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 applicable f o r t e s t i n g o f d i s t r i b u t i o n transformers 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 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
4.
Measurement o f impedance v o l t a g e and load l o s s
5.
Measurement of no-load
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
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
l o s s and c u r r e n t
3-1.
..3-2
5-1.
..S-3
6-1.
. .6-2
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 of the i n s u l a t i n g o i l
Pages
CONTENTS
14.
Temperature-rise test
15.
Lightning impulse test
16.
Test with lightning impulse, chopped on the tail X )
17.
Switching impulse test
18.
Partial discharge measurement
19.
Measurement o f acoustic sound level X ) on request
List of equipment
X) X)
X) X) X)
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 on 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 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 o f rwlnalng U- C 300 kV ~!iform i n s u l a t i o n
Tests -Separate-source
Sectlon withstand t e s t
-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 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 ) -Lightning impulse t e s t -for l i n e t e r m i n a l s ( t y p e t e s t ) -for neutral terminal (special
7 6
15
test U- c 3 0 0 kV m Non-uniform insulation
-Separate-source w i t h s t a n d t e s t corresponding t o i n s u l a t i o n level of neutral (routine t e s t )
7
-Induced l i n e - t o - e a r t h overvoltage 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 )
6
1)
-Lightning i n p u l s e t e s t -for l i n e terminals (type t e s t ) -for neutral terminal (special
15
test U-% 300 kV ~ % i - u n i of m insulation
T e s t i n g according t o method 1 , s e e IEC 76-3 U , 3 300 kV Non-unif orm insulation
-
T e s t i n g according L
!,G
- - L , - .
t ! ! r
7
l-rwcr
L.
IEC 76-3
"chopped-wave
SYP
-Separate-source w i t h s t a n d t e s t corresponding t o i n s u l a t i o n level o f neutral (routine t e s t )
7
-Induced l i n e - t o - e a r t h o v e r 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 single-phase t e s t s (routine t e s t )
6
l) -Lightning x p u l s e t e s t -for l i n e terminals ( r o u t i n e -for neutral terminal (special test) -Separate-source w i t h s t a n d t e s t corresponding t o 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 )
15
-Switching impulse t e s t f o r l i n e terminals (routine t e s t )
17
7
l) 15 -Lightning i x p u l s e t e s t -for l i n e terminals ( r o u t i n e t e s t ) -fcr n e u t r a l t e r m l n a l ( s p e c i a l test J - P a r t i a l d i s c h a r g e measurement 18 (routine testj2)
Lightning i e p u l s e :ast
' ) ~ a r o t h e r c a r e g o r i s s o f windings :he a special t e s t .
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.
p a r t i a l Cischarge 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 is t h e r a t i o of v o l t a g e s ( i n three-phase 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 of 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 value 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 transformer standard ( g e n e r a l l y 0.5 %). The connection symbol of t h e transformer i s checked a t t h e same time. 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 is 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 corresponding 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 provided 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 balanced, t h e v o l t a g e r a t i o o f t h e decade transformer 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 the voltage 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 zero1 sequence v o l t m e t e r , U, supply voltage o f t h e bridge; U secondary 2 voltage o f t h e transformer.
S i n c e t h e measuring d e v i c e i s a single-phase 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 time. I t is 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 bridge does n o t alway: correspond 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 important t o observe 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 voltage. For example, the v o l t a g e r a r i o o f a 120/21 kV Yd-connected t r a n s f o r m e r is 120000: \/3/21000 V = 3.299. The r e a d i n g o ~ t a i n e afrom 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 lof t h e transformer is 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 specified ratios.
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 Terminals
U/V
LV-side Main voltage Terminals
U/V
Voltage between HV and LV terminals Terminals
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
WwWw
1
WwWv=Ww
2
WwWv
3
WwWv
4
WwWvUV
5
Ww=Vw>WvUV
6
Ww>Vw=WvUV
7
Ww>VwUV
8
Ww>VwWw>UV
9
Ww>VwWw≥UV
10
Ww>VwWw
11
Ww=VwWw
The vector group symbol is _____________________________________
U/V
3 . NEASUREMENT OF WINDING 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 bridge.
Fig. 3-1 Resistance measurement using Thomson-bridge.
R dec
RV
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 X and R a r e compared by 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 N a s t a n d a r d r e s i s t a n c e -. 7 The a c c G a c y i s b e t t e r than +-0.1 % and 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 is 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 is 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 is o b t a i n e d by determining t h e average o i l temperature. The average o i l temperature is o b t a i n e d 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 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 resistance of the 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 is 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 nt 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. Test r e s u l t The r e s i s t a n c e v a i ~ e sand 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 . Literature (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 The ohmic r e s i s t a n c e o f e a c h w i n d i n g is measured between 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 phase and a t a l l t a p p i n g s of 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 is m e a s u r e d . Test 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 measurement 2 3
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 Ammeter Voltmeter
4
Transformer under test
1
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 ohmic winding 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 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 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 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 t h e measurement The measurement i s c a r r i e d o u t t o determine t h e load-losses 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 furthermore 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 load-loss measurement. 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 transformer t o be 1 1 2 t e s t e d , T current transformers, T voltage transformers, P wattgecers, 3 4 1 P ammeters (r.rn.s. v a l u e ) , P voltmeters (r.m.s. v a l u e ) , C c a p a c i t o r 2 3 1 bank. G
The supply 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 ) . Current 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 - ~ epower s u p p l i e d by 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 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 1 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 e = power r e a d from t h e meters
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 load 1 K = correction
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) is shown a s a s e t of 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 are generally different. 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 the N' power P 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 km 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 The c o r r e c t i o n s a r e made a s f o l l o w s :
20
Fig. 4-2 The correction caused by the phase displacement of instrument transformers.
181
Q02
10 8
Q03 Q04
6
0.06
4
K correction in percent, 6 6.1 phase displacement U
0.08 0,lO
-
in minutes, cos power factor of the measurement. The sign of K is the same as that of SU
- hi.
OJ5
2
0.20 1 Q8
0.30
0,L0
0.6
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. at the measuring temperature are calculated using The d.c. losses P 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
am
at the measuring temperature are
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)
-% and U
'km
= 100 km
n'
is the measured short-circuit voltage corrected according to 'km Equation (4.3); U is the rated voltage and S the rated power. The ! t losses and X is the short circuit reactance Xk does not depend on e 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 to 75OC are obtained as follows:
235O for Copper 225O for Aluminium
and the short circuit impedance Z Now the short circuit resistance Rkc kc at the reference temperature can be determined:
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 additional losses P ac load losses Pkc short circuit resistance R short circuit reactance X kc short circuit impedance Zkc kc
Literature
( PDC ) (PA) (PK (RK) (XK) (ZK)
5. MEASUREMENT OF NO-LOAD
LOSS AND CURRENT
Pur~oseof 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 3 1.11).
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, t h e y a r e g e n e r a l l y ignored. 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 = kl = = = U' =
measured i r o n l o s s e s coefficient relating t o hysteresis losses 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 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 voltmeter s c a l e d t o r e a d t h e r . m . s . value 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
P
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 shape 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 supply. 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 , it i s n e c e s s a r y t o apply 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 sinusoidal. 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 value 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 correspond 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 p1 P2
= 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
conditions = r a t i o , expressed 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
losses t o t o t a l iron losses = r a t i o , expressed a s a p e r c e n t a g e , o f eddy-current
losses
t o t o t a l iron losses
The l o s s v a l u e corresponding t o s t a n d a r d c o n d i t i o n s i s obtained from t h e measured v a l u e P a s f o l l o w s : 0
I t is assumed t h a t f o r o r i e n t e d s h e e t s p
1
= p2 = 50 %.
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 phases a r e u s u a l l y d i f f e r e n t ( t h e power can 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 nonuniformly 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 120 seconds. rated frequency 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.
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 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.
6-1-B TESTING OF POWER TRANSFORMERS
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 withstand t e s t on non-uniformly i n s u l a t e d winding o f three-phase transformer 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 3 value ) , P v o l t m e t e r ( p e a t value?. G
4
The t e s t connection shown i n Fig. 6-1 i s a p p l i c a b l e t o three-phase transformers 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 terminal 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 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 phases i n succession. During each 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 is e q u a l t o t h e r a t e d withstand voltage. 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 conjunction 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 according t o t h i s v o l t m e t e r . Test report 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 report
.
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, measurements should be taken at U2 = 1,3 Um.
during
the
phase-to-phase
test,
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:
6-2-A TESTING OF POWER TRANSFORMERS
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.
6-2-B TESTING OF POWER TRANSFORMERS
7 . SEPARATE-SOURCE
VOLTAGE WITHSTAND 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 overvoltages which may occur i n s e r v i c e . Test c i r c u i t
Fig. 7-1.
T e s t c i r c u i t f o r separate-source v o l t a g e w i t h s t a n d t e s t supply 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 transformer under t e s t , c1 u r r e n t t r a n s f o r m e r , L 1compensating reactor: E v o l t a g e d i v i d e r , ammeter, P voltmeter ( 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 G
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 with 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 values. The peak-voltmeter i n d i c a t e s t h e peak value 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 according 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 list (Section 20). Performance The t e s t i s made with single-phase 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 seconds 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 tank of 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 of 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 according t o S e c t i o n 6.5. Test report The t e s t v o l t a g e , frequency and t e s t d u r a t i o n a r e s t a t e d i n t h e 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 supply 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 transformer 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 with 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 , with 85 % o f t h e r a t e d a u x i l i a r y supply 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 transformer 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 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 possible t h e r a t e d c u r r e n t of 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 with 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 is as 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 tapping.
9. MEASUREMENT OF ZERO-SEQUENCE IMPEDANCE
Purpose o f t h e measurement The zero-sequence impedance is u s u a l l y measured f o r a l l star-connected 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 supplying 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 times t h e impedance measured i n t h i s way. The zerosequence 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 current calculations. 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 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 currenk t r a n s f o r m e r , P v o l t m e t e r , ammeter, I t e s t 3 2 3 current.
6
The zero-sequence impedance i s dependent on t h e c u r r e n t flowing 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 is 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 of 3 . I . However, t h i s i s not 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 temperature 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 . Result 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 transformer h a s a three-limb c o r e and no delta-connected windings, t h e zero-sequence impedance is 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 times 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 and extreme t a p p i n g s a r e s t a t e d .
I.
10. CAPACITANCE MEASUREMENT
Purpose of the measurement 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.
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 measured. when wiz&
-
The capacitance K from the interconnected windings No. 1 and 12 2 to earth is measured.
between earth and winding No. 2 is No. 1 is earthed.
.
and CZ0 a r e determined by solving the The p a r t i a l capacitances C C s e t of equations ( 10. l 1. 3 3 . For transformers with three 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
..+PO.
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 t h e measurement a r e divided by 3 . Literature (10.1)
Bertula, T . , Palva , V . : Transformer capacitances, S a k o E l e c t r i c i t y i n 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 t h e measurement is 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 . This i s a f u n c t i o n o f t h e moisture and impurity 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 transformer. This measurement g i v e s information 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 adequately small. 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 is 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 connecting 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 , while 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 readings and R a r e taken 15 S and 60 S a f t e r connecting t h e v o l t a g e . The 60 a sorbtion r a t i o R i s normally 1 , 2 3 i n d r i e d tranformers. The 60:~15 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 stated i n the report.
...
The readings should be taken after 15s160s1180s and 600s! The absorbtion ratio ~ 6 0 \R1 : 5 is not covered by any recognised standard. The polarisation index R600 :R60 (in IEEE called R10 : R I ) shall be higher than 1.l ! \
Readings shall be refered to 20 "C by multiplying the reading at ambient temperature T(hbienoby correction factor given in the table below.
Guide for information only! Not covered by any recognised standard.
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 other. 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.
Fig. 12-1
Valid f o r t h e series-connection:
Obtained f o r t h e p a r a l l e l connection tans 2
P
=
= a
1
S
.
R
!,+tan26, c ? tan-
=
l "
=
S
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.
12-2-A
TESTING OF POWER TRANSFORMERS
C R P A C I TQNCE & POWER FkCTOR
Measuring Bridge : l.
Teitex
- 5chering
kasurmeni o f t h e resulting caoacitances 50 H: at 25°C kasuring voltace 10 hV ,
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
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%
DF
It ransforber-ran~ earthed)
CRPRCI TANCE R POWER FRCTDR
1.5 Hi@rvoltaoe t o g w - II LV-Windirpc! earthed HV-Windity
2.
tan
= 0.32t:
tan
= 6.314s
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)
d
(=
2511 &/Phase)
6310 DF
(=
2IW OFIPhase)
212 pr'
(=
71 $;Phase)
1237 d
(=
402 $/Phase)
C=:;
(EX (:)+CX (2)-CX (4))E=
C4= (CX(l)+CT(6)
E= (CX (4)+CX (5)-CX (6)-EX (2)1/l= C&= (CX(4)+CX (5)-CX (1)-CX13)) /L-
753,'
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 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 (13.1). 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 being 2 kV/s. The e l e c t r i c s t r e n g t h is t h e average 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 is about 70 kV. Literature (13.1)
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 of t h e e l e c t r i c strength of i n s u l a t i n g o i l s .
(13.2)
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 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 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 standards
.
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. 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 .
The
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
calculated on the basis of t h e "hot'' and "cold" r e s i s t a n c e values and the 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 the temperature r i s e t e s t is generally carried o u t s e v e r a l times i n order 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 the temperature of the cooling medium is 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 standards approximately half-way up the transformer. For forced-air cooled transformers t h e temperature of t h e 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 t h e 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 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 leading t o t h e cooler near t h e transformer. Furthermore the 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 interestrng. 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
to
is
r a t e d l o s s e s 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 o f o i l going i n t o t h e c o o l e r 6 3 = temperature o f o i l coming from t h e c o o l e r Temperature r i s e o f windings The average temperature of o i l iS
'
0
before t h e h o t - r e s i s t a n c e measurement
The average temperature o f winding+_ is
23S°C f o r Copper 22S°C f o e Aluminium cold r e s i s t a n c e hot r e s i s t a n c e t h e average temperature o f o i l during c o l d r e s i s t a n c e measurement The average temperature r i s e 8 of t h e winding above t h e oil r0 temperature i s
= r a t e d c u r r e n t o f the winding It = t e s t current y = exponent according t o t h e s t a n d a r d
N'
The average temperature r i s e a of the winding above t h e ambient r temperature i s
The temperature r i s e 8 o f t h e hot spot o f t h e winding above t h e hS ambient temperature i s
The winding temperature i n d i c a t o r , i f any, w i l l be adjusted on t h e b a s i s o f t h e temperature r i s e 8 hs ' Results
-
c o l d r e s i s t a n c e values and the corresponding o i l temperature
-
temperatures o f o i l and cooling medium i n thermal equilibrium and t h e corresponding l o s s e s
-
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
R 1 charging resistor C R series resistor S R a low-ohmic discharging
resistor for switching impulse,
R high-ohmic discharging resistor for switching impulse
...F main spark-gaps, ... an auxiliary spark-gaps . Fal 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 ( 1 2 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~aarrilc~sri tile ~.eqillr'ed v ~ ~ l u eLr.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 theC ' circuit by the auxiliary spark-gap F the pgtential difference across a1 ' the breakdown of F is the spark-gap F rises considerably and
-
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.
Cr resulting impulse capacitance, Rsr resulting series resulting discharge resistance, L L stray resistance, R inductances, input capacitance of transformer,'lp l transformer inhctance, C capacitance of voltage dlvider, F
er
----l.
'. ---- -. rrpa
-0
:--..l
.,,tp...==
--
protective resistor.
1
---,.--&A-
=ca,C..
.".,
Fa calibratf i i i ; p k t ~ ~ ~ g n' ~p,
The required impulse shape is obtained by selecting the series and discharge resistors of the generator suitably.
2
The front time can be calculated approximately from the equation:
and the time to half value from the equation:
1 and Cr. The factor k depends on the quantities Rsr* Rar' L.
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 P' 2 peak oscilloscope, voltmeter, R terminal the resistance measuring cable, R damping resistor oh voltage divider, C high 1 voltage capacitor of voltage divider, C2 low voltage capacitor of divider.
OF
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(15.G!. d
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 singleterminal 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~gha 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 F r o n t time Time t o h a l f - v a l u e
T1 = 1.2
"S
+- 30 %.
T2 = 50 bs +- 20 %.
I n p r a c t i c e t h e impulse shape may d e v i a t e from t h e s t a n d a r d impulse when t e s t i n g low-voltage windings o f h i g h r a t e d power and windings o f h i g h input capacitance. The v o l t a g e measurement i s based on t h e r e a d i n g o f t h e peak v o l t m e t e r . If r e q u i r e d t h e v o l t a g e measuring system, i n c l u d i n g t h e peak v o l t m e t e r , i s c a l i b r a t e d by means o f sphere-gap, i n c o n n e c t i o n with t h e t e s t i n g o f the f i r s t l i n e terminal. I n t e s t i n g the o t h e r terminals the voltage. measurement i s based on t h e r e a d i n g of t h e c a l i b r a t e d peak v o l t m e t e r . The v o l t a g e c a l i b r a t i o n i s performed a t 60 % o f t h e v o l t a g e t e s t l e v e l . O s c i l l o g r a p h i c r e c o r d s a r e made o f t h e a p p l i e d v o l t a g e and t h e v o l t a g e a c r o s 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 d u r i n g c a l i b r a t i o n a t 62.5 % of t h e v o l t a g e t e s t l e v e l and d u r i n g t h e 100 % v o l t a g e a p p l i c a t i o n s . A t f u l l t e s t v o l t a g e each l i n e t e r m i n a l i s t e s t e d w i t h a s many impulses a s i s r e q u i r e d by t h e s t a n d a r d . I n o r d e r t o f a c i l i t a t e t h e d e t e c t i o n o f n n c c i b l e d i c c r e ~ a n c i ein r--.-~ the c s c i l l o g- r ~ z f i i c r $ ~ c r 5t h~e ~~ = ~ i a t t e n u a t i o n i s a d j u s t e d such t h a t t h e c u r v e s r e c o r d e d d u r i n g 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 c o i n c i d e w i t h t h o s e o b t a i n e d d u r i n g the calibration.
1
Unless agreed otherwise d i f f e r e n t t a p p i n g s a r e s e l e c t e d f o r t h e impulse t e s t s on t h e t h r e e phases o f a three-phase transformer, u s u a l l y t h e two extreme t a p p i n g s and t h e p r i n c i p a l t a p p i n g . Test report The summary o f t e s t r e s u l t s i s given on a form termed "Report o f impulse v o l t a g e withstand t e s t on transformer". The o s c i l l o g r a p h i c r e c o r d s and measurement r e c o r d s a r e s t o r e d i n t h e a r c h i v e s , where t h e y a r e a v a i l a b l e when r e q u i r e d . Literature (15.1)
IEC Publ. 60-3 ( 1 9 7 6 ) : High-voltage P a r t 3: Measuring d e v i c e s .
t e s t techniques.
(15.2)
I E C Publ. 60-4 ( 1 9 7 7 ) : High-voltage t e s t t e c h n i q u e s . P a r t 4: A p p l i c a t i o n guide f o r measuring d e v i c e s .
(15.3)
IEC Publ 52 ( 1960) : Recommendations f o r v o l t a g e measurement by means o f s p h e r e gaps.
(15.4)
I E C Publ. 60-2 ( 1 9 7 3 ) : High-voltage t e s t techniques. P a r t 2 : T e s t procedures.
(15.5)
I E C Publ. 76-3 ( 1 9 8 0 ) : Power t r a n s f o r m e r s . 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 i s t o s e c u r e t h a t t h e t r a n s f o r m e r i n s u l a t i o n s w i t h s t a n d t h e v o l t a g e s t r e s s e s caused by chopped l i g h t n i n g impulse, which may o c c u r i n s e r v i c e . T e s t i n g 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 d e t e c t i o n c o n n e c t i o n s a r e used a s f o r t h e s t a n d a r d l i g h t n i n g impulse t e s t . The impulse i s chopped by means o f a t r i g g e r e d - t y p e chopping gap connected t o t h e t e r m i n a l t o which t h e impulse i s a p p l i e d . The d e l a y 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 i s a d j u s t a b l e , t h u s t h e time T from t h e s t a r t o f t h e impulse t o t h e chopping can be a d j u s t e d (fig. 16-11. Performance o f t h e t e s t The t e s t is performed w i t h impulses o f n e g a t i v e p o l a r i t y . The d u r a t i o n T from t h e b e g i n n i n g o f t h e impulse t o t h e chopping can vary w i t h i n t h e C range o f 2 . . . 6 PS ( F i g . 16-1). According t o t h e s t a n d a r d ( 1 6 . 1 ) t h e amount o f overswing t o o p p o s i t e p o l a r i t y s h a l l b e l i m i t e d t o n o t more t h a n 30 % o f t h e a m p l i t u d e o f t h e chopped impulse ( F i g . 16-1). If n e c e s s a r y t h e overswing amplitude w i l l be l i m i t e d t o t h e v a l u e 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.ltimes the amplitude of full impulse.
F i g . 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 v o l t a g e measurement is based on t h e peak voltmeter i n d i c a t i o n . 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 o f a sphere-gap. The t e s t with chopped l i g h t n i n g impulse is combined with t h e t e s t c a r r i e d o u t with standard impulse. The following o r d e r of p u l s e a p p l i c a t i o n s is recommended by t h e s t a n d a r d (16.1)
-
one one one two two
62.5 % f u l l impulse 100 % f u l l impulse o r more 62.5 % chopped impulses 100 % chopped impulses 100 % f u l l impulses
The f a u l t d e t e c t i o n is a l s o f o r chopped impulses p r i m a r i l y based on t h e comparison o f voltages and winding c u r r e n t s obtained a t 62.5 % c a l i b r a t i o n v o l t a g e s and 100 % t e s t voltages. I n o r d e r t o make t h e comparison o f f a u l t d e t e c t i o n oscillograms obtained a t 100 % v o l t a g e 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 t h e f a u l t d e t e c t i o n i s a d d i t i o n a l l y secured s i n c e t h e t e s t sequence i n c l u d e s t h e a p p l i c a t i o n o f two s t a n d a r d impulses a f t e r t h e a p p l i c a t o n o f t h e chopped impulses. A t high t e s t v o l t a g e s (> 750 kV) t h e r e is a small delay i n t h e i g n i t i o n s of t h e chopping-gap, which causes d i f f e r e n c e s i n t h e f a u l t d e t e c t i o n and c a l i b r a t i o n oscillograms o f v o l t a g e s and winding c u r r e n t s . I n t h i s c a s e t h e f a u l t d e t e c t i o n must be based p r i m a r i l y on t h e recordings obtained a t t h e a p p l i c a t i o n of f u l l impulses. When c a r r y i n g o u t t h e chopped-impulse t e s t , u n l e s s otherwise agreed, d i f f e r e n t tappings a r e s e l e c t e d f o r t h e t e s t s on t h e t h r e e phases o f a three-phase transformer, u s u a l l y t h e two extreme tappings and t h e p r i n c i p a l tapping. Test r e o o r t The t e s t v o l t a g e values, impulse shapes, tappings and t h e number o f 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 t h e r e p o r t . The o s c i l l o g r a p h i c records and measurement r e c o r d s a r e s t o r e d i n t h e a r c h i v e s , where they a r e a v a i l a b l e when r e q u i r e d . Literature (16.1)
I E C Publ. 76-3 (1980): Power transformers, 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 .
17. SWITCHING IMPULSE TEST Purpose o f t h e t e s t The purpose o f t h e switching impulse 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 s between windings, between windings and e a r t h , between l i n e terminals and e a r t h and between d i f f e r e n t t e r m i n a l s withstand t h e switching o v e r v o l t a g e s , which may occur i n s e r v i c e . Performance of t h e 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 i g h t n i n g impulse t e s t a r e used h e r e . According t o t h e s t a n d a r d ( 1 7 . 1 ) t h e s w i t c h i n g impulse t e s t is c a r r i e d o u t on each l i n e t e r m i n a l of a three-phase winding i n sequence. A single-phase no-load t e s t connection i s used i n accordance with Fig. 17-1. The voltage developed between l i n e t e r m i n a l s during t h e t e s t is approximately 1 . 5 times t h e t e s t voltage between l i n e and n e u t r a l terminals. The f l u d e n s i t y i n t h e magnetic c i r c u i t i n c r e a s e s considerably during t h e t e s t . When t h e c o r e reaches s a t u r a t i o n t h e winding impedance i s d r a s t i c a l l y reduced and a chopping o f t h e a p p l i e d voltage t a k e s p l a c e ( F i g . 17-2). The time t o s a t u r a t i o n determines t h e d u r a t i o n o f t h e switching impulse. Because t h e remanent f l u x can amount t o even 70 t o 80 % of t h e s a t u r a t i o n f l u x , t h e i n i t i a l remanence of t h e c o r e has a g r e a t i n f l u e n c e on t h e v o l t a g e d u r a t i o n . By i n t r o d u c i n g remanent f l u x of opposite p o l a r i t y i n r e l a t i o n t o t h e f l u x caused by t h e switching impulse, t h e maximum p o s s i b l e switching impulse d u r a t i o n can be increased. The remanence o f opposite p o l a r i t y i s introduced i n t h e core by applying low v o l t a g e impulses of o p p o s i t e p o l a r i t y t o t h e transformer before each f u l l v o l t a g e t e s t impulse.
Fig. 17-1 Transformer switching impulse t e s t i n g and f a u l t d e t e c t i o n 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 Time above 90 % Time to the first zero passage
O T1 > ~ O Ys Td > 200 ks Tz > 500 PS
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
d i s t u r b a n c e s caused by corona d i s c h a r g e s i n t h e t e s t c i r c u i t may be found on t h e c u r r e n t o s c i l l o g r a m s recorded a t t e s t v o l t a g e .
The f a u l t d e t e c t i o n is mainly based on t h e v o l t a g e o s c i l l o g r a m s . The t e s t is s u c c e s s f u l i f no sudden c o l l a p s e o f v o l t a g e caused by f l a s h o v e r o r breakdown i s i n d i c a t e d on t h e v o l t a g e o s c i l l o g r a m s and no abnormal sound e f f e c t s a r e observed. ' h e n t h e core r e a c h e s s a t u r a t i o n a s l i g h t n o i s e caused by m a g n e t o s t r i c t i o n can be heard from t h e t r a n s f o r m e r . Test report The t e s t v o l t a g e v a l u e s , impulse shapes, and number o f impulses a t d i f f e r e n t v o l t a g e 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 o s c i l l o g r a p h i c r e c o r d s a r e s t o r e d i n t h e a r c h i v e s , where they a r e a v a i l a b l e when required. Literature (17.1)
IEC Publ. 76-3 (1980): Power t r a s f o r m e r s . 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. 18l ) .*
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 v o l t a g e U a c r o s s C h a s i n c r e a s e d enough, t h e spark-gap C i g n i t e s . The c a p a c ~ t a n c eC s i s c h a r g e s and t h e v o l t a g e d i f f e r e n c e a c r o s s C t h e c a v i t y vanishes w i t h i n 1...1000 n s . The d i s c h a r g e magnitude o r apparent charge q and t h e v o l t a g e Uc a r e r e l a t e d by t h e following equation:
The d i s c h a r g e g i v e s r i s e t o a c u r r e n t p u l s e , which c a u s e s a f a s t v o l t a g e change a t t h e t e r m i n a l s o f t h e t r a n s f o r m e r ; t h i s change can be measured by means o f 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 and a p u l s e t r a n s f o r m e r .
Fig. 18-2 Analogue c i r c u i t of a gas-filled cavity.
The p a r t i a l d i s c h a r g e s do n o t l e a d t o an immediate breakdown. They have, however, o t h e r e f f e c t s on t h e i n s u l a t i n g medium:
-
t h e s u r f a c e o f t h e d i e l e c t r i c is bombarded by i o n e s , which cause t e m p e r a t u r e - r i s e 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 m a t e r i a l Chemical changes may g i v e r i s e t o m a t e r i a l components, which speed up ageing. On t h e o t h e r hand t h e p a r t i a l d i s c h a r g e s may a l s o be e x t i n g u i s h e d by t h e i n f l u e n c e o f some o t h e r degradation p r o d u c t s
-
d i s c h a r g e s 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 d i s c h a r g e site.
These phenomena r e s u l t i n d e g r a d a t i o n of t h e d i e l e c t r i c p r o p e r t i e s o f t h e i n s u l a t i n g medium, and i n c r e a s e of l o s s e s . The o b j e c t of t h e p a r t i a l d i s c h a r g e s measurement i s t o r e v e a l t h e above mentioned weak p a r t s o f t h e d i e l e c t r i c , ,which may cause d e s t r u c t i o n o f J-he t-..--*--.l-r -A." I* -.." " lI a l l b .
--
;
...A
----.4
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.
feeding generator t r a n s f o r m e r t o be t e s t e d pulse transformer s t e p up t r a n s f o r m e r compensating r e a c t o r s low-pass f i l t e r s 1 Z t e r m i n a l r e s i s t o r s o f measuring c a b l e 2 W measuring c a b l e s 1 E c a p a c i t i v e voltage divider G 1 T 1 T 2 T 3 L 1 Z
P ammeters 1 P volt-meter (peak v a l u e ) 2 P oscilloscope 3 P volt-meter 4 Z reactance 3
The f e e d i n g and measuring instruments used a r e described on a s e p a r a t e measuring instrument l i s t ( S e c t i o n 2 0 ) . Performance o f t h e measurement The measurement i s based on observing and e v a l u a t i n g t h e apparent charge LEC 76-3. The neasuring system i s i n a c c ~ r d z n c cx i t n tne standard ( : Y . t j j b a s i c a l l y a wide-band system, but a narrow-band instrument can be connected t o t h e system i f necessary.
Stability test Due t o i n t e r n a l c a p a c i t a n c e s , t h e v o l t a g e on t h e high v o l t a g e s i d e o f t h e t r a n s f o r m e r under t e s t may r i s e t o a n unacceptably high v a l u e when connecting t h e g e n e r a t o r t o t h e f e e d i n g c i r c u i t . For t h i s r e a s o n t h e s t a b i l i t y o f t h e g e n e r a t o r v o l t a g e c o n t r o 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 v o l t a g e e q u a l t o h a l f t h e measurement v o l t a g e . Therefore, spark-gaps a r e connected between t h e high v o l t a g e t e r m i n a l s and e a r t h . The spark-gaps a r e s e t a c c o r d i n g t o t h e maximum permissable 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 t h e c a l i b r a t i o n measurement ( F i g . 18-31 a n a p p a r e n t charge q i s i n j e c t e d between each high v o l t a g e t e r m i n a l and e a r t h . The v o l t g g e pulse caused by t h e i n j e c t e d charge i s measured by means o f an o s c i l l o p e with t h e a i d o f p u l s e t r a n s f o r m e r s connected t o t h e t e s t t a p o f t h e bushings. The highThe r e a d i n g on t h e o s c i l l o s c o p e corresponds t o t h e charge q v o l t a g e s i d e o f t h e step-up t r a n s f o r m e r is e a r t h e d d u r i n g tRis measurement.
.
F i e . 18-3
Calibration. calibration generator, wfiich produces c h a r g e p u l s e s of magnitude q .
C
0
P a r t i a l d i s c h a r g e measureaent
The v o l t a g e i s i n c r e a s e d s t e p w i s e , f i r s t up t o t h e measuring v o l t a g e U 2' when t h e occurance o f d i s c h a r g e s i s checked. The t e s t v o l t a g e i s i n c r e a s e d t o t h e p r e - s t r e s s v o l t a g e l e v e l U and h e l d t h e r e f o r a 1
d u r a t i o n o f 5 seconds. The p r e - s t r e s s v o l t a g e is a p p l i e d i n o r d e r t o i g n i t e t h e d i s c h a r g e s . T h e r e a f t e r , the v o l t a g e is r a p i d l y reduced t o U2 and maintained a t t h i s value f o r t h e agreed d u r a t i o n o f time t (Fig. 18-51. During t h i s p e r i o d t h e occurence of d i s c h a r g e s is beingm%ecked a t t h e t e r m i n a l s o f t h e transformer. I f d i s c h a r g e s occur, t h e r e s u l t s a r e recorded i n o r d e r t o determine t h e d i s c h a r g e magnitudes. I f t h e r e a r e d i s c h a r g e s a t t h e v o l t a g e l e v e l U t h e v o l t a g e i s decreased stepwise a f t e r t h e d u r a t i o n o f time Tmes i n o r g e r t o determine t h e e x t e n s i o n voltage. The v o l t a g e measurement i s c a r r i e d o u t a t t h e h i g h v o l t a g e s i d e 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 T e s t v o l t a g e U pre-stress voltage 1 U measuring v o l t a g e 2
U. p a r t i a l discharge inception voltage l U p a r t i a l discharge extinction voltage e
According :G tha standard (18.6) I F Z 76-3 tile ~ e s is i c a r r i e r i c u r , ..:?g t h e following v a l u e s of t e s t v o l t a g e s between l i n e and n e u t r a l ter-:nais and t e s t p e r i o d d u r a t i o n s :
-
-
U1 U2
t, +L
mes
= um = e l t h e r 1.3 urn/V3 with q or l . 5 urnf13 with q
= 5 min = 30 min
< ? -
300 pc 500 pc
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 a duration of at least 60 min when Um ≥ 300 kV or 30 min for Um < 300 kV 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. The duration of the test, except for the enhancement level U1, shall be independent of the test frequency.
A B C D E
= = = = =
5 min 5 min test time 60 min for Um ≥ 300 kV or 30 min for Um < 300 kV 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 / √3 U2 = 1,5 Um / √3 NOTE For network conditions where transformers are severely exposed to over-voltages, values for U1 and U2 can be 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 a long time, the test is regarded as non-destructive. A failure to meet the partial discharge acceptance criteria shall therefore not warrant immediate rejection, but lead to consultation between purchaser and supplier about further investigations as described in IEC 60076-3. 18-5-A
TESTING OF POWER TRANSFORMERS
When t h e t e s t i s c a r r i e d o u t a s a s p e c i a l t e s t , t h e t e s t procedure can be s e p a r a t e l y agreed upon.
Test r e p o r t
Test is special test up to Um=170kV, and is routine ltest for windings with Um=245kV and above.
A summary o f 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 i s s t o r e d i n t h e a r c h i v e s , and i s then a v a i l a b l e when requested. Literature ELECTRA No. 19, November, 1971. ELECTRA No. 11, December, 1969. Brown, R . D . , Corona measurement on high v o l t a g e apparatus using t h e bushing capacitance t a p . IEEE Trans. Power Apparatus and Systems 84 ( 1 9 6 5 ) , 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 t h e picocoulomb and microvolt f o r corona measurements on h.v. transformers and o t h e r apparatus. IEEE Paper T 72086-2, 1972. IEC P u b l i c a t i o n 270, 1968. P a r t i a l d i s c h a r g e measurements. I E C Publ. 76-3 ( 1 9 8 0 ) : Power Transformers. 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 .
Auswertung der Me8ergebnissebei sdrmalbandigw M m g mit dem RIV
Berucks~chtigungder Freauem der PrGfsoannung und der Frequent d e s fi-m-rt nacb CISPR) I
Wertetabelle Mr qo = 500 PC
@wmrWn9sbJf'J~
19. MEASUREMENT OF ACOUSTIC SOUND LEVEL Puraose o f t h e measurement The purpose o f t h e sound l e v e l measurement i s t o check t h a t t h e sound l e v e l o f t h e t r a n s f o r m e r 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 t a n d a r d s , e.g. ( 1 9 . 1 ) o r ( 1 9 . 2 ) , o r g u a r a n t e e v a l u e s g i v e n by t h e t r a n s f o r m e r manufacturer. A sound spectrum a n a l y s i s i s c a r r i e d o u t f o r t h e transformer a t t h e c u s t o m e r ' s r e q u e s t . The sound spectrum i n d i c a t e s t h e magnitude o f sound components (measured a t a given band-width) a s a f u n c t i o n of frequency. Measurine e a u i ~ m e n t A p r e c i s i o n sound l e v e l meter complying w i t h s t a n d a r d s (19.11, ( 1 9 . 2 )
and ( 1 9 . 3 ) i s used i n t h e sound l e v e l measurements. The measilrements a r e performed u s i n g t h e weightning curve A . The sound spectrum a n a l y s i s o f t h e t r a n s f o r m e r i s c a r r i e d o u t by r e c o r d i n g t h e sound band l e v e l s a u t o m a t i c a l l y a s a f u n c t i o n o f frequency. T h i s i s done with t h e a i d of an a n a l y s e r , which i s both mechanically and e l e c t r i c a l l y connected t o t h e r e c o r d e r 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 d e s c r i b e d i n a s e p a r a t e l i s t of equipment (Section 20). Performance o f t h e measurement The measurement i s c a r r i e d o u t a t measuring p o s i t i o n s l o c a t e d around t h e t r a n s f o r m e r a s d e t a i l e d i n t h e s t a n d a r d s ( 1 9 . 1 ) , (19.2) and ( 1 9 . 3 ) . According t o t h e s t a n d a r d s ( 1 9 . 1 ) and ( 1 9 . 3 ) t h e microphone p o s i t i o n i n t h e v e r t i c a l d i r e c t i o n s h a l l be on h o r i z o n t a l p l a n e s a t one t h i r d and two t h i r d s o f one t r a n s f o r m e r t a n k h e i g h t , when t h e h e i g h t o f t h e tank i s e q u a l t o o r g r e a t e r t h a n 2 . 5 m . When t h e t a n k h e i g h t is l e s s t h a n 2 . 5 m , and when t h e measurement i s c a r r i e d o u t i n accordance w i t h t h e s t a n d a r d (19.21, t h e measuring p l a n e i s l o c a t e d a t h a l f t h e t a n k h e i g h t . The microphone i s d i r e c t e d p e r p e n d i c u l a r l y a g a i n s t t h e s u r f a c e o f t h e transformer ( 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 t h e t r a n s f o r m e r sound l e v e l measurement t h e background n o i s e l e v e l i s measured. P r e f e r a b l y t h e background l e v e l s h o u l d 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 t h a n 9 dB(A) b u t n o t l e s s than 3 dB(A), a c o r r e c t i o n f o r background l e v e l w i l l be a p p l i e d according t o s t a n d a r d s (19.11, ( 1 9 . 2 ) and ( 1 9 . 3 ) . The t r a n s f o r m e r w i l l be l o c a t e d a t t h e 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 a n c e from t h e t r a n s f o r m e r 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 large. The measurement i s c a r r i e d o u t a t r a t e d v o l t a g e 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 . C o r r e c t i o n s f o r background Level and environmental c o r r e c t i o n a r e made t o t h e mean v a l u e .
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 s o u n d l e v e l Acoustic sound l e v e l measurements, 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 s o u n d p r e s s u r e l e v e l o r A-weighted s o u n d 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 with IEC 551. T h i s I E C ~ e g u l a t i o nc o r r e s p o n d s . t o D I N 4 5 6 3 5 , 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 m o s t i m p o r t a n t c o n d i t i o n s f o r s o u n d l e v e l m e a s u r e m e n t s a r e a s follows: 1 . T h e t e s t o b j e c t m u s t be e x c i t e d w i t h r a t e d v o l t a g e a n d r a t e d
frequency, i.e. f o r transformers:
no-load
excitation.
2 . The f a n s a n d pumps ( i f f i t t e d ) o f t h e c o o l i n g s y s t e m 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 e s 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 measuring 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 test o b j e c t is e x c i t e d i n accordance with l . ,
and t h e transformer cooling devices described i n 2. a r e i n operation, t h e measuring d i s t a n c e i s 2 m.
Microphone positions
5 . The h e i g h t o f t h e m i c r o p h o n e H i s f o r t a n k h e i g h t s h h h ~ 2 . 5m: H = -Z, and f o r t a n k h e i g h t s h 2 2 . 5 m: H = and
$h
6 . The s o u n d l e v e l meter meets t h e r e q u i r e m e n t s o f I E C 651 i n a c c o r d a n c e w i t h D I N 45 6 3 3 .
Attenuator I
Attenuator
I1
Block d i a g r a m o f t h e s o u n d p r e s s u r e m e t e r 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 m e a s u r e m e n t s 7.1 The a m b i e n t A-weighted s o u n d l e v e l s h o u l d b e a t l e a s t 1 0 dB below t h e s o u n d l e v e l o f 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 o f t h e test object.
