1. ELECTRICAL AND CHEMICAL DIAGNOSTICS OF TRANSFORMER INSULATION

2. CONTENTS INTRODUCTION EXPERIMENTAL TECHNIQUES
CONVENTIONAL ELECTRIC TESTS
IPS MEASUREMENTS
GPC ANALYSIS
RESULTS AND DISCUSSIONS
CASE STUDY 1:POWER TRANSFORMER
CONVENTIONAL ELECTRIC TESTS RESULT
INTERFACIAL POLARIZATION SPECTRA(IPS) RESULTS
GPC TEST RESULTS
RESULTS FROM PRESSBOARD SAMPLES

3. CASE STUDY 2:DISTRIBUTION TRANSFORMERS
CONVENTIONAL ELECTRIC TESTS RESULTS
INTERFACIAL POLARIZATION TEST RESULTS
GPC TEST RESULTS
Advantages and disadvantages of IPS measurements
Advantages and disadvantages of GPC
Goals of future diagnostic methods
CONCLUSION
REFERENCES

4. INTRODUCTION
Reliability of a power system depends on trouble free transformer operation
Significant no of power transformers are operating beyond their design life
Need improved diagnostic techniques for determining the condition of the insulation in aged transformers
Insulation system consists of cellulosic materials and mineral oil
Cellulosic materials degrade with time
In the final stage of the ageing the electric insulation system breaks down and the power equipment falls out.
Condition of the paper and pressboard insulation has been monitured by
A) bulk measurements
B) measurements on samples removed from the transformer
DC method is used for measuring the interfacial polarization spectrum

5. FACTORS AFFECTING THE DEGRADATION AND AGEING OF PAPER
MOISTURE IN PAPER
OXYGEN IN PAPER
TEMPERATURE

6. EXPERIMENTAL TECHNIQUES
CONVENTIONAL ELECTRIC TESTS
Dissipation factor & capacitance measured using a schering bridge
Power frequency breakdown strength was measured by using the step by step method
Negative lightning impulse breakdown strength was measured

7. Figure 1. The Schering Bridge
The capacitive reactance of a capacitor C is 1/2πfC.
R3 = C1R2 / C2.
C3 = R1C2 / R2.
A Schering Bridge is a bridge circuiit used for measuring an unknown electrical capacitance and its dissipation factor. The dissipation factor of a capacitor is the the ratio of its reesistance to its capacitive reactance

8. 2)INTERFACIAL POLARIZATION SPECTRA(IPS) MEASUREMENTS
designed and built at School of Information technology and Electrical Engineering, University of Queensland by Dr Saha and team.
Using return voltage measurements
automatic IPS measurement system

9. IPS system consists of
1) An optic isolation power control interface used for control two relays.
2) A programmable high voltage DC power supply.
3) A communication protocol interface.
4) A PC to control the entire set of instruments

10. THEORY
When a direct voltage is applied to a dielectric for a long period of time, and it is then short circuited for a short period, the charge bounded by the polarization will turn into free charges after opening the short circuit, a voltage will build up between the electrodes on the dielectric. This phenomena is called the return voltage.
P(t) = P0 F(t),
P0 = αE
Ja(t) = P(t) = d/dt P (t).
Ja (0) = βE

11. Initial slope is the slope of the return voltage graph (with linear approximation) for
first few seconds
central time constant -the time at which the return voltage is maximum
a larger value of the maximum initial slope indicates the sample has more moisture content.
central time constant with smaller value indicates the sample has more moisture content and is more degraded and more aged.

12. 3) GEL PERMEATION CHROMATOGRAPHY(GPC) ANALYSIS
Gel permeation chromatography provides a detailed molecular weight distribution of the polymer.
chromatographic technique uses highly porous, non-ionic gel beds for the separation of polydispersed polymers in solution.
separates polymer molecules on the basis of their hydrodynamic volume.
Separation occurs via the porous beds packed in a column
total volume: Vt = Vg + Vi + Vo
If a sample has a broad molecular weight range it may be necessary to use several GPC columns in tangent with one another to fully resolve the sample

13. Gels GPC instrument includes Eluent sample holder Gels Eluent Pump
Detector
Gels
used as stationary phase for GPC
absence of ionizing groups
low affinity for the substances to be separated
Sephadex Bio-Gel , agarose gel , Styragel
Eluent
The eluent should be a good solvent for the polymer
should wet the packing surface.
THF, o-dichlorobenzene at room temperature
trichlorobenzene at 130–150 °C for crystalline polyalkines
m-cresol and o-chlorophenol at 90 °C for crystalline condensation polymers such as polyamides and polyesters.

14. Pump Detector constant supply of fresh eluent to the column
for uniform delivery of relatively small liquid volume of eluent
Detector
To detect weight of polymer in the eluting solvent
concentration sensitive detectors
Molecular weight sensitive detectors

15. waters chromatograph A typical Waters GPC instrument including
A. sample holder,
B.Column
C.Pump
D. Refractive Index Detector
E. UV-vis Detector

16. The inside of sample holder of Waters GPC instrument
cellulose tricarbanilate derivative was prepared
The molecular weight distribution of the cellulose tricarbanilate was measured using a Waters Chromatograph equipped with a variable wavelength tunable absorbance detector. Four ultrastyragel columns were used in series in the Chromatograph, with THF as the eluent. The elution profile was acquired by interfacing to an IBM computer.

17. RESULTS AND DISCUSSIONS
CASE STUDY 1
POWER TRANSFORMER

18. A 25 year old, 25 MVA, 132/11 kV transformer from Kareeya power station, was used to investigate the quality of the insulation using electrical and chemical testing techniques.
To examine the differences that exist between the high stress and low stress insulation samples, the samples were collected from top, middle and bottom coils of low voltage and high voltage windings of the transformer
With two insulated conductors placed side by side to form the specimen, the thickness of paper insulation between them was 1.0 mm and 0.8 mm for the HV and LV specimens respectively. Pressboard (of 0.2 mm thickness) samples were collected from the main bulk insulation between the high voltage and low voltage winding

19. Figure 2: Schematic diagram of one phase of LV winding of 25 MVA Kareeya transformer

20. CONVENTIONAL ELECTRIC TEST RESULTS
Table 1: Results of conventional electrical tests on samples from LV A phase winding of Kareeya transformer

21. Table 2: Results of conventional electrical tests on samples from HV B phase of Kareeya transformer.

22. IPS TEST RESULTS
Table 3: Results of IPS measurements on samples from LV A phase winding of Kareeya transformer

23. Table 4: Results of IPS measurements on samples from HV B phase winding of Kareeya transformer

24. GPC TEST RESULTS
Figure 3: GPC chromatogram of insulating paper samples obtained from new stock and aged transformer

25. Figure 4: The simulation chromatogram of the new insulating paper

26. Table 5: Results from GPC analysis on samples from LV A phase of Kareeya transformer

27. Table 6: Results from GPC analysis on samples from HV B phase of Kareeya transformer

28. Table 7: Results obtained from aged and new press board:
RESULTS FROM PRESSBOARD SAMPLES
Table 7: Results obtained from aged and new press board:

29. Table 9: GPC Results obtained from aged and new pressboard Sample
Table 8: Results of IPS measurements on new and aged pressboard samples
Table 9: GPC Results obtained from aged and new pressboard Sample

30. CASE STUDY 2 DISTRIBUTION TRANSFORMERS

31. CONVENTIONAL ELECTRICAL TEST RESULTS
Results of Dissipation factors and capacitances of distribution transformers

32. IPS TEST RESULTS
Results of IPS measurements of the single phase distribution transformers
Results of IPS measurements of the three phase distribution transformers

33. GPC TEST RESULTS
Table 14: GPC Results of cellulose samples from distribution transformers

34. Advantages and disadvantages of IPS measurements
less easily disturbed by interference in the field
a non destructive test
disadvantages
takes quite a significant amount of time for simulation and plotting.
Adsorbed moisture and temperature of the oil-paper insulation adversely affects the return voltage measurement. So the return voltage measurement was always conducted at a known and low oil-paper moisture content and at ambient environmental conditions (20 — 25° C).

35. Advantages and Disadvantages of GPC
has a well-defined separation time
a lower chance for analyte loss to occur
most samples can be thoroughly analyzed in an hour or less
quick and relatively easy estimation of molecular weights and distribution for polymer samples
Disadvantages
limited number of peaks that can be resolved within the short time scale
GPC requires around at least a 10% difference in molecular weight for a reasonable resolution of peaks to occur
that filtrations must be performed before using the instrument
pre-filtration of the sample has the possibility of removing higher molecular weight sample

36. Goals of future diagnostic methods
Political goals:
1.Reduction of environmental risks by an early sign for a developing fault.
2.Technical information in time.
3.More safety and less stress for service personnel.
Economic goals:
1.Adaptive maintenance
2.Reduction of maintenance personnel
3.Planned service interruption and investments
4.Minimum period of outrage
5.Minimum costs for outrage
Technical goals:
1.Optimization of apparatus or systems
2.Registration of sporadic fault-behaviour by continuous monitoring
3.Quantitative information about certain parameters
4.Correlation of measured parameters with maintenance intervals and remaining
lifetime
5.Improvements in insulation coordination

37. CONCLUSION
The globalisation of the energy market results in an increasing cost consciousness of utilities and other electrical power equipment operators. Power transformers are among the most expensive parts of a power network. Therefore, utilities try to postpone replacement investments for those equipment and even try to cutback maintenance costs. On the other hand, numerous transformers have reached a considerable age well beyond 30 years. Further operation means an increasing risk of an outage. A damage of a transformers means not only costs for replacement but also considerable extra costs for lost sales of energy and environmental damages

38. REFERENCES IEEE transactions on power delivery October 1997.
“Degradation of electrical insulating paper monitored with high performance liquid chromatography” IEEE transaction on electrical insulation august 1990.
“Thermal ageing of cellulose paper insulation” IEEE transaction on electrical insulation February 1997.