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DELTA 4000

ZM-AH02E

3 TESTING POWER SYSTEM COMPONENTS

Dry-type transformers

Testing notes

Test voltages should be limited to line-to-ground ratings

of the transformer windings. Insulation power factor tests

should be made from windings to ground and between

windings. Temperature at the time of testing should be at

or near 20°C. ANSI/IEEE C57.12.91 - 1997 recommends

correcting results other than 20°C. However, there is very

little data available for temperature correction of dry-type

transformers. Repeat tests should be performed as near as

possible, in the same conditions as the original test.
Higher overall power factor results may be expected on dry-

type transformers. The majority of test results for power

factor is found to be below 2.0%, but can range up to 10%.

The insulation materials necessary for dry-type construc-

tion, must meet the thermal and stress requirements.
If power factor results appear to be unacceptable, an ad-

ditional Tip-Up Test can be performed if a 10kV test set

is used. This test can be performed to evaluate whether

moisture or corona is present in the insulation system.

The applied test voltage is varied starting at about 1kV and

increased in intervals up to 10 kV or the line-to-ground rat-

ing of the winding insulation. If the power factor does not

change as the test voltage is increased, moisture is suspected

to be the probable cause. If the power factor increases as

the voltage is increased, carbonization of the insulation or

ionization of voids is the cause.

Note

DELTA 4000 has a specific feature where the

test set recognizes voltage dependence and
will automatically indicate a non-linear behav-
ior and by this indicate to the user to perform
a tip-up test

Bushings

Introduction

Bushings provide an insulated path for energized conduc-

tors to enter grounded electrical power apparatus. Bushings

are a critical part of the electrical system that transforms

and switches ac voltages ranging from a few hundred volts

to several thousand volts. Bushings not only handle high

electrical stress, they could be subjected to mechanical

stresses, affiliated with connectors and bus support, as well.

Although a bushing may be thought of as somewhat of a

simple device, its deterioration could have severe conse-

quences.
All modern bushings rated 23 kV and higher have a power

factor or a capacitance tap which permits dissipation factor

testing of the bushing while it is in place on the apparatus

without disconnecting any leads to the bushing. The dis-

sipation factor is measured by the ungrounded specimen

test (UST) which eliminates the influence of transformer

winding insulation, breaker arc-interrupters, or support

structures which are connected to the bushing terminal.
Figure 11 shows the test connections between the test set

and bushing when using the UST test mode.

Connect test ground to apparatus ground.

Connect the high-voltage lead to the terminal at
the top of the bushing and the low-voltage lead
(red) to the power factor tap.

Ground the apparatus tank. The tap is normally
grounded through a spring and it is necessary,
when making measurements, to remove the
plug which seals and grounds the tap. Use the
UST measure red, ground blue test mode setting
(UST-R).

The UST test also can be used for making measurements

on bushings which have provisions for flange isolation. The

normal method of isolating the flange from the apparatus

cover is to use insulating gaskets between the flange and

cover and insulating bushings on all but one of the bolts

securing the mounting flange to the cover. During normal

operation, the flange is grounded by a single metal bolt;

however, when testing the bushing, this bolt is removed.

The measurement is identical to that when testing bushings

which have a power factor tap except that the low-voltage

lead, red in this case, is connected to the isolated bushing

flange.