ELECTRICAL AND CHEMICAL DIAGNOSTICS OF TRANSFORMER INSULATION
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
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
1) 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
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
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
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 = aE
Ja(t) = P(t) = d/dt P (t).
Ja (0) = ÃƒÅ¸E
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
GPC instrument includes
sample holder 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
The eluent should be a good solvent for the polymer
should wet the packing surface.
THF, o-dichlorobenzene at room temperature
trichlorobenzene at 130â€œ150 Ã‚Â°C for crystalline polyalkines
m-cresol and o-chlorophenol at 90 Ã‚Â°C for crystalline condensation polymers such as polyamides and polyesters.
constant supply of fresh eluent to the column
for uniform delivery of relatively small liquid volume of eluent
To detect weight of polymer in the eluting solvent
concentration sensitive detectors
Molecular weight sensitive detectors
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.
CASE STUDY 1
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
CASE STUDY 2
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
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
Goals of future diagnostic methods
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.
2. Reduction of maintenance personnel
3. Planned service interruption and investments
4. Minimum period of outrage
5. Minimum costs for outrage
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
5. Improvements in insulation coordination
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