LEGEND FOR CODE LEGEND FOR INTERPRETATION SENSITIVITY OF TESTS
First Letter – Nature of the defect or fault, function affected
Second letter – component 1 Good identification
T – Thermal B – Bushing 2 Fair identification
D – Dielectric C – Core 3 Good detection and rough identification
M – Mechanical D – Diverter, tap changer 4 Fair detection
C – Contamination or aging O – Oil 5 Rough detection
S – Selector, tap changer (on-load and off-load) 6 Complementary test T – Tank and accessories
W- Winding, major insulation and leads
NOTE: The following table is sorted first by Defect/Fault Code and then by Component Code. Using Microsoft Word, it could also be sorted first by Component and then by Defect. To do this, place the cursor inside the table, select Table/Sort, and then specify "Sort by" Component and "Then by" Defect.
Defect/
Fault Code
Component Code
Description INDIVIDUAL
TESTS
(Recommended test or group of tests noted by asterisks)
INTERPRETATION/
SENSITIVITY OF INDIVIDUAL
TESTS
METHODOLOGY AND LIMITATIONS Ref.
C B 1-Aged insulation Power factor/tan delta* (1) (IEC 137)
DGA* (2) (IEC 567)
Partial discharge
5 6 4
Power factor/tan delta part of routine test program. DGA only performed if not a risk in sampling and re-sealing the bushing. Sampling restrictions due to limited oil volume. Compare results of power factor/tan delta tests between different phases and with commissioning tests.
Defect/
(Recommended test or group of tests noted by asterisks)
INTERPRETATION/
SENSITIVITY OF INDIVIDUAL
TESTS
METHODOLOGY AND LIMITATIONS Ref.
C B 2-Internal surface Insulation resistance (1)
Power factor/tan delta and IR would be normal test supported by DGA where considered appropriate. Sampling restrictions due to limited oil volume. Compare results of power factor/tan delta tests between different phases and with commissioning tests.
C B 3-External surface Power factor/tan delta*
(IEC 137)
Routine test. Compare results of power factor/tan delta tests between different phases and with commissioning tests.
Limitation: Separation of bushing from transformer
Further test would be performed after high Power factor/tan delta reading or suspected moisture entry (e.g., cracked gauge glass).
Compare results of power factor/tan delta tests between different phases and with commissioning tests.
Reliable only if C2 is the capacitance between the last capacitive layer and the flange.
[100]
Limitation: Less suitable for detecting wear of contacts
Defect/
(Recommended test or group of tests noted by asterisks)
INTERPRETATION/
SENSITIVITY OF INDIVIDUAL
TESTS
METHODOLOGY AND LIMITATIONS Ref.
C O 1-Oxidation and
Temperature of oil should be measured in order to determine % saturation.
C O 3-Abnormal Electric or acoustic PD Pump bearing monitor*
2 3 3 6
If transformer has oil pumps, particle count should be made after they have been turned on.
[100]
Limitation: Less suitable for detecting wear of contacts. Some experience that 100 amps
measuring current is needed for satisfactory result
C T 1-Leaks Visual
DGA
Neutralization number Dissolved metals
2
4 DGA will detect presence of oxygen and nitrogen.
Defect/
(Recommended test or group of tests noted by asterisks)
INTERPRETATION/
SENSITIVITY OF INDIVIDUAL
TESTS
METHODOLOGY AND LIMITATIONS Ref.
C T 2-Corrosion Visual 3
Moisture level in paper*
Estimate through Power
Moisture in oil should measure dissolved and bounded water.
Water heat run test in Case 1 of reference #100 was for 54 hours with oil temperature of 65º C.
Power factor/tan delta and RVM spectrum can be influenced by ion conductivity of oil.
[100]
Electric or acoustic PD Insulation resistance analysis will validate if CO is from paper.
DP requires internal access-paper samples limited to accessible parts of winding.
[100]
Defect/
(Recommended test or group of tests noted by asterisks)
INTERPRETATION/
SENSITIVITY OF INDIVIDUAL
TESTS
METHODOLOGY AND LIMITATIONS Ref.
C W 5-Oil particle Electric or acoustic PD Dissolved metals
D B 1-Tracking Power factor/tan delta*
DGA*
Insulation resistance Change in power factor/tan delta, losses, and C1* Change in power factor/tan delta, leakage current and sum current*
For ON cooling, oil samples from different locations may assist in locating the fault.
D W 3-Partial discharge
For ON cooling, oil samples from different locations may assist in locating the fault.
D W 4-Partial discharge
For ON cooling, oil samples from different locations may assist in locating the fault.
D W 5-Streaming
electrificaion
DGA*
Electric or acoustic PD*
4 4
Defect/
(Recommended test or group of tests noted by asterisks)
INTERPRETATION/
SENSITIVITY OF INDIVIDUAL
TESTS
METHODOLOGY AND LIMITATIONS Ref.
D W 6-Inter-turn problem Turns ratio*
Magnetizing current*
Winding resistance DGA
Electric or acoustic PD*
FRA
Transformer usually tripped from protection for this type of fault
D W 7-Inter-strand
DGA may not always detect cellulose
involvement, fault will only be evident when on load.
Acoustic transducers may be able to detect gas bubbles and help in locating the fault area.
[102]
Infrared if external, DGA if internal
M C 1-Open circuit in
On-line monitor: motor amps at 2 kHz, relay timing*
3 6
M S 1-Mechanical
problems
Visual
On-line monitor: motor amps at 2 kHz, relay timing*
3
4 Occurrence of through-faults may cause damage to shielding.
[100]
[105]
M W 1-Loose clamping Leakage reactance*
Capacitance change
Single phase reactance measurement recommended.
[100]
Defect/
(Recommended test or group of tests noted by asterisks)
INTERPRETATION/
SENSITIVITY OF INDIVIDUAL
TESTS
METHODOLOGY AND LIMITATIONS Ref.
M W 2-Axial movement,
Single phase reactance measurement recommended.
[102]
M W 3-Radial movement Leakage reactance*
FRA/Transfer function analysis*
5 4
Single phase reactance measurement recommended.
M W 4-Spiral movement Leakage reactance*
FRA/Transfer function analysis*
5 4
Single phase reactance measurement recommended.
power factor/tan delta C1
reduction at 10kV power factor/tan delta C1
imbalance current
Defect/
(Recommended test or group of tests noted by asterisks)
INTERPRETATION/
SENSITIVITY OF INDIVIDUAL
TESTS
METHODOLOGY AND LIMITATIONS Ref.
T S 1-Pyrolytic carbon
growth (bad contact, coking)
DGA*
DC resistance (transient)*
Electric or acoustic PD
On-line temperature differential*
Infrared scan of tank
4 5
Excluding GIC case, monitoring gas levels at different loads will assist in identifying the problem. DGA (consumption of oxygen)
Visual*
Measure oil temperature
4 1
1 Monitor temperature vs load for cooling conditions.
Combustible gas generation increases with load.
Operation at no load can also provide information.
To aid in locating a localized hotspot, it may be necessary to review the transformer design (estimated winding and lead temperatures).
[100]
T W 4-Overheated joint DGA*
DC resistance*
Electric or acoustic PD*
3 3 6
Generally, gas level increases with load.