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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



WwWwUV



9



Ww>VwWw≥UV



10



Ww>VwWw 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.