*{r

What is transformer Noise 7

The main source of sound in a transformer is a magneto-striction. This is the property ofelectrical sheet steel which causes it to elongate and contract in the presence of the magneticfield. These periodic mechanical movements produce sound vibrations in the core, having afundamental frequency equal to twice the flux frequency. Due to a non-linear relationshipbetween magneto-striction and flux density a number fo harmonically related higherfrequency are also produced. This results in a noise spectrum, composed of fundamentalfrequencies of 120 cycles per second and harmonics of 240,360,480, 600 etc.

The pulsations of the core also travel through other parts of the transformer, which have a

tendency to vibrate at one or other of these same frequencies. These add to the overall soundlevel in the form of interlaminar core and coil vibrations, enclosure, and sub-assemblyvibrations. The amount of this addition is dependent upon the surface area and resonantfrequency of each component part.

How to control transformer sound and minimize its transmission - As outlined above. thereare fbur main sources of sound in a transformer:

1.

2.aJ.

4.

Core vibrationsInterlaminar core and gap vibrations and coil vibrationsSubassembly vibrationsEnclosure vibrations

Never Make Megger Tests under Vacuum or Without Oil

The great the vacuum (unless it's perfect) the lower the spark-over level. At the vacuumvalues normally reached in the field, it is very likely the megger readings will be zero; inother words, creeping or flashing to gound will occur. Breakdown strength under vacuum is afunction of both pressure and distance.

It is not our intent at this point to go into a long discourse on the phenomenon of voltagebreakdown but simply to CAUTION you not to megger a unit while under vacuum.

It is also recommended that you do not megger a unit in air because the values may lead towrong conclusions and will not agree with readings taken at alater date when they are takenunder oil. Readings taken in oil are about ll2 to 114 that of the insulation resistance in air.

Evaluation of Insulation

Megger readings, power factor readings, and oil dielectric breakdown values are importantindicators as to whether or not a transformer is wet. The dielectric strength of oil lor highvoltage power transformers must be 30kv or better before it is put into a new unit in the field.The megger readings on high voltage power transformers, when corrected to 20oC, should be1 megohm per kv or higher.

*V

.fu'

On IIHV transformers a minimum value of 3 megohms per kv is requircd. Several oneminutes readings should be taken with a motor driven megger and the megohm resultscompared with factory data. Power factor should be less than 2Yo on oil-filled transformerswith two windings and less than 2-ll2oh on units with load tap changers w.hen corrected to20"c

Insulation

There is not much that can be done in the way of maintenance of the solid insulation of oil-insulated transformers and regulators, but correct load and temperature control can do muchto keep insulation aging within normal limits. Aging of insulation is affected by the operatin_etemperature of the windings and the condition of the oil in which it is submerged. Oil of highacidity will accelerate again of the solid insulation. The aging rate of organic insulationdoubts for approximately each 8oC increase in its temperature the lower the operatingtemperature, the longer the insulation life.

Since aging is so much more rapid at the higher range of temperature, excessively hightemperatures should be avoided as much as possible. On the other hand, little is gained byhaving insulation life exceed the obsolescence life of the apparatus. Maximum lif'e is assuredonly under uniform, normal loading. Therefore, loading should be controllcd as much aspossible to obtain that objective.

The Secondary of a current transformer (CT) should not be open-circuitccl under anycircumstances. Why?

If the CT secondary becomes open circuited, a high voltage will exit across the secondarybecause the large ratio of secondary to primary turn causes the transformer to act as a step-uptransformer. Also, since the counter ampere-turns of the secondary no longer exist, the flux inthe core, instead of being due to the difference of the primary and secondary ampere-tLlrns,will now be due to the total primary ampere-turns acting alone. This causes a large incr-easein the flux, producing excessive core loss and heating, as well as a high voltage across thesecondary terminals.

/-- - a {l

d' ,r

ri i',^t i'' i 'i-{.'i .,,

,,1 t ..i't ,, o{' -{ r. t,s 1 r y i'',u'"

'' ,

i ',r'r" ! ,.t . { I ii. ,';-,,. ,,r,!,

, ,' ,. ,, ., : ii( ,.-i r , r.,;*. .' '

-t'i/i+, { :.r ,tr\\

Jt

;,; ' Va,')

-".1t! ..* i,..1,/,:/ i,f i"r.' dL- . ^) .,

.,,.,-.";:, y/,., j,-* r*l.i.,1 1. - 1 g j'.r d/rr 1..

; /^,l! , , t !

. '*.;, Irkr '' "

. ,tri

lr

j4.'f '

lr

sir

f,,