Megger DELTA 4000, Reference Manual

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28 DELTA 4000 ZM-AH02E
3 TESTING POWER SYSTEM COMPONENTS
Circuit breakers
Introduction
Circuit-breakers are designed to interrupt either normal
or short-circuit currents. They behave like big switches
that may be opened or closed by local push-buttons or by
distant telecommunication signals emitted by the system
protection. Thus, circuit-breakers will automatically open
a circuit whenever the line current, line voltage, frequency,
etc. exceeds their limit values. The most common types of
circuit-breakers are:
▪
Oil Circuit-Breakers (OCB’s)
▪
Air-Blast Circuit-Breakers
▪
SF
6
Circuit-Breakers
▪
Vacuum Circuit-Breakers
▪
Air Magnetic Circuit-Breakers
▪
Oil Circuit Reclosers
The nameplate on a circuit breaker usually indicates (1) the
maximum steady-state current it can carry, (2) the maxi -
mum interrupting current, (3) the maximum line voltage,
and (4) the interrupting time in cycles. It is critical that large
currents are interrupted quickly. High speed interruption
limits the damage to transmission lines and equipment and,
equally important, it helps to maintain the stability of the
system when a contingency occurs. Also of critical impor-
tance is the insulation of the bushings and tank members
of the circuit-breakers. Power and dissipation factor testing
provides a means of verifying the integrity of the insula-
tion.
The most important insulation in medium and high-voltage
outdoor power switch gear is that of the bushings them-
selves, the guide assembly, the lift rods, and, in the case of
oil circuit breakers, the oil. Measurements should be made
from each bushing terminal to the ground tank with the
breaker open, and from each phase (each pair of phase
bushing terminals) to the grounded tank with the breaker
closed. When an individual bushing assembly is tested in
each phase, the other bushing terminal in that phase should
be guarded. It is also desirable to test samples of the liquid
insulation.
Oil circuit-breakers
Oil circuit-breakers are composed of a steel tank filled with
insulating oil. A typical three-phase oil circuit breaker has
six bushings. Three bushings channel the three-phase line
currents to a set of fixed contacts. Three movable contacts,
actuated simultaneously by an insulated rod, open and
close the circuit. When the circuit breaker is closed, the line
current for each phase penetrates the tank by way of one
bushing, flows through the first fixed contact, the move -
able contact, the second fixed contact, and then out by the
second bushing.
Test connections
There are six overall tests performed when the breaker is
open. Each bushing is individually tested in the overall GST
test mode. If the bushing is equipped with a test tap, the
C
1
main insulation test can be performed in the UST mode
along with the overall GST test without making a lead
change. Three overall tests are performed with the breaker
closed in the GST test mode.
Table 3.4
Dead tank circuit breaker test connections
Te
st
N
o
.
CB In
su
la
ti
o
n
te
ste
d
Low voltage lead
configuration
Test con-
nections
to
bushings
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
O
P
E
N
C1G GST-GND Red & Blue 1
Bushing 2
floating
2 C2G
GST-
GND
Red & Blue 2
Bushing 1
floating
3 C3G
GST-
GND
Red & Blue 3
Bushing 4
floating
4 C4G GST-GND Red & Blue 4
Bushing 3
floating
5 C5G GST-GND Red & Blue 5
Bushing 6
floating
6 C6G GST-GND Red & Blue 6
Bushing 5
floating
7 C12 UST-R Red Blue 1 2
8 C34 UST-R Red Blue 3 4
9 C56 UST-R Red Blue 5 6
10 C
L
O
S
E
D
C1G
+
C2G
GST-
GND
Red & Blue
1
or
2
11
C3G
+
C4G
GST-
GND
Red & Blue
3
or
4
12
C5G
+
C6G
GST-
GND
Red & Blue
5
or
6
Diagram
Insulation tested
1 to 6 = Bushing terminals
G = Ground
Note: No. in High Voltage column is bushing energized. Tests 1
through 6, 10, 11, and 12 all other bushings must be floating.
Connect a ground wire from the test set to the grounded
frame of the breaker.