 %. 1

The c u r r e n t and power r e a d i n g s o f d i f f e r e n t p h a s e s a r e u s u a l l y d i f f e r e n t ( t h e power c a n even be n e g a t i v e i n some p h a s e ) . T h i s i s due t o t h e

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 in tables 2 or 3 of IEC 60076-3 (2000). As a rule, the test voltage across an untapped winding of the transformer shall be as close as possible to twice the rated voltage. Normally, no partial discharge measurements are performed during this test.

The test shall be commenced at a voltage not greater than one-third of the test value and the voltage shall be increased to the test value as rapidly as is consistent with measurement. At the end of the test, the voltage shall be reduced rapidly to less than one-third of the test value before switching 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 discharge measurement. The phase-to-phase test voltages shall not exceed the rated AC withstand voltages of tables 2, 3 or 4 of IEC 60076-3. As a rule, the test voltage across an untapped winding 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 the application 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 and 4, 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 a

duration 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.

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 partial

discharge 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 to ensure that any inception and extinction of partial discharge can be detected and recorded. The above-mentioned value 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 exceed

300 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 and supplier about further investigations (annex A of IEC 60076-3). In such cases, a long-duration induced AC voltage test (see clause 18 hereinafter) may be performed. If the transformer meets the requirements of 12.4 of IEC 60076-3, the test shall be considered successful.

b . Non-uniformly i n s u l a t e d windings

r-lN

F i g . 6-1. T e s t c i r c u i t f o r induced o v e r v o l t a g e w i t h s t a n d t e s t on non-uniformly i n s u l a t e d winding o f three-phase t r a n s f o r m e r G supply g e n e r a t o r , T step-up t r a n s f o r m e r , T t r a n s f o r m e r u n d e r t e s t , 1

T c u r r e n t transformer: T v o l t a g e transformer: L compensating r e a c t o r ,

3 4

E v o l t a g e d i v i d e r , P ammeter, P v o l t m e t e r , P v o l t m e t e r (r.m.s. 1

v a l u e )

,

P v o l t m e t e r ( p e a t v a l u e ? . 3

4

The t e s t c o n n e c t i o n shown i n F i g . 6-1 i s a p p l i c a b l e t o three-phase

t r a n s f o r m e r s i f t h e i n s u l a t i o n l e v e l o f t h e n e u t r a l t e r m i n a l i s 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 h e t e r m i n a l s . The t e s t v o l t a g e i s a p p l i e d t o t h e i n d i v i d u a l p h a s e s i n s u c c e s s i o n . During e a c h a p p l i c a t i o n t h e t e s t v o l t a g e from t e r m i n a l t o e a r t h i s e q u a l t o t h e r a t e d w i t h s t a n d v o l t a g e . The v o l t a g e i s measured w i t h a c a p a c i t i v e v o l t a g e d i v i d e r i n c o n j u n c t i o n with v o l t m e t e r s r e s p o n s i v e t o peak a n d r . m . s . v a l u e s . The peak v o l t m e t e r i n d i c a t e s t h e peak v a l u e d i v i d e d by

b

2: The t e s t v o l t a g e i s a d j u s t e d a c c o r d i n g t o t h i s v o l t m e t e r .

T e s t r e p o r t

The t e s t v o l t a g e , f r e q u e n c y , t e s t d u r a t i o n and t a p p i n g a r e s t a t e d i n t h e r e p o 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-uniformly insulated 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 to

tables 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 between

phases according to tables 2, 3 or 4 with partial discharge measurement. The test shall be carried 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 carried out 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 as far as possible. In exceptional cases, see clause 6 of IEC 60076-3, the voltage on the neutral terminal may be raised by connection to an auxiliary booster transformer. In such cases, the neutral should be insulated accordingly.

The test sequence for a three-phase transformer consists of three single-phase applications of test voltage with different points of the winding connected to earth at each time. Recommended test connections which avoid excessive over-voltage between line terminals are shown in figure 6.2. There are also other possible methods.

Other separate windings shall generally be earthed at the neutral if they are star-connected, and at 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 transformer with respect to operating conditions or test plant limitations. The test time and the time sequence for 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 kV

and 550 kV with AC test values of 460 kV or 510 kV, the partial discharge evaluation level should be reduced to U2 = 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 voltage

according 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 complete connection of all windings during the test be reviewed between supplier and purchaser at the contract stage, in order that 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 stresses between phases. If this test is specified, the clearances between phases should be adjusted accordingly and specified at the contract stage.

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

The continuous level of ‘apparent charge’ at U2 during the second 5 min does not exceed

500 pC on all measuring terminals for single-phase tests at U2 = 1,5 Um / √3 line-to-earth, or

300 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) on transformers with non-uniform insulation

Connection a) may be used when the neutral is designed to withstand at least one-third of the voltage 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 core form).

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

Connection c) shows an auxiliary booster transformer, which gives a bias voltage Ut at the

neutral terminal of an auto-transformer under test. Rated voltages of the two auto-connected windings are Ur1, Ur2, and the corresponding test voltages U, Ux. This connection may also be

used for a three-phase transformer without unwound magnetic return paths having the neutral insulation designed for less than one-third of the voltage U.

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 t h e t e s t i s t o s e c u r e t h a t t h e i n s u l a t i o n between t h e windings and t h e i n s u l a t i o n between windings and e a r t h e d p a r t s ,

w i t h s t a n d t h e temporary o v e r v o l t a g e s and s w i t c h i n g o v e r v o l t a g e s which may occur i n s e r v i c e . T e s t c i r c u i t F i g . 7-1. T e s t c i r c u i t f o r s e p a r a t e - s o u r c e v o l t a g e w i t h s t a n d t e s t G s u p p l y g e n e r a t o r , T t e s t t r a n s f o r m e r , T t r a n s f o r m e r under t e s t , 1 1 c u r r e n t t r a n s f o r m e r , L compensating r e a c t o r : E v o l t a g e d i v i d e r , 3 ammeter, P v o l t m e t e r ( r . m . s . v a l u e ) , P v o l t m e t e r (peak v a l u e ) . P1 2 3 The v o l t a g e i s measured u s i n g a c a p a c i t i v e v o l t a g e d i v i d e r i n

c o n j u n c t i o n w i t h v o l t m e t e r s r e s p o n s i v e t o r . m . s . and peak v a l u e s . The peak-voltmeter i n d i c a t e s t h e peak v a l u e d i v i d e d by

\'2.

The t e s t v o l t a g e is a d j u s t e d a c c o r d i n g t o t h i s meter.

The g e n e r a t o r s and t h e equipment a r e d e s c r i b e d i n t h e t e s t equipment l i s t ( S e c t i o n 2 0 ) .

Performance

The t e s t i s made w i t h s i n g l e - p h a s e v o l t a g e o f r a t e d frequency. The t e s t v o l t a g e i s a p p l i e d f o r 60 s e c o n d s between t h e winding under t e s t and a l l t e r m i n a l s o f t h e remaining windings, c o r e and t a n k o f t h e t r a n s f o r m e r , connected t o g e t h e r t o e a r t h ( F i g . 7-1).

The t e s t i s s u c c e s s f u l i f no c o l l a p s e o f t h e t e s t v o l t a g e o c c u r s . The l i n e t e r m i n a l s o f non-uniformly i n s u l a t e d windings a r e t e s t e d b y induced t e s t a c c o r d i n g t o S e c t i o n 6 . 5 .

T e s t r e p o r t

8 . OPERATION TESTS ON ON-LOAD TAP-CHANGER

After t h e tap-changer i s f u l l y assembled on t h e t r a n s f o r m e r , t h e f o l l o w i n g t e s t s a r e performed a t ( w i t h e x c e p t i o n o f b ) 130 % o f t h e r a t e d a u x i l i a r y s u p p l y v o l t a g e : a 8 complete o p e r a t i n g c y c l e s w i t h t h e t r a n s f o r m e r n o t e n e r g i z e d b 1 complete o p e r a t i n g c y c l e w i t h t h e t r a n s f o r m e r n o t e n e r g i z e d , w i t h 85 % o f t h e r a t e d a u x i l i a r y s u p p l y v o l t a g e C 1 complete o p e r a t i n g c y c l e w i t h t h e t r a n s f o r m e r e n e r g i z e d and r a t e d v o l t a g e and frequency a t no l o a d d ) 1 0 tap-change o p e r a t i o n s w i t h +- 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 t a p p i n g w i t h a s f a r a s p o s s i b l e t h e r a t e d c u r r e n t o f t h e t r a n s f o r m e r , w i t h one winding s h o r t - c i r c u i t e d . I n p r a c t i c e t h e o p e r a t i n g t e s t w i t h t h e r a t e d c u r r e n t i s u s u a l l y performed by one complete o p e r a t i n g c y c l e from one extreme t a p p i n g t o a n o t h e r . The c u r r e n t i s a s f a r a s p o s s i b l e t h e r a t e d c u r r e n t of each t a p p i n g .

9. MEASUREMENT OF ZERO-SEQUENCE IMPEDANCE Purpose o f t h e measurement

The zero-sequence impedance i s u s u a l l y measured f o r a l l s t a r - c o n n e c t e d windings o f t h e t r a n s f o r m e r . The measurement i s c a r r i e d o u t by s u p p l y i n g a c u r r e n t o f r a t e d frequency between t h e p a r a l l e l connected phase

t e r m i n a l s and t h e n e u t r a l t e r m i n a l . The zero-sequence impedance p e r phase i s t h r e e t i m e s t h e impedance measured i n t h i s way. The zero- sequence impedance i s needed f o r e a r t h - f a u l t p r o t e c t i o n and e a r t h - f a u l t c u r r e n t c a l c u l a t i o n s .

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

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

G supply g e n e r a t o r , T t r a n s f o r m e r t o be t e s t e d , T v o l t a g e 1

t r a n s f o r m e r , T c u r r e n k t r a n s f o r m e r , P v o l t m e t e r ,

6

ammeter, I t e s t

c u r r e n t . 3 2 3

The zero-sequence impedance i s dependent on t h e c u r r e n t f l o w i n g througn t h e winding. Usually t h e v a l u e corresponding t o r a t e d c u r r e n t I i s

N s t a t e d . This i m p l i e s t h a t t h e measurement i s c a r r i e d o u t w i t h a t e s t c u r r e n t o f 3

.

I

.

However, t h i s i s n o t always p o s s i b l e i n p r a c t i c e s i n c e t h e curren! must be l i m i t e d t o avoid e x c e s s i v e t e m p e r a t u r e o f m e t a l l i c c o n s t r u c t i o n a l p a r t s . The zero-sequence impedance i s measured a s f u n c t i o n o f t e s t c u r r e n t , and when n e c e s s a r y t h e f i n a l r e s u l t i s o b t a i n e d by e x t r a p o l a t i o n .

R e s u l t

The zero-sequence impedance i s u s u a l l y g i v e n a s a percentage o f t h e r a t e d phase impedance. When t h e t r a n s f o r m e r h a s a three-limb c o r e and no delta-connected windings, t h e zero-sequence impedance i s a b o u t 3 0 . . . C ' . % . When t h e t r a n s f o r m e r h a s a delta-connected winding, t h e zero-sequencn impedance i s 0 . 8

...

1 . 0 t i m e s t h e corresponding s h o r t - c i r c u i t impedanc->. I n t h e t e s t r e p o r t t h e zero-sequence impedance v a l u e s a t t h e p r i n c l c I .

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

The p a r t i a l capacitances C

.

C and CZ0 a r e determined by s o l v i n g t h e s e t of equations ( 10. l 1.

. .+PO.

3 3 . For transformers with t h r e e o r more windings a s i m i l a r method is used. The number nk of p a r t i a l capacitances

(and measurement combinations) is

n

= t h e number o f windings

Test r e p o r t

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

L i t e r a t u r e

(10.1) Bertula, T . , Palva

,

V . : Transformer capacitances, S a k o -

11. INSULATION RESISTANCE MEASUREMENT W p o s e o f t h e measurement

The purpose o f t h e measurement i s t o determine t h e leakage c u r r e n t r e s i s t a n c e o f t h e i n s u l a t i o n . T h i s i s a f u n c t i o n o f t h e moisture and i m p u r i t y c o n t e n t s o f t h e i n s u l a t i o n and o f i t s temperature such t h a t when t h e s e parameters a r e i n c r e a s e d t h e i n s u l a t i o n r e s i s t a n c e , a s measured a t a c o n s t a n t v o l t a g e d i f f e r e n c e a c r o s s t h e i n s u l a t i o n , depend on t h e s t r e n g t h o f t h e e l e c t r i c f i e l d d u r i n g t h e measurement and t h u s on t h e s i z e and c o n s t r u c t i o n o f t h e t r a n s f o r m e r . T h i s measurement g i v e s i n f o r m a t i o n about t h e c o n d i t i o n o f t h e i n s u l a t i o n and s e c u r e s t h a t t h e leakage c u r r e n t i s a d e q u a t e l y s m a l l . Performance o f t h e measurement

The i n s u l a t i o n r e s i s t a n c e i s measured by means o f an i n s u l a t i o n r e s i s t a n c e meter a t a v o l t a g e o f 5000 V d . c . Each winding i s measured s e p a r a t e l y by c o n n e c t i n g t h e v o l t a g e between t h e winding t o be t e s t e d and e a r t h , w h i l e t h e o t h e r windings a r e e a r t h e d . The r e s i s t a n c e r e a d i n g s

and R a r e t a k e n 15 S and 60 S a f t e r connecting t h e v o l t a g e . The 60

a s o r b t i o n r a t i o R

6 0 : ~ 1 5 i s normally 1 , 2

...

3 i n d r i e d t r a n f o r m e r s . The t y p e o f meter used, t h e measuring v o l t a g e , temperature, RI5, R60 and R60/R15 a r e s t a t e d i n t h e r e p o r 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

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 o u t t o check t h e c o n d i t i o n of i n s u l a t i o n s between t r a n s f o r m e r windings and between windings and e a r t h e d p a r t s . The r e s u l t ( l e a k a g e c u r r e n t r e s i s t a n c e ) o b t a i n e d 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) d e s c r i b e s i n t h e 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 c u r r e n t v o l t a g e . 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 v o l t a g e . The l o s s f a c t o r is p r i m a r i l y a c h a r a c t e r i s t i c

q u a n t i t y f o r t h e 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 o b t a i n e d f o r t r a n s f o r m e r s o f 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 b r i d g e and a s t a n d a r d c a p a c i t o r . The measuring v o l t a g e i s u s u a l l y 5 kV o r 10 kV. The c a p a c i t a n c e CS o f each winding end t h a t o i pair-wise connected windings and t h e l o s s f a c t o r t a n 4 a g a i n s t e a r t h

( c o n n e c t i o n s a s i n item 1 0 ) a r e g e n e r a l l y d e f i n e d i n t h e measurement. P a r a l l e l - c o n n e c t i o n o r s e r i e s - c o n n e c t i o n ( F i g . 12-11 can b e used a s e q u i v a l e n t c i r c u i t o f t h e 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. V a l i d f o r t h e s e r i e s - c o n n e c t i o n : Obtained f o r t h e 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 d i , s t a n c e a t t h e AC v o l t a g e . The r e s i s t a n c e depends zn t h e

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 insulating materials and their structure, moisture content, etc. Values of correction factor K listed in the Table 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. When insulation power factor is measured at a relatively high temperature and the corrected values are unusually high, the transformer should be allowed to cool; and the measurements should be repeated 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 on insulating systems using mineral oil as an insulating liquid. Other insulation liquids may have different correction factors.

C R P A C I TQNCE &

POWER FkCTOR

Measuring Bridge : Teitex

-

5chering

l. k a s u r m e n i o f t h e resulting 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)

C R P R C I 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)

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 i s g i v e n by t h e breakdown v o l t a g e , measured using an e l e c t r o d e system i n accordance w i t h IEC 156 ( 1 3 . 1 ) . The

e l e c t r o d e s a r e s p h e r i c a l s u r f a c e d with 25 mm r a d i u s and a r e 2 . 5 mm a p a r t . The measurement is c a r r i e d o u t a t 50 Hz, t h e r a t e of i n c r e a s e o f t h e v o l t a g e b e i n g 2 kV/s. The e l e c t r i c s t r e n g t h i s t h e a v e r a g e of s i x breakdown v o l t a g e v a l u e s .

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 s h o u l d be a t l e a s t 60 kV. O i l which does n o t w i t h s t a n d t h i s v o l t a g e may c o n t a i n a i r bubbles, d u s t o r moisture. I n p r a c t i c e t h e breakdown v o l t a g e i s a b o u t 70 kV. L i t e r a t u r e ( 1 3 . 1 ) I E C 156 (1963) Method f o r t h e d e t e r m i n a t i o n o f t h e e l e c t r i c s t r e n g t h o f i n s u l a t i n g o i l s . ( 1 3 . 2 ) I E C 296 (19821, S p e c i f i c a t i o n forunusedmineralinsulating o i l s f o r t r a n s f o r m e r s and s w i t c h g e a r .

14.

TEMPERATURE-RISE

TEST

Purpose o f t h e measurement

The

purpose

i s

t o check t h a t t h e temperature rises of t h e 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 t a n d a r d s

.

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 h e same a s i n load l o s s measurement ( S e c t i o n 4 ) and i n t h e r e s i s t a n c e measurement ( S e c t i o n 3 ) . I n a d d i t i o n thermometers a r e used f o r t h e measurement of t h e t e m p e r a t u r e o f t h e o i l , c o o l i n g medium and t h e ambient temperature and f u r t h e r a temperature r e c o r d e r and Pt-100 r e s i s t i v e s e n s o r s are used f o r t h e measurement o f c e r t a i n t e m p e r a t u r e s and f o r e q u i l i b r i u m c o n t r o l .

Performance o f t h e measurement

The t e s t is performed by u s i n g 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 t e m p e r a t u r e a r e measured.

R e s i s t a n c e s a r e measured between l i n e t e r m i n a l s e . g . , A-B and 2A-2B. The winding temperature i s t h e same a s t h e o i l t e m p e r a t u r e .

Determination o f t h e t e m p e r a t u r e r i s e o f o i l

--------p--------

-The power t o be s u p p l i e d t o t h e transformer i s t h e sum of t h e no-load l o s s e s and l o a d l o s s e s on t h e tapping on which t h e temperature-rise t e s t i s t o be performed ( g e n e r a l l y t h e maximum l o s s t a p p i n g ) . With t h i s power t h e t r a n s f o r m e r i s warmed up t o thermal e q u i l i b r i u m . The supply v a l u e s and t h e temperatures o f d i f f e r e n t p o i n t s a r e r e c o r d e d 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 t e m p e r a t u r e r i s e above t h e c o o l i n g medium temperarur can be c a l c u l a t e d from :he equilibrium t e m p e r a t u r e s .

Determination o f t h e t e m p e r a t u r e r i s e o f windings

---

-

-

Without i n t e r r u p t i n g t h e supply t h e c u r r e n t i s reduced t o r a t e d c u r e ? ? f o r 1 h. The supply v a l u e s and t h e t e m p e r a t u r e s 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 t h e h o t - r e s i s t a n c e measurement i s performed. The t e s t c o n n e c t i o n i s changed f o r c a r r y i n g o u t t h e

r e s i s t a n c e measurement and a f t e r t h e i n d u c t i v e e f f e c t s have d i s a p p e a r e d t h e resistance-time-curves a r e measured f o r a s u i t a b l e p e r i o d o f tr-a

( z e r o time i s t h e 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 o b t a i n e d by

e x t r a p o l a t i n g t h e r e s i s t a n c e - t i m e -curves t o t h e i n s t a n t o f s w i t e h l r ; o f f . The temperature r r s e s of t h e windings above t h e o i l temperature : r e

c a l c u l a t e d on the b a s i s of t h e "hot'' and "cold" r e s i s t a n c e values and t h e o i l temperature. The temperature r i s e s o f t h e windings above t h e cooling medium temperature a r e found by adding t h e temperature r i s e of o i l above the cooling medium temperature t o t h e before mentioned winding temperature r i s e s .

For multi-winding transformers t h e l a t t e r p a r t of t h e temperature r i s e t e s t is generally c a r r i e d o u t s e v e r a l times i n o r d e r t o determine the individual winding temperature r i s e s a t the s p e c i f i e d loading

combination.

For air-cooled transformers with n a t u r a l a i r c i r c u l a t i o n t h e temperature of the cooling medium i s t h e same a s the ambient temperature. The

ambient temperature is measured by means of a t l e a s t t h r e e thermometers, which a r e placed a t d i f f e r e n t p o i n t s around t h e transformer a t a

distance defined by the s t a n d a r d s approximately half-way up the

transformer. For forced-air cooled transformers t h e temperature of t h e ingoing a i r i s measured. If water is used a s cooling medium, t h e water temperature a t the intake o f t h e cooler

is

t h e r e f e r e n c e temperature. The top o i l temperature i s measured by a thermometer placed i n a n o i l - f i l l e d thermometer pocket on t h e cover o r i n t h e tube leading t o - t h e coolers. If transformer has s e p a r a t e cooler, t h e top o i l temperature is measured from the tube l e a d i n g t o t h e cooler n e a r t h e transformer.

Furthermore t h e temperatures of t h e o i l coming from o r going t o t h e transformer a r e measured and a l s o some other temperatures which may be i n t e r e s t r n g .

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

Results

The temperature r i s e s a r e c a l c u l a t e d a s follows: O i l temperature r i s e

-

-

The temperature r i s e of t o p o i l

a

i s t o r a t e d l o s s e s P 75OC

+

PO k

power s u p p l i e d during the t e s t exponent according t o the standard top o i l temperature