1)
BDV of Transformer Oil
The oil sample is subjected to a steadily increasing
alternating voltage until breakdown occurs in a BDV test kit. The breakdown
voltage is the voltage reached at the time of the first spark appears between
the electrodes. The test is carried out six times on the same cell filling and
the electric strength of the oil is the arithmetic means of the six results
obtained. The electrodes are mounted on a horizontal axis with a test spacing
of 2.5 mm.
The recommended values as follows:
Electric Strength (BDV)
|
50 KV (Min)
|
Above
170 KV to 420 KV
|
40 KV (Min)
|
Above 72.5KV to 170KV
|
|
30 KV (Min)
|
Upto 72.5KV
|
2)
Specification for uninhibited
Mineral Insulating Oil-New/ Unused Before Filling In Transformer/ Switchgear
Sl.No
|
Characteristics
/ Property
|
Standard Values
|
1
|
Appearance
|
Clear & transparent Free
from suspended matter or sediments
|
2
|
Density at 29.5º C, Max.
|
0.89 g/cm3
|
3
|
Kinematic Viscosity at 27º C,
Max.
|
27 cst
|
4
|
Interfacial Tension (IFT) 29.5º C,
Min.
|
0.04 N/m
|
5
|
Flash point, Pensky Martin
(Closed), Min.
|
140º C
|
6
|
Pour point, Max.
|
-6º C
|
7
|
Acidity, Neutralisatin Value
-
Total
Acidity, Max.
-
Inorganic
acidity/Alkalinity
|
0.03 mg KOH/g
NIL
|
8
|
Corrosive Sulphur
|
Non-corrosive
|
9
|
Di-electric Strength (Breakdown
Voltage) Min.
- New unfiltered oil
- After filtration
|
30 KV, rms
60 KV, rms
|
10
|
Dielectric Dissipation Factor
(Tan δ)
DDF at 90º C, Max.
|
0.002
|
11
|
Specific Resistance
(resistivity)
At 90º C, Min.
At 27º C, Min.
|
35 x 1012 ohm -cm
1500 x 1012 ohm-cm
|
12
|
Oxidation Stability
Neutralisation value after
oxidation, Max.
|
0.40 mg KOH/gm
|
13
a
b
c
d
|
Total sludge after oxidation,
Max.
Ageing Characteristics after
accelerated ageing (Open Breaker method with copper catalyst)
Specific Resistance
(resistivity)
- At 27º C, Min.
- At 90º C, Min.
DDF at 90º C, Max.
Total Acidity, Max.
Total Sludge Value, Max. % by
weight
|
0.10% by weight
2.5 x 1012 ohm -cm
0.2 x 1012 ohm -cm
0.2
0.05
0.05
|
14
|
Presence of Oxidation Inhibitor
|
Max. 0.05% treated as absence of
oxidative inhibitor
|
15
|
Water content
- New unfiltered oil
- After filtration
|
50 ppm
15 ppm
|
16
|
PCB Content
|
< 2 ppm
|
17
|
SK Value
|
4 to 8%
|
18
|
Dissolved Gas Analysis (DGA)
|
Not applicable
|
Recommended Limits of unused Mineral Oil in New Power Transformers
as per Standard
Sl.No
|
Property
|
Power transformer upto 72.5 KV
|
Power transformer above 72.5 KV To 170 KV
|
Power transformer above 170 Kv
|
01
|
40 KV
|
50 KV
|
||
02
|
Flash
point
|
140° C
|
140° C
|
140° C
|
03
|
Pour
point
|
- 6 ° C
|
- 6 ° C
|
- 6 ° C
|
04
|
Water
content
|
20 ppm
|
15 ppm
|
10 ppm
|
05
|
Neutralisation
value ( mg KOH/ g)
|
0.03
|
0.03
|
0.03
|
06
|
Di –
electric dissipation factor at 90° C
|
0.015
|
0.015
|
0.010
|
Measurement of Insulation
Resistance
Recommended Values :-
Related
Voltage Class
of
Winding
|
Min.
IR value at
one
minute
|
11
KV
|
300
Meg. Ohm
|
33
KV
|
400
Meg. Ohm
|
66 KV
and above
|
500
Meg. Ohm
|
Measurement of Polarisation Index
If moisture is present in a
system, the leakage current increases at a faster rate than the absorption
current and the mega ohm readings will not increase with time as fast with
insulation in poor condition as with insulation in good condition. This results
in a lower PI.
Polarisation Index
|
Insulation Co-ordination
|
<
1
|
Dangerous
|
1.0
to 1.1
|
Poor
|
Above
1.1 to 1.25
|
Questionable
|
Above
1.25 to 2.0
|
Fair
|
Above
2
|
Good
|
