Megger DELTA 4000, Reference Manual

Page: 30 / 48

30 DELTA 4000 ZM-AH02E
3 TESTING POWER SYSTEM COMPONENTS
Table 3.5
Tank-loss index of oil circuit breakers (Equivalent
to 10 kV losses)
Tank
loss
index Test remarks Probable problem Insulation
rating
<±0.16 W Normal results
for both open
CB tests
None Good
>+0.16 W Normal results
for both open
CB tests
1. Tank oil
2. Tank liner
3. Lift rod
4. Auxiliary contact
insulation Investigate
>-0.16 W High losses for
both open CB
tests
Closed CB test
near normal
1. Cross guide assembly
2. Isolated cross guide
3. Contact assembly
insulation
4. Lift rod upper section
(moisture contami-
nated) Investigate
<±0.16 W Normal results
for one open
CB test
Other has high
losses
1. Bushing with high
loss reading
2. Arc interruption as-
sembly Investigate
<±0.16 W High losses
for both open
CB tests and
closed CB test
1. Bushings
2. Arc interruption as-
sembly
3. Tank oil
4. Tank liner
5. Lift rod
6. Auxiliary contact
insulation
7. Cross guide assembly
8. Isolated cross guide
9. Contact assembly
insulation Investigate
Oil circuit breakers are composed of many different materi-
als each having its own temperature coefficient. For this
reason it may be difficult to correct tank-loss indexes for a
standard temperature. On this basis, an attempt should be
made to conduct tests at approximately the same time of
the year to minimize temperature variations. The measure-
ments on the bushings, however, may readily be corrected
to the base temperature; usually 20°C.
Comparison of tank-loss indexes taken when an oil circuit-
breaker is new and initially installed will give the general
range of values to expect from a good unit. This practice
also will avoid condemning a good unit as the result of the
inherent design of a particular manufacturer that normally
may show tank-loss indexes without the unit being defec-
tive or deteriorated. The losses in an oil circuit-breaker are
different between an open circuit test and a closed circuit
test because the voltage stress on the insulating members is
distributed differently.
Air-blast circuit-breakers
These circuit-breakers interrupt the circuit by blowing com-
pressed air at supersonic speed across the opening contacts.
Compressed air is stored in reservoirs and is replenished by
a compressor located in the substation. There are two inter-
rupters mounted in a live tank which is then mounted on an
insulated column. The interrupting capacity of the circuit
determines the height of the column and the number of
tanks per-phase connected in series. The most powerful
circuit-breakers can typically open short-circuit currents
of 40 kA at a line voltage of 765 kV in a matter of 3 to 6
cycles on an AC line. Other designs of live tank breakers
may be of a T or Y design with one interrupter mounted in
each arm of the porcelain housing.
Test connections
Air and gas circuit breakers vary so much in construction
that specific instructions and interpretation would be too
lengthy. This section, however, does contain a detailed test
connection chart (Table 3.7) outlining the normal series of
measurements performed on a General Electric Type ATB
Air-Blast Circuit Breaker. Table 3.8 outlines the normal
series of measurements performed on a three-column live
tank breaker.
Table 3.7
General Electric Air-blast type circuit breaker test
connections
Te
st
N
o
.
In
su
la
ti
o
n
te
ste
d
Low voltage lead
configuration
Test
connec-
tions to
breaker
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 C2 + B2 UST-R Red Blue D F A A Grounded
2
C1 + B1
+ I1
UST-B Blue Red D F A F Grounded
3
C2 + B2
+ C1 +
B1 + I1 UST
Red
&
Blue
D F A
4
R (or R
+ I3)
GST
Red
&
Blue
D F A
F & A
Guarded
5 I2 + T * GST Red Blue A F D
D Guarded
F Grounded
* Test performed only on units with current transformer.