Megger DELTA 4000, Reference Manual

Page: 17 / 48

ZM-AH02E DELTA 4000 17
3 TESTING POWER SYSTEM COMPONENTS
Figure 10: Two-Winding Transformer Test
Test connections
For all transformer testing, including spare transformers,
ensure the following safety conditions are observed:
Test connections are defined in table 3.1.
Table 3.1
Two-winding transformer test connections
Te
st
N
o
.
In
su
la
ti
o
n
te
ste
d
Low voltage
lead configuration
Test
lead
connec-
tions to
wind-
ings
R
em
ark
s
Te
st
m
o
d
e
M
ea
sur
e
G
ro
un
d
G
uar
d
H
ig
h
v
o
lt
ag
e
Re
d
B
lu
e
1
CHG+
CHL GST-
GND Red &
Blue
H L – L Grounded
2 CHG GSTg-RB
Red &
Blue
H L – L Guarded
3 CHL UST-R Red Blue H L –
4 CHL – Test 1 minus Test 2 – – –
Calculated
intercheck
5
CLG +
CHL GST-
GND Red &
Blue
L H –
H Ground-
ed
6 CLG GSTg-RB
Red &
Blue
L H – H Guarded
7 CLH UST-RB Red Blue L H –
8 LH – Test 5 minus Test 6 – – –
Calculated
intercheck
Equivalent circuit
Note: Short each winding on itself.
Measurement Inter-checks (Calcu-
lated)
Capacitance Watts
C
4
= C
1
- C
2
W
4
= W
1
- W
2
C
8
= C
5
- C
6
W
8
= W
5
- W
6
Note: Subscripts are test numbers
H
High-voltage winding
L
Low-voltage winding
G
Ground
1]
The transformer must be taken out of service and
isolated from the power system.
2]
Ensure the transformer is properly grounded to the
system ground.
3]
Before applying any voltage on the transformer
make sure that all bushing current transformers
are shorted out.
4]
Never perform electrical tests of any kind on a unit
under vacuum. Flashovers can occur at voltages as
low as 250 volts.
5]
If the transformer is equipped with a load tap
changer, set the unit to some step off of neutral.
Some load tap changers are designed with arrester
type elements that are not effectively shorted out
in the neutral position even with all the bushings
shorted.
6]
Connect a ground wire from the test set to the
transformer ground.
7]
Short all bushings of each winding including the
neutral of a wye-connected winding. The neutral
ground must also be removed. The shorting wire
must not be allowed to sag.
8]
Connect the high voltage lead to the high side
bushings for tests 1, 2, and 3. Ensure that the high
voltage cable extends out away from the bushing.
9]
Connect the red low voltage lead to the low volt-
age bushings.
10]
For tests 5, 6 and 7, connect the high voltage lead
from the test set to the low voltage bushings of
the transformer and the red low voltage lead from
the test set to the high voltage bushings.
11]
Individual tests should be performed on each bush-
ing. Bushings equipped with a potential/test tap
should have the UST test performed and the GST
on those without test taps.
Hot collar tests can if necessary be performed on
both types.