Biddle MEGGER - Page 49

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‘ansformers

Acceptable

insulation

resistance

values

for

dry

and

com-

ound-filled

transformers

should

be

comparable

to

those

for

lass

A

rotating

machinery,

although

no

standard

minimum

alues

are

available.

|

Oil-filled

transformers

or

voltage

regulators

present

a

special

roblem

in

that

the

condition

of

the

oil

has

a

marked

influence

n

the

insulation

resistance

of

the

windings.

In

the

absence

of

more

reliable

data

the

following

formula

is

aggested:

τος

R=minimum

l-minute

500-volt

d-c

insulation

resistance

in

megohms

from

winding

to

ground,

with

other

winding

or

windings

guarded,

or

from

winding

to

winding

with

core

guarded.

_C=a

constant

for

20°C

measurements.

|

E=voltage

rating

of

winding

under

test.

va=rated

capacity

of

winding

under

test.

|

For

tests

of

winding

to

ground

with

the

other

winding

or

vindings

grounded,

the

values

will

be

much

less

than

that

లు

by

the

formula.

R

in

this

formula

is

based

on

dry,

acid-

ree,

sludge-free

oil,

and

bushings

and

terminal

boards

that

τε

in

good

condition.

Values

of

C

at

20°C.

60-cycle

25-cycle

|

Tanked

oil-filled

type

1.5

1.0

|

Untanked

oil-filled

type

30.0

20.0

|

Dry

or

compound-filled

type

30.0

20.0

|

The

foregoing

formula

is

intended

for

single-phase

trans-

ormers.

If

the

transformer

under

test

is

of

the

three-phase

type,

ind

the

three

individual

windings

are

being

tested

as

one,

hen:

E=voltage

rating

of

one

of

the

single-phase

windings

(phase

to

phase

for

delta

connected

units

and

phase

to

|

neutral

for

star

connected

units.)

|

kva=rated

capacity

of

the

complete

three-phase

winding

under

test.

|

[471