Maintenance Guidelines for Power Transformers


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
 
BDV
 30 KV
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.  

           PI = 10 minutes IR value/1 minute IR value

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